Posts Tagged ism

Recent Postings from ism

Investigating Nearby Star-Forming Galaxies in the Ultraviolet with HST/COS Spectroscopy. I: Spectral Analysis and Interstellar Abundance Determinations

This is the first in a series of three papers describing a project with the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure abundances of the neutral interstellar medium (ISM) in a sample of 9 nearby star-forming galaxies. The goal is to assess the (in)homogeneities of the multiphase ISM in galaxies where the bulk of metals can be hidden in the neutral phase, yet the metallicity is inferred from the ionized gas in the HII regions. The sample, spanning a wide range in physical properties, is to date the best suited to investigate the metallicity behavior of the neutral gas at redshift z=0. ISM absorption lines were detected against the far-ultraviolet spectra of the brightest star-forming region(s) within each galaxy. Here we report on the observations, data reduction, and analysis of these spectra. Column densities were measured by a multi-component line-profile fitting technique, and neutral-gas abundances were obtained for a wide range of elements. Several caveats were considered including line saturation, ionization corrections, and dust depletion. Ionization effects were quantified with `ad-hoc’ CLOUDY models reproducing the complex photoionization structure of the ionized and neutral gas surrounding the UV-bright sources. An `average spectrum of a redshift z=0 star-forming galaxy’ was obtained from the average column densities of unsaturated profiles of neutral-gas species. This template can be used as a powerful tool for studies of the neutral ISM at both low and high redshift.

Eyes in the sky: Interactions between AGB winds and the interstellar magnetic field

We aim to examine the role of the interstellar magnetic field in shaping the extended morphologies of slow dusty winds of Asymptotic Giant-branch (AGB) stars in an effort to pin-point the origin of so-called eye shaped CSE of three carbon-rich AGB stars. In addition, we seek to understand if this pre-planetary nebula (PN) shaping can be responsible for asymmetries observed in PNe. Hydrodynamical simulations are used to study the effect of typical interstellar magnetic fields on the free-expanding spherical stellar winds as they sweep up the local interstellar medium (ISM). The simulations show that typical Galactic interstellar magnetic fields of 5 to 10 muG, are sufficient to alter the spherical expanding shells of AGB stars to appear as the characteristic eye shape revealed by far-infrared observations. The typical sizes of the simulated eyes are in accordance with the observed physical sizes. However, the eye shapes are of transient nature. Depending on the stellar and interstellar conditions they develop after 20,000 to 200,000yrs and last for about 50,000 to 500,000 yrs, assuming that the star is at rest relative to the local interstellar medium. Once formed the eye shape will develop lateral outflows parallel to the magnetic field. The "explosion" of a PN in the center of the eye-shaped dust shell gives rise to an asymmetrical nebula with prominent inward pointing Rayleigh-Taylor instabilities. Interstellar magnetic fields can clearly affect the shaping of wind-ISM interaction shells. The occurrence of the eyes is most strongly influenced by stellar space motion and ISM density. Observability of this transient phase is favoured for lines-of-sight perpendicular to the interstellar magnetic field direction. The simulations indicate that shaping of the pre-PN envelope can strongly affect the shape and size of PNe.

Supernovae Driven Turbulence In The Interstellar Medium

I model the multi-phase interstellar medium (ISM) randomly heated and shocked by supernovae, with gravity, differential rotation and other parameters we understand to be typical of the solar neighbourhood. The simulations are 3D extending horizontally 1 x 1 kpc squared and vertically 2 kpc, symmetric about the galactic mid-plane. They routinely span gas number densities 1/10000 to 100 per cubic cm, temperatures 100 to 100 MK, speeds up to 10000 km/s and Mach number up to 25. Radiative cooling is applied from two widely adopted parameterizations, and compared directly to assess the sensitivity of the results to cooling. There is strong evidence to describe the ISM as comprising well defined cold, warm and hot regions, which are statistically close to thermal and total pressure equilibrium. This result is not sensitive to the choice of parameters considered here. The distribution of the gas density within each can be robustly modelled as lognormal. Appropriate distinction is required between the properties of the gases in the supernova active mid-plane and the more homogeneous phases outside this region. The connection between the fractional volume of a phase and its various proxies is clarified. An exact relation is then derived between the fractional volume and the filling factors defined in terms of the volume and probabilistic averages. These results are discussed in both observational and computational contexts. The correlation scale of the random flows is calculated from the velocity autocorrelation function; it is of order 100 pc and tends to grow with distance from the mid-plane. The origin and structure of the magnetic fields in the ISM is also investigated in non-ideal MHD simulations. A seed magnetic field, with volume average of roughly 4 nG, grows exponentially to reach a statistically steady state within 1.6 Gyr.

Supernovae Driven Turbulence In The Interstellar Medium [Replacement]

I model the multi-phase interstellar medium (ISM) randomly heated and shocked by supernovae, with gravity, differential rotation and other parameters we understand to be typical of the solar neighbourhood. The simulations are 3D extending horizontally 1 x 1 kpc squared and vertically 2 kpc, symmetric about the galactic mid-plane. They routinely span gas number densities 1/10000 to 100 per cubic cm, temperatures 100 to 100 MK, speeds up to 10000 km/s and Mach number up to 25. Radiative cooling is applied from two widely adopted parameterizations, and compared directly to assess the sensitivity of the results to cooling. There is strong evidence to describe the ISM as comprising well defined cold, warm and hot regions, which are statistically close to thermal and total pressure equilibrium. This result is not sensitive to the choice of parameters considered here. The distribution of the gas density within each can be robustly modelled as lognormal. Appropriate distinction is required between the properties of the gases in the supernova active mid-plane and the more homogeneous phases outside this region. The connection between the fractional volume of a phase and its various proxies is clarified. An exact relation is then derived between the fractional volume and the filling factors defined in terms of the volume and probabilistic averages. These results are discussed in both observational and computational contexts. The correlation scale of the random flows is calculated from the velocity autocorrelation function; it is of order 100 pc and tends to grow with distance from the mid-plane. The origin and structure of the magnetic fields in the ISM is also investigated in non-ideal MHD simulations. A seed magnetic field, with volume average of roughly 4 nG, grows exponentially to reach a statistically steady state within 1.6 Gyr.

Clues on the Galactic evolution of sulphur from star clusters

(Abridged) The abundances of alpha-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur, being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-alpha galaxies and extragalactic HII regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg. The measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur. We find <A(S)>NLTE=6.11+/-0.04 for M 4, <A(S)>NLTE=7.17+/-0.02 for NGC 2477, and <A(S)>NLTE=7.13+/-0.06 for NGC 5822. For the only star studied in Trumpler 5 we find A(S)NLTE=6.43+/-0.03 and A(S)LTE=6.94+/-0.05. Our measurements show that, by and large, the S abundances in Galactic clusters trace reliably those in field stars. The only possible exception is Trumpler 5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than observed in field stars of comparable metallicity, even though its LTE sulphur abundance is in line with abundances of field stars. Moreover the LTE sulphur abundance is consistent only with the abundance of another alpha-element, Mg, in the same star, while the low NLTE value is consistent with Si and Ca. The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05 dex.

Spatially resolved kinematics, galactic wind, and quenching of star formation in the luminous infrared galaxy IRAS F11506-3851

We present a multi-wavelength integral field spectroscopic study of the low-z LIRG IRAS F11506-3851, on the basis of VIMOS and SINFONI (ESO-VLT) observations. The morphology and the 2D kinematics of the gaseous (neutral and ionized) and stellar components have been mapped using the NaD doublet, the H$\alpha$ line, and the near-IR CO(2-0) and CO(3-1) bands. The kinematics of the ionized gas and the stars are dominated by rotation, with large observed velocity amplitudes and centrally peaked velocity dispersion maps. The stars lag behind the warm gas and represent a dynamically hotter system, as indicated by the observed dynamical ratios. Thanks to these IFS data we have disentangled the contribution of the stars and the ISM to the NaD feature, finding that it is dominated by the absorption of neutral gas clouds in the ISM. The neutral gas 2D kinematics shows a complex structure dominated by two components. On the one hand, the thick slowly rotating disk lags significantly compared to the ionized gas and the stars, with an irregular and off-center velocity dispersion map. On the other hand, a kpc-scale neutral gas outflow is observed along the semi-minor axis of the galaxy, as revealed by large blueshifted velocities (30-154 km/s). We derive an outflowing mass rate in neutral gas of about 48 $\dot{M_{\rm w}}$/yr. Although this implies a global mass loading factor of 1.4, the 2D distribution of the ongoing SF suggests a much larger value of mass loading factor associated with the inner regions (R$<$200 pc), where the current SF represents only 3 percent of the total. All together these results strongly suggest that we are witnessing (nuclear) quenching due to SF feedback in IRAS F11506-3851. However, the relatively large mass of molecular gas detected in the nuclear region via the H2 1-0 S(1) line suggests that further episodes of SF may take place again.

The morphology of the Milky Way - I. Reconstructing CO maps from simulations in fixed potentials

We present an investigation into the morphological features of the Milky Way. We use smoothed particle hydrodynamics (SPH) to simulate the interstellar medium (ISM) in the Milky Way under the effect of a number of different gravitational potentials representing spiral arms and bars, assuming the Milky Way is grand design in nature. The gas is subject to ISM cooling and chemistry, enabling us to track the evolution of molecular gas. We use a 3D radiative transfer code to simulate the emission from the SPH output, allowing for the construction of synthetic longitude-velocity (l-v) emission maps as viewed from the Earth. By comparing these maps with the observed emission in CO from the Milky Way, we infer the arm/bar geometry that provides a best fit to our Galaxy. We find that it is possible to reproduce nearly all features of the l-v diagram in CO emission. There is no model, however, that satisfactorily reproduces all of the features simultaneously. Models with 2 arms cannot reproduce all the observed arm features, while 4 armed models produce too bright local emission in the inner Galaxy. Our best fitting models favour a bar pattern speed within 50-60km/s/kpc and an arm pattern speed of approximately 20km/s/kpc, with a bar orientation of approximately 45 degrees and arm pitch angle between 10-15 degrees.

Evolution of dust and molecular hydrogen in the Magellanic System

We investigate the evolution of the interstellar medium (ISM) in self-consistent, chemodynamical simulations of the Magellanic Clouds (MCs) during their recent (z<0.3) past. An explicit modelling of dust and molecular hydrogen lifecycles enables the comparison of our models against the observed properties of the ISM, including elemental depletion from the gas-phase. Combining this model with a tidal-dominated paradigm for the formation for the Magellanic Stream and Bridge, we reproduce the age-metallicity relations, long gas depletion timescales, and presently observed dust and molecular hydrogen masses of the MCs to within their respective uncertainties. We find that these models’ enrichment depends sensitively on the processing of dust within the ISM and the dynamical influence of external tides/stellar bars. The ratio of characteristic dust destruction timescales in our SMC and LMC models, a governing parameter of our models’ evolution, is consistent with estimates based on observed supernova (SN) rates. Our reference MC models tend to exhibit the disputed universal dust-to-metal ratio, which we argue stems from the adoption of high SNe II condensation efficiencies. Our models are the first to reproduce the one-tenth solar metallicity of the Stream/Leading Arm following tidal stripping of the SMC; the hypothesis that the LMC contributes a metal-rich filament to the Stream, as implied by recent kinematic and abundance analyses, is also appraised in this study.

Three-dimensional extinction mapping using Gaussian random fields [Replacement]

We present a scheme for using stellar catalogues to map the three-dimensional distributions of extinction and dust within our Galaxy. Extinction is modelled as a Gaussian random field, whose covariance function is set by a simple physical model of the ISM that assumes a Kolmogorov-like power spectrum of turbulent fluctuations. As extinction is modelled as a random field, the spatial resolution of the resulting maps is set naturally by the data available; there is no need to impose any spatial binning. We verify the validity of our scheme by testing it on simulated extinction fields and show that its precision is significantly improved over previous dust-mapping efforts. The approach we describe here can make use of any photometric, spectroscopic or astrometric data; it is not limited to any particular survey. Consequently, it can be applied to a wide range of data from both existing and future surveys.

Three-dimensional extinction mapping using Gaussian random fields [Replacement]

We present a scheme for using stellar catalogues to map the three-dimensional distributions of extinction and dust within our Galaxy. Extinction is modelled as a Gaussian random field, whose covariance function is set by a simple physical model of the ISM that assumes a Kolmogorov-like power spectrum of turbulent fluctuations. As extinction is modelled as a random field, the spatial resolution of the resulting maps is set naturally by the data available; there is no need to impose any spatial binning. We verify the validity of our scheme by testing it on simulated extinction fields and show that its precision is significantly improved over previous dust-mapping efforts. The approach we describe here can make use of any photometric, spectroscopic or astrometric data; it is not limited to any particular survey. Consequently, it can be applied to a wide range of data from both existing and future surveys.

Three-dimensional extinction mapping using Gaussian random fields [Replacement]

We present a scheme for using stellar catalogues to map the three-dimensional distributions of extinction and dust within our Galaxy. Extinction is modelled as a Gaussian random field, whose covariance function is set by a simple physical model of the ISM that assumes a Kolmogorov-like power spectrum of turbulent fluctuations. As extinction is modelled as a random field, the spatial resolution of the resulting maps is set naturally by the data available; there is no need to impose any spatial binning. We verify the validity of our scheme by testing it on simulated extinction fields and show that its precision is significantly improved over previous dust-mapping efforts. The approach we describe here can make use of any photometric, spectroscopic or astrometric data; it is not limited to any particular survey. Consequently, it can be applied to a wide range of data from both existing and future surveys.

Three-dimensional extinction mapping using Gaussian random fields

We present a scheme for using stellar catalogues to map the three-dimensional distributions of extinction and dust within our Galaxy. Extinction is modelled as a Gaussian random field, whose covariance function is set by a simple physical model of the ISM that assumes a Kolmogorov-like power spectrum of turbulent fluctuations. As extinction is modelled as a random field, the spatial resolution of the resulting maps is set naturally by the data available; there is no need to impose any spatial binning. We verify the validity of our scheme by testing it on simulated extinction fields and show that its precision is significantly improved over previous dust-mapping efforts. The approach we describe here can make use of any photometric, spectroscopic or astrometric data; it is not limited to any particular survey. Consequently, it can be applied to a wide range of data from both existing and future surveys.

Nuclear enhancement of the photon yield in cosmic ray interactions

The concept of the nuclear enhancement factor has been used since the beginning of gamma-ray astronomy. It provides a simple and convenient way to account for the contribution of nuclei (A>1) in cosmic rays (CRs) and in the interstellar medium (ISM) to the diffuse gamma-ray emission. An accurate treatment of the dominant emission process, such as hadronic interactions of CRs with the ISM, enables one to study CR acceleration processes, CR propagation in the ISM, and provides a reliable background model for searches of new phenomena. The Fermi Large Area Telescope (Fermi-LAT) launched in 2008 provides excellent quality data in a wide energy range 30 MeV – 1 TeV where the diffuse emission accounts for the majority of photons. Exploiting its data to the fullest requires a new study of the processes of gamma-ray production in hadronic interactions. In this paper we point out that several commonly used studies of the nuclear enhancement factor miss to account for the spectrally averaged energy loss fraction which ensures that the energy fraction transferred to photons is averaged properly with the spectra of CR species. We present a new calculation of the spectrally averaged energy loss fraction and the nuclear enhancement factor using the QGSJET-II-04 and EPOS-LHC interaction models.

The effects of galaxy shape and rotation on the X-ray haloes of early-type galaxies - II. Numerical simulations

By means of high resolution 2D hydrodynamical simulations, we study the evolution of the hot ISM for a large set of early-type galaxy models, characterized by various degrees of flattening and internal rotation. The galaxies are described by state-of-the-art axisymmetric two-component models, tailored to reproduce real systems; the dark matter haloes follow the Navarro-Frenk-White or the Einasto profile. The gas is produced by the evolving stars, and heated by Type Ia SNe. We find that, in general, the rotation field of the ISM in rotating galaxies is very similar to that of the stars, with a consequent negligible heating contribution from thermalization of the ordered motions. The relative importance of flattening and rotation in determining the final X-ray luminosity $L_x$ and temperature $T_x$ of the hot haloes is a function of the galactic mass. Flattening and rotation in low mass galaxies favour the establishment of global winds, with the consequent reduction of $L_x$. In medium-to-high mass galaxies, flattening and rotation are not sufficient to induce global winds, however, in the rotating models the nature of the gas flows is deeply affected by conservation of angular momentum, resulting in a reduction of both $L_x$ and $T_x$.

Collapse and fragmentation of molecular clouds under pressure

Recent analytical and numerical models show that AGN outflows and jets create ISM pressure in the host galaxy that is several orders of magnitude larger than in quiescent systems. This pressure increase can confine and compress molecular gas, thus accelerating star formation. In this paper, we model the effects of increased ambient ISM pressure on spherically symmetric turbulent molecular clouds. We find that large external pressure confines the cloud and drives a shockwave into it, which, together with instabilities behind the shock front, significantly accelerates the fragmentation rate. The compressed clouds therefore convert a larger fraction of their mass into stars over the cloud lifetime, and produce clusters that are initially more compact. Neither cloud rotation nor shear against the ISM affect this result significantly, unless the shear velocity is higher than the sound speed in the confining ISM. We conclude that external pressure is an important element in the star formation process, provided that it dominates over the internal pressure of the cloud.

Exploring the Origins of Carbon in Terrestrial Worlds

Given the central role of carbon in the chemistry of life, it is a fundamental question as to how carbon is supplied to the Earth, in what form and when. We provide an accounting of carbon found in solar system bodies, in particular a comparison between the organic content of meteorites and that in identified organics in the dense interstellar medium (ISM). Based on this accounting identified organics created by the chemistry of star formation could contain at most ~15% of the organic carbon content in primitive meteorites and significantly less for cometary organics, which represent the putative contributors to starting materials for the Earth. In the ISM ~30% of the elemental carbon is found in CO, either in the gas or ices, with a typical abundance of ~10^-4 (relative to H2). Recent observations of the TW Hya disk find that the gas phase abundance of CO is reduced by an order of magnitude compared to this value. We explore a solution where the volatile CO is destroyed via a gas phase processes, providing an additional source of carbon for organic material to be incorporated into planetesimals and cometesimals. This chemical processing mechanism requires warm grains (> 20 K), partially ionized gas, and sufficiently small <10 micron grains, i.e. a larger total grain surface area, such that freeze-out is efficient. Under these conditions static (non-turbulent) chemical models predict that a large fraction of the carbon nominally sequestered in CO can be the source of carbon for a wide variety of organics that are present as ice coatings on the surfaces of warm pre-planetesimal dust grains.

On the properties of the interstellar medium in extremely metal-poor blue compact dwarf galaxies: GMOS-IFU spectroscopy and SDSS photometry of the double-knot galaxy HS 2236+1344

The main goal of this study is to carry out a spatially resolved investigation of the warm interstellar medium (ISM) in the extremely metal-poor Blue Compact Dwarf (BCD) galaxy HS 2236+1344. Special emphasis is laid on the analysis of the spatial distribution of chemical abundances, emission-line ratios and kinematics of the ISM, and to the recent star-forming activity in this galaxy. This study is based on optical integral field unit spectroscopy data from Gemini Multi-Object Spectrograph at the Gemini North telescope and archival Sloan Digital Sky Survey images. The data were obtained in two different positions across the galaxy, obtaining a total 4 arcsec X 8 arcsec field which encompasses most of its ISM. Emission-line maps and broad-band images obtained in this study indicate that HS 2236+1344 hosts three Giant HII regions. Our data also reveal some faint curved features in the BCD periphery that might be due to tidal perturbations or expanding ionized-gas shells. The ISM velocity field shows systematic gradients along the major axis of the BCD, with its south-eastern and north-western half differing by ~80 km/s in their recessional velocity. The Ha and Hb equivalent width distribution in the central part of HS 2236+1344 is consistent with a very young (~3 Myr) burst. Our surface photometry analysis indicates that the ongoing starburst provides ~50% of the total optical emission, similar to other BCDs. It also reveals an underlying lower-surface brightness component with moderately red colors, which suggest that the galaxy has undergone previous star formation. We derive an integrated oxygen abundance of 12+log(O/H)=7.53\pm0.06 and a nitrogen-to-oxygen ratio of log(N/O)=-1.57\pm0.19. Our results are consistent, within the uncertainties, with a homogeneous distribution of oxygen and nitrogen within the ISM of the galaxy. (abridged)

The IRAM M33 CO(2-1) Survey - A complete census of the molecular gas out to 7 kpc

In order to study the ISM and the interplay between the atomic and molecular components in a low-metallicity environment, we present a complete high angular and spectral resolution map and data cube of the 12CO(2-1) emission from the Local Group galaxy M33. Its metallicity is roughly half-solar, such that we can compare its ISM with that of the Milky Way with the main changes being the metallicity and the gas mass fraction. The data have a 12" angular resolution (50pc) with a spectral resolution of 2.6 km/s and a mean noise level of 20 mK per channel in antenna temperature. A radial cut along the major axis was also observed in the 12CO(1-0) line. The CO data cube and integrated intensity map are optimal when using HI data to define the baseline window and the velocities over which the CO emission is integrated. Great care was taken when building these maps, testing different windowing and baseline options and investigating the effect of error beam pickup. The total CO(2-1) luminosity is 2.8e7 K km/s pc2, following the spiral arms in the inner disk. There is no clear variation in the CO(2-1/1-0) intensity ratio with radius and the average value is roughly 0.8. The total molecular gas mass is estimated, using a N(H2)/Ico(1-0)=4e20 cm-2/(K km/s) conversion factor, to be 3.1e8 Msol. The CO spectra in the cube were shifted to zero velocity by subtracting the velocity of the HI peak from the CO spectra. Hence, the velocity dispersion between the atomic and molecular components is extremely low, independently justifying the use of the HI line in building our maps. Stacking the spectra in concentric rings shows that the CO linewidth and possibly the CO-HI velocity dispersion decrease in the outer disk. Using the CO(2-1) emission to trace the molecular gas, the probability distribution function of the H2 column density shows an excess at high column density above a log normal distribution.

3D modelling of clumpy PDRs - Understanding the Orion Bar stratification

Aims. We aim to construct a self-consistent numerical PDR model to simulate full spectral cubes of line emission from arbitrary PDRs in three dimensions (3D). The model is to reproduce the intensity of the main cooling lines from the Orion Bar PDR and the observed layering structure of the different transitions. Methods. Using a fractal description of the ISM combined with the KOSMA-{\tau} PDR model, we build up a 3D compound, made of voxels ("3D pixels"), resembling the internal structure of a PDR. Each voxel contains "clumps" mimicking the fractal ISM. The local FUV field strength is calculated self-consistently for each voxel. Line emissivities and opacities of individual clumps, provided by the KOSMA-{\tau} PDR model, are used to calculate voxel-averaged emissivities and opacities that are finally used to simulate full spectral cubes computing the radiative transport through the compound. To test the new model we try to simulate the structure of the Orion Bar PDR and compare the results to observations from HIFI/Herschel and from the Caltech Submillimetre Observatory (CSO). Results. Our model is able to qualitatively reproduce the line intensities and the observed stratification of the emission structure in the various tracers based on the representation of the Orion Bar PDR by a clumpy edge-on cavity wall. In contrast, the model of a convex filament can be ruled out. In the cavity wall, a large fraction of the total mass needs to be contained in clumps. The mass of the interclump medium is constrained by the FUV penetration. Furthermore, the stratification profile cannot be reproduced by a model having the same amount of clump and interclump mass in each voxel, but dense clumps have to be removed from the PDR surface to reproduce the observed intensities and spatial structure.

Properties of bow-shock sources at the Galactic center [Replacement]

There are an enigmatic population of massive stars around the Galactic Center (GC) that were formed some Ma ago. A fraction of these stars has been found to orbit the supermassive black hole, SgrA*, in a projected clockwise disk, which suggests that they were formed in a formerly existing dense disk around SgrA*. We focus on the extended, near-infrared (NIR) sources IRS1W, IRS5, IRS10W, and IRS21 that have been suggested to be young, massive stars that form bow-shocks through their interaction with the ISM. Their nature has impeded accurate determination of their orbital parameters. We aim at establishing their nature and kinematics to test whether they form part of the clockwise disk. We performed NIR multi-wavelength imaging using adaptive optics (AO) and sparse aperture masking (SAM). We introduce a new method for self-calibration of the SAM PSF in dense stellar fields. The emission mechanism, morphology and kinematics of the targets were examined via 3D bow-shock models. We confirm previous findings that IRS21, IRS1W, and IRS5 are bow-shocks created by the interaction between mass-losing stars and the interstellar gas. The nature of IRS10W remains unclear. Our modeling shows that the bow-shock-emission is caused by thermal emission while the scattering of stellar light does not play any significant role. IRS 1W appears to be a bow-shock produced by an anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit models provide an estimate of the local proper motion of the ISM in the NA in agreement with the published models. Assuming that all of the sources are tied to SgrA*, their orbital planes were obtained via a Monte-Carlo simulation. Our orbital analysis suggests that they are not part of any of the clockwise disk. We thus add more evidence to recent findings that a large part of the massive stars show apparently random orbital orientations.

The properties of bow-shock sources at the Galactic Center

There are an enigmatic population of massive stars around the Galactic Center (GC) that were formed some Ma ago. A fraction of these stars has been found to orbit the supermassive black hole, SgrA*, in a projected clockwise disk, which suggests that they were formed in a formerly existing dense disk around SgrA*. We focus on the extended, near-infrared (NIR) sources IRS1W, IRS5, IRS10W, and IRS21 that have been suggested to be young, massive stars that form bow-shocks through their interaction with the ISM. Their nature has impeded accurate determination of their orbital parameters. We aim at establishing their nature and kinematics to test whether they form part of the clockwise disk. We performed NIR multi-wavelength imaging using adaptive optics (AO) and sparse aperture masking (SAM). We introduce a new method for self-calibration of the SAM PSF in dense stellar fields. The emission mechanism, morphology and kinematics of the targets were examined via 3D bow-shock models. We confirm previous findings that IRS21, IRS1W, and IRS5 are bow-shocks created by the interaction between mass-losing stars and the interstellar gas. The nature of IRS10W remains unclear. Our modeling shows that the bow-shock-emission is caused by thermal emission while the scattering of stellar light does not play any significant role. IRS 1W appears to be a bow-shock produced by an anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit models provide an estimate of the local proper motion of the ISM in the NA in agreement with the published models. Assuming that all of the sources are tied to SgrA*, their orbital planes were obtained via a Monte-Carlo simulation. Our orbital analysis suggests that they are not part of any of the clockwise disk. We thus add more evidence to recent findings that a large part of the massive stars show apparently random orbital orientations.

Tracing the ISM magnetic field morphology: The potential of multi-wavelength polarization measurements

$\textit{Aims.}$ We present a case study to demonstrate the potential of multi-wavelength polarization measurements. The aim is to investigate the effects that dichroic polarization and thermal re-emission have on tracing the magnetic field in the interstellar medium (ISM). Furthermore, we analyze the crucial influence of imperfectly aligned compact dust grains on the resulting synthetic continuum polarization maps.$\\ \textit{Methods.}$ We developed an extended version of the well-known 3D Monte-Carlo radiation transport code MC3D for multi-wavelength polarization simulations running on an adaptive grid.We investigated the interplay between radiation, magnetic fields and dust grains. Our results were produced by post-processing both ideal density distributions and sophisticated magnetohydrodynamic (MHD) collapse simulations with radiative transfer simulations. We derived spatially resolved maps of intensity, optical depth, and linear and circular polarization at various inclination angles and scales in a wavelength range from 7 $\mu m$ to 1 $mm$.$\\ \textit{Results.}$ We predict unique patterns in linear and circular polarization maps for different types of density distributions and magnetic field morphologies for test setups and sophisticated MHD collapse simulations. We show that alignment processes of interstellar dust grains can significantly influence the resulting synthetic polarization maps. Multi-wavelength polarization measurements allow one to predict the morphology of the magnetic field inside the ISM. The interpretation of polarization measurements of complex structures still remains ambiguous because of the large variety of the predominant parameters in the ISM.

Transport of charged dust grains into the galactic halo

We develop a 3D dynamical model of dust outflows from galactic discs. The outflows are initiated by multiple SN explosions in a magnetized interstellar medium (ISM) with a gravitationally stratified density distribution. Dust grains are treated as particles in cells interacting collisionally with gas, and forced by stellar radiation of the disc and Lorenz force. We show that magnetic field plays a crucial role in accelerating the charged dust grains and expelling them out of the disc: in 10–20~Myr they can be elevated at distances up to 10~kpc above the galactic plane. The dust-to-gas ratio in the outflowing medium varies in the range $5 \cdot 10^{-4} – 5 \cdot 10^{-2}$ along the vertical stream. Overall the dust mass loss rate depends on the parameters of ISM and may reach up to $3\times 10^{-2}$~\Msun~yr$^{-1}$

First time-dependent study of H2 and H3+ ortho-para chemistry in the diffuse ISM

The chemistry in the diffuse interstellar medium initiates the gradual increase of molecular complexity during the life cycle of matter. A key molecule that enables build-up of new molecular bonds and new molecules via proton-donation is H$_3^+$. Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H$_2$ and H$_3^+$ in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H$_3^+$, unlike H$_2$, seems to be out of thermal equilibrium, contrary to the predictions of modern astrochemical models. We conduct the first time-dependent modeling of the para-fractions of H$_2$ and H$_3^+$ in the diffuse ISM and compare our results to a set of line-of-sight observations, including new measurements presented in this study. We isolate a set of key reactions for H$_3^+$ and find that the destruction of the lowest rotational states of H$_3^+$ by dissociative recombination largely control its ortho/para ratio. A plausible agreement with observations cannot be achieved unless a ratio larger than 1:5 for the destruction of (1,1)- and (1,0)-states of H$_3^+$ is assumed. Additionally, an increased CR ionization rate to $10^{-15}$ s$^{-1}$ further improves the fit whereas variations of other individual physical parameters, such as density and chemical age, have only a minor effect on the predicted ortho/para ratios. Thus our study calls for new laboratory measurements of the dissociative recombination rate and branching ratio of the key ion H$_{3}^{+}$ under interstellar conditions.

Joint XMM-Newton and Chandra Observations of the NGC 1407/1400 Complex: a Tail of an Early-Type Galaxy and a Tale of a Nearby Merging Group

The nearby group centered on its bright central galaxy NGC 1407 has been suggested to be an unusually dark system from previous kinematic studies. It is also known for hosting a bright galaxy, NGC 1400, with a large radial velocity (1200 km s$^{-1}$) with respect to the group center. Previous ROSAT X-ray observations revealed an extended region of enhanced surface brightness just eastward of NGC 1400. We investigate the NGC 1407/1400 complex with XMM-Newton and Chandra observations. We find that the temperature and metallicity of the enhanced region are different (cooler and more metal rich) than those of the surrounding group gas, but consistent with those of the ISM in NGC 1400. The relative velocity of NGC 1400 is large enough that much of its ISM could have been ram pressure stripped while plunging through the group atmosphere. We conclude that the enhanced region is likely to be hot gas stripped from the ISM of NGC 1400. We constrain the motion of NGC 1400 using the the pressure jump at its associated stagnation front and the total mass profile of the NGC 1407 group. We conclude that NGC 1400 is moving within ~$30^{\circ}$ of the line-of-sight with Mach number $\mathcal{M}\lesssim3$. We do not detect any obvious shock features in this complex, perhaps due to the highly line-of-sight motion of NGC 1400. With an {\sl XMM-Newton} pointing on the relatively relaxed eastern side of NGC 1407, we derive a hydrostatic mass for this group of ~$\times 10^{13}$ $M_\odot$ within 100 kpc. The total mass extrapolated to the virial radius (681 kpc) is 3.8$\times 10^{13}$ $M_\odot$, which puts an upper limit of ~300 $M_\odot/L_{B_\odot}$ on the mass-to-light ratio of this group. This suggests that the NGC 1407 group is not an unusually dark group.

Three Dimensional Hydrodynamic Simulations of Multiphase Galactic Disks with Star Formation Feedback: II. Synthetic HI 21 cm Line Observations

We use three-dimensional numerical hydrodynamic simulations of the turbulent, multiphase atomic interstellar medium (ISM) to construct and analyze synthetic HI 21 cm emission and absorption lines. Our analysis provides detailed tests of 21 cm observables as physical diagnostics of the atomic ISM. In particular, we construct (1) the "observed" spin temperature, $T_{s,obs}(v_{ch})\equiv T_B(v_{ch})/[1-e^{-{\tau}(v_{ch})}]$, and its optical-depth weighted mean T_s,obs; (2) the absorption-corrected "observed" column density, $N_{H,obs}\propto \int dv_{ch} T_B(v_{ch}){\tau}(v_{ch})/[[1-e^{-{\tau}(v_{ch})}]$; and (3) the "observed" fraction of cold neutral medium (CNM), $f_{c,obs}\equiv T_c/T_{s,obs}$ for T_c the CNM temperature; we compare each observed parameter with true values obtained from line-of-sight (LOS) averages in the simulation. Within individual velocity channels, T_s,obs(v_ch) is within a factor 1.5 of the true value up to ${\tau}(v_{ch})\approx10$. As a consequence, N_H,obs and T_s,obs are respectively within 5% and 12% of the true values for 90% and 99% of LOSs. The optically thin approximation significantly underestimates N_H for ${\tau}>1$. Provided that T_c is constrained, an accurate observational estimate of the CNM mass fraction can be obtained down to 20%. We show that T_s,obs cannot be used to distinguish the relative proportions of warm and thermally-unstable atomic gas, although the presence of thermally-unstable gas can be discerned from 21 cm lines with 200K<$T_{s,obs}(v_{ch})$<1000K. Our mock observations successfully reproduce and explain the observed distribution of the brightness temperature, optical depth, and spin temperature in Roy et al. (2013a). The threshold column density for CNM seen in observations is also reproduced by our mock observations. We explain this observed threshold behavior in terms of vertical equilibrium in the local Milky Way’s ISM disk.

Grain opacity and the bulk composition of extrasolar planets. I. Results from scaling the ISM opacity

The opacity due to grains in the envelope of a protoplanet regulates the accretion rate of gas during formation, thus the final bulk composition of planets with primordial H/He is a function of it. Observationally, for exoplanets with known mass and radius it is possible to estimate the bulk composition via internal structure models. We first determine the reduction factor of the ISM grain opacity f_opa that leads to gas accretion rates consistent with grain evolution models. We then compare the bulk composition of synthetic low-mass and giant planets at different f_opa with observations. For f_opa=1 (full ISM opacity) the synthetic low-mass planets have too small radii, i.e., too low envelope masses compared to observations. At f_opa=0.003, the value calibrated with the grain evolution models, synthetic and actual planets occupy similar mass-radius loci. The mean enrichment of giant planets relative to the host star as a function of planet mass M can be approximated as Z_p/Z_star = beta*(M/M_Jup)^alpha. We find alpha=-0.7 independent of f_opa in synthetic populations in agreement with the observational result (-0.71+-0.10). The absolute enrichment level decreases from beta=8.5 at f_opa=1 to 3.5 at f_opa=0. At f_opa=0.003 one finds beta=7.2 which is similar to the observational result (6.3+-1.0). We thus find observational hints that the opacity in protoplanetary atmospheres is much smaller than in the ISM even if the specific value of the grain opacity cannot be constrained here. The result for the enrichment of giant planets helps to distinguish core accretion and gravitational instability. In the simplest picture of core accretion where first a critical core forms and afterwards only gas is added, alpha=-1. If a core accretes all planetesimals inside the feeding zone, alpha=-2/3. The observational result lies between these values, pointing to core accretion as the formation mechanism.

The scale height of gas traced by [CII] in the Galactic plane

The distribution of various interstellar gas components and the pressure in the interstellar medium (ISM) is a result of the interplay of different dynamical mechanisms and energy sources on the gas in the Milky Way. The scale heights of the different gas tracers, such as HI and CO, are a measure of these processes. The scale height of [CII] emission in the Galactic plane is important for understanding those ISM components not traced by CO or HI. We determine the average distribution of [CII] perpendicular to the plane in the inner Galactic disk and compare it to the distributions of other key gas tracers, such as CO and HI. We calculated the vertical, z, distribution of [CII] in the inner Galactic disk by adopting a model for the emission that combines the latitudinal, b, spectrally unresolved BICE survey, with the spectrally resolved $Herschel$ Galactic plane survey of [CII] at b = 0 deg. Our model assumed a Gaussian emissivity distribution vertical to the plane, and related the distribution in z to that of the latitude b using the spectrally resolved [CII] Herschel survey as the boundary solution for the emissivity at b=0 deg. We find that the distribution of [CII] perpendicular to the plane has a full-width half-maximum of 172 pc, larger than that of CO, which averages ~110 pc in the inner Galaxy, but smaller than that of HI, ~230 pc, and is offset by -28 pc. We explain the difference in distributions of [CII], CO, and HI as due to [CII] tracing a mix of ISM components. Models of hydrostatic equilibrium of clouds in the disk predict different scale heights, for the same interstellar pressure. The diffuse molecular clouds with [CII] but no CO emission likely have a scale height intermediate between the low density atomic hydrogen HI clouds and the dense CO molecular clouds.

Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?

The recently discovered coreshine effect can aid in exploring the core properties and in probing the large grain population of the ISM. We discuss the implications of the coreshine detected from the molecular cloud core L1506C in the Taurus filament for the history of the core and the existence of a primitive ISM component of large grains becoming visible in cores. The coreshine surface brightness of L1506C is determined from IRAC Spitzer images at 3.6 micron. We perform grain growth calculations to estimate the grain size distribution in model cores similar in gas density, radius, and turbulent velocity to L1506C. Scattered light intensities at 3.6 micron are calculated for a variety of MRN and grain growth distributions to compare with the observed coreshine. For a core with the overall physical properties of L1506C, no detectable coreshine is predicted for an MRN size distribution. Extending the distribution to grain radii of about 0.65 $\mu$m allows to reproduce the observed surface brightness level in scattered light. Assuming the properties of L1506C to be preserved, models for the growth of grains in cores do not yield sufficient scattered light to account for the coreshine within the lifetime of the Taurus complex. Only increasing the core density and the turbulence amplifies the scattered light intensity to a level consistent with the observed coreshine brightness. The grains could be part of primitive omni-present large grain population becoming visible in the densest part of the ISM, could grow under the turbulent dense conditions of former cores, or in L1506C itself. In the later case, L1506C must have passed through a period of larger density and stronger turbulence. This would be consistent with the surprisingly strong depletion usually attributed to high column densities, and with the large-scale outward motion of the core envelope observed today.

Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"

Dark gas in the interstellar medium (ISM) is believed to be not detectable either in CO or HI radio emission, but it is detected in the other means including gamma-rays, dust emission and extinction. In these analyses, the 21-cm HI emission is usually assumed to be completely optically thin. We have reanalyzed the HI emission from the whole sky at |b|>15 degrees by considering temperature stratification in the ISM inferred from the Planck/IRAS analysis of the dust properties. The results indicate that the HI emission is saturated with an optical depth ranging from 0.5 to 5 for 80 % of the local HI gas. This optically thick HI is characterized by spin temperature in the range 15 K – 70 K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of HI toward radio continuum sources. The distribution and the column density of the HI are consistent with those of the dark gas suggested by gamma-rays, and we infer that the dark gas in the Galaxy is dominated by optically thick cold HI gas. This result implies that the average density of HI is two times higher than that derived on the optically-thin assumption in the local interstellar space.

The Milky Way as a Star Formation Engine

The cycling of material from the interstellar medium (ISM) into stars and the return of stellar ejecta into the ISM is the engine that drives the “galactic ecology” in normal spirals, a cornerstone in the formation and evolution of galaxies through cosmic time. Major observational and theoretical challenges need to be addressed in determining the processes responsible for converting the low-density ISM into dense molecular clouds, forming dense filaments and clumps, fragmenting them into stars, OB associations and bound clusters, and characterizing the feedback that limits the rate and efficiency of star formation. This formidable task can be now effectively attacked thanks to the combination of new global-scale surveys of the Milky Way Galactic Plane from infrared to radio wavelengths, offering the possibility of bridging the gap between local and extragalactic star formation studies. The Herschel, Spitzer and WISE mid to far infrared continuum surveys, complemented by analogue surveys from ground-based facilities in the millimetre and radio wavelengths, enables us to measure the Galactic distribution and physical properties of dust on all scales and in all components of the ISM from diffuse clouds to filamentary complexes and tens of thousands of dense clumps. A complementary suite of spectroscopic surveys in various atomic and molecular tracers is providing the chemical fingerprinting of dense clumps and filaments, as well as essential kinematic information to derive distances and thus transform panoramic data into a 3D representation. The latest results emerging from these Galaxy-scale surveys are reviewed. New insights into cloud formation and evolution, filaments and their relationship to channeling gas onto gravitationally-bound clumps, the properties of these clumps, density thresholds for gravitational collapse, and star and cluster formation rates are discussed.

The Milky Way as a Star Formation Engine [Replacement]

The cycling of material from the interstellar medium (ISM) into stars and the return of stellar ejecta into the ISM is the engine that drives the "galactic ecology" in normal spirals, a cornerstone in the formation and evolution of galaxies through cosmic time. Major observational and theoretical challenges need to be addressed in determining the processes responsible for converting the low-density ISM into dense molecular clouds, forming dense filaments and clumps, fragmenting them into stars, OB associations and bound clusters, and characterizing the feedback that limits the rate and efficiency of star formation. This formidable task can be now effectively attacked thanks to the combination of new global-scale surveys of the Milky Way Galactic Plane from infrared to radio wavelengths, offering the possibility of bridging the gap between local and extragalactic star formation studies. The Herschel, Spitzer and WISE mid to far infrared continuum surveys, complemented by analogue surveys from ground-based facilities in the millimetre and radio wavelengths, enables us to measure the Galactic distribution and physical properties of dust on all scales and in all components of the ISM from diffuse clouds to filamentary complexes and tens of thousands of dense clumps. A complementary suite of spectroscopic surveys in various atomic and molecular tracers is providing the chemical fingerprinting of dense clumps and filaments, as well as essential kinematic information to derive distances and thus transform panoramic data into a 3D representation. The latest results emerging from these Galaxy-scale surveys are reviewed. New insights into cloud formation and evolution, filaments and their relationship to channeling gas onto gravitationally-bound clumps, the properties of these clumps, density thresholds for gravitational collapse, and star and cluster formation rates are discussed.

Interstellar gamma-ray emission from cosmic rays in star-forming galaxies

Fermi/LAT observations of star-forming galaxies in the ~0.1-100GeV range have made possible a first population study. Evidence was found for a correlation between gamma-ray luminosity and tracers of the star formation activity. Studying galactic cosmic rays (CRs) in various global conditions can yield information about their origin and transport in the interstellar medium (ISM). This work addresses the question of the scaling laws that can be expected for the interstellar gamma-ray emission as a function of global galactic properties, with the goal of establishing whether the current experimental data in the GeV range can be constraining. I developed a 2D model for the non-thermal emissions from steady-state CR populations interacting with the ISM in star-forming galaxies. Most CR-related parameters were taken from Milky Way studies, and a large number of galaxies were then simulated with sizes from 4 to 40kpc, several gas distributions, and star formation rates (SFR) covering six orders of magnitude. The evolution of the gamma-ray luminosity over the 100keV-100TeV range is presented, with emphasis on the contribution of the different emission processes and particle populations, and on the transition between transport regimes. The model can reproduce the normalisation and trend inferred from the Fermi/LAT population study over most of the SFR range. This is obtained with a plain diffusion scheme, a single diffusion coefficient, and the assumption that CRs experience large-scale volume-averaged interstellar conditions. There is, however, no universal relation between high-energy gamma-ray luminosity and star formation activity, as illustrated by the scatter introduced by different galactic global properties and the downturn in gamma-ray emission at the low end (abridged).

The New Model of Chemical Evolution of r-process Elements Based on The Hierarchical Galaxy Formation I: Ba and Eu

We investigate the chemical enrichment of r-process elements in the early evolutionary stages of the Milky Way halo within the framework of hierarchical galaxy formation using a semi-analytic merger tree. In this paper, we focus on heavy r-process elements, Ba and Eu, of extremely metal-poor (EMP) stars and give constraints on their astronomical sites. Our models take into account changes of the surface abundances of EMP stars by the accretion of interstellar matter (ISM). We also consider metal-enrichment of intergalactic medium (IGM) by galactic winds and the resultant pre-enrichment of proto-galaxies. The trend and scatter of the observed r-process abundances are well reproduced by our hierarchical model with $\sim 10\%$ of core-collapse supernovae in low-mass end ($\sim 10M_{\odot}$) as a dominant r-process source and the star formation efficiency of $\sim 10^{-10} \hbox{yr}^{-1}$. For neutron star mergers as an r-process source, their coalescence timescale has to be $ \sim 10^7$yrs, and the event rates $\sim 100$ times larger than currently observed in the Galaxy. We find that the accretion of ISM is a dominant source of r-process elements for stars with [Ba/H] < -3.5. In this model, a majority of stars at [Fe/H] < -3 are formed without r-process elements but their surfaces are polluted by the ISM accretion. The pre-enrichment affects $\sim 4\%$ of proto-galaxies, and yet, is surpassed by the ISM accretion in the surface of EMP stars.

Radiative and mechanical feedback into the molecular gas of NGC 253

Starburst galaxies are undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With SPIRE on the Herschel Space Observatory we have observed the rich molecular spectrum towards the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed and together with low-J line fluxes from ground based observations, these lines trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the $^{12}$CO and $^{13}$CO observations to large velocity gradient models and PDR models we find three main ISM phases. We estimate the density, temperature,and masses of these ISM phases. By adding $^{13}$CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these are preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; $6\times 10^7$ M$_\odot$ for phase 1 (low-J CO lines), $3\times 10^7$ M$_\odot$ for phase 2 (mid-J CO lines), and $9\times 10^6$ M$_\odot$ for phase 3 (high-J CO lines) for a total system mass of $1\times10^{8}$ M$_\odot$.

The Evolution of ISM Mass Probed by Dust Emission -- ALMA Observations at z = 0.3 to 2 [Replacement]

The use of submm dust continuum emission to probe the mass of interstellar dust and gas in galaxies is empirically calibrated using samples of local star forming galaxies, Planck observations of the Milky Way and high redshift submm galaxies (SMGs). All of these objects suggest a similar calibration, strongly supporting the view that the Rayleigh-Jeans (RJ) tail of the dust emission can be used as an accurate and very fast probe of the ISM in galaxies. We present ALMA Cycle 0 observations of the Band 7 (350 GHz) dust emission in 107 galaxies from z = 0.2 to 2.5. Three samples of galaxies with a total of 101 galaxies were stellar mass-selected from COSMOS to have $M_* \simeq10^{11}$\msun: 37 at z$\sim0.4$, 33 at z$\sim0.9$ and 31 at z$=2$. A fourth sample with 6 IR luminous galaxies at z = 2 was observed for comparison with the purely mass-selected samples. From the fluxes detected in the stacked images for each sample, we find that the ISM content has decreased a factor $\sim 6$ from $1 – 2 \times 10^{10}$\msun at both z = 2 and 0.9 down to $\sim 2 \times 10^9$\msun at z = 0.4. The IR luminous sample at z = 2 shows a further $\sim 4$ times increase in M$_{ISM}$ compared to the equivalent non-IR bright sample at the same redshift. The gas mass fractions are $\sim 2\pm0.5, 12\pm3, 14\pm2 ~\rm{and} ~53\pm3$ $%$ for the four subsamples (z = 0.4, 0.9, 2 and IR bright galaxies).

Dust origin in late-type dwarf galaxies: ISM growth vs. type II supernovae [Replacement]

We re-evaluate the roles of different dust sources in dust production as a function of metallicity in late-type dwarf galaxies, with the goal of understanding the relation between dust content and metallicity. The dust content of late-type dwarf galaxies with episodic star formation is studied with a multicomponent model of dust evolution, which includes dust input from AGB stars, type II SNe and dust mass growth by accretion of gas species in the ISM. Dust growth in the ISM becomes an important dust source in dwarf galaxies, on the timescale of 0.1 – few Gyrs. It increases the dust-to-gas ratio (DGR) during post-burst evolution, unlike type II SNe, which eject grains into the ISM only during starbursts. Before the dust growth in the ISM overtakes the dust production, AGB stars can be major sources of dust in metal-poor dwarf galaxies. Our models reproduce the relation between the DGR and oxygen abundance, derived from observations of a large sample of dwarf galaxies. The steep decrease in the DGR at low O values is explained by the relatively low efficiency of dust condensation in stars. The scatter observed at higher O values is determined mainly by different critical metallicities for the transition from stardust- to ISM-growth dominated dust production, depending on the star formation history. In galaxies with episodic star formation, additional dispersion in the DGR is introduced by grain destruction during starbursts, followed by an increase of the dust mass due to dust growth in the ISM during post-burst evolution. We find that the carbon-to-silicate ratio changes dramatically, when the ISM growth becomes the dominant dust source, therefore this ratio can be used as an indicator of the transition. The observed DGR-O relation in dwarf galaxies favours low condensation efficiencies in type II SNe, together with an increase in the total dust mass by means of dust growth in the ISM.

Dust origin in late-type dwarf galaxies: ISM growth vs. type II supernovae

We re-evaluate the roles of different dust sources in dust production as a function of metallicity in late-type dwarf galaxies, with the goal of understanding the relation between dust content and metallicity. The dust content ol late-type dwarf galaxies with episodic star formation is studied with a multicomponent model of dust evolution, which includes dust input from AGB stars, type II SNe and dust growth by accretion of atoms in the ISM. Dust growth in the ISM becomes an important dust source in dwarf galaxies, on the timescale of 0.1 – a few Gyrs. It increases the dust-to-gas ratio (DGR) during post-burst evolution, unlike type II SNe, which eject grains to the ISM only during starbursts. Before the dust growth in the ISM overtakes the dust production, AGB stars can be major sources of dust in metal-poor dwarf galaxies. Our models reproduce the relation between the DGR and oxygen abundance, derived from observations of a large sample of dwarf galaxies. The steep decrease in the DGR at low O values is explained by the relatively low efficiency of dust condensation in stars. The scatter observed at higher O values is determined mainly by different critical metallicities for the transition from stardust- to ISM-growth dominated dust production, depending on the star formation history. In galaxies with episodic star formation, additional dispersion in the DGR is introduced by grain destruction during starbursts, followed by an increase of the dust mass due to dust growth in the ISM during post-burst evolution. We find that the carbon-to-silicate ratio changes dramatically, when the ISM growth becomes the dominant dust source, therefore this ratio can be used as an indicator of the transition. The observed DGR-O relation in dwarf galaxies favours low condensation efficiencies in type II SNe, together with an increase in the total dust mass by means of dust growth in the ISM.

Abundance Patterns in the Interstellar Medium of Early-type Galaxies Observed with Suzaku

We have analyzed 17 early-type galaxies, 13 ellipticals and 4 S0′s, observed with Suzaku, and investigated metal abundances (O, Mg, Si, and Fe) and abundance ratios (O/Fe, Mg/Fe, and Si/Fe) in the interstellar medium (ISM). The emission from each on-source region, which is 4 times effective radius, r_e, is reproduced with one- or two- temperature thermal plasma models as well as a multi-temperature model, using APEC plasma code v2.0.1. The multi-temperature model gave almost the same abundances and abundance ratios with the 1T or 2T models. The weighted averages of the O, Mg, Si, and Fe abundances of all the sample galaxies derived from the multi-temperature model fits are 0.83+-0.04, 0.93+-0.03, 0.80+-0.02, and 0.80+-0.02 solar, respectively, in solar units according to the solar abundance table by Lodders (2003). These abundances show no significant dependence on the morphology and environment. The systematic differences in the derived metal abundances between the version 2.0.1 and 1.3.1 of APEC plasma codes were investigated. The derived O and Mg abundances in the ISM agree with the stellar metallicity within a aperture with a radius of one r_e derived from optical spectroscopy. From these results, we discuss the past and present SN Ia rates and star formation histories in early-type galaxies.

HST Imaging of Decoupled Dust Clouds in the Ram Pressure Stripped Virgo Spirals NGC 4402 and NGC 4522

We present the highest-resolution study to date of the ISM in galaxies undergoing ram pressure stripping, using HST BVI imaging of NGC 4522 and NGC 4402, Virgo Cluster spirals that are well-known to be experiencing ICM ram pressure. We find that throughout most of both galaxies, the main dust lane has a fairly well-defined edge, with a population of GMC-sized (tens- to hundreds-of-pc scale), isolated, highly extincting dust clouds located up to ~1.5 kpc radially beyond it. Outside of these dense clouds, the area has little or no diffuse dust extinction, indicating that the clouds have decoupled from the lower-density ISM material that has already been stripped. Several of the dust clouds have elongated morphologies that indicate active ram pressure, including two large (kpc-scale) filaments in NGC 4402 that are elongated in the projected ICM wind direction. We calculate a lower limit on the HI + H_2 masses of these clouds based on their dust extinctions and find that a correction factor of ~10 gives cloud masses consistent with those measured in CO for clouds of similar diameters, probably due to the complicating factors of foreground light, cloud substructure, and resolution limitations. Assuming that the clouds’ actual masses are consistent with those of GMCs of similar diameters (~10^4-10^5 M_sun), we estimate that only a small fraction (~1-10%) of the original HI + H_2 remains in the parts of the disks with decoupled clouds. Based on H-alpha images, a similar fraction of star formation persists in these regions, 2-3% of the estimated pre-stripping star formation rate. We find that the decoupled cloud lifetimes may be up to 150-200 Myr.

A thin diffuse component of the Galactic Ridge X-ray emission and heating of the interstellar medium contributed by the radiation of Galactic X-ray binaries [Replacement]

We predict a thin (scale height $\sim$ 80 pc) diffuse component of the Galactic Ridge X-ray emission (GRXE) arising from the scattering of the radiation of bright X-ray binaries (XBs) by the interstellar medium (ISM). The morphology of this scattered component is expected to trace the clumpy molecular and HI clouds. We calculate this contribution to the GRXE from known Galactic XBs assuming that they are all persistent. The known XBs sample is incomplete, however, because it is flux limited and spans the lifetime of X-ray astronomy ($\sim 50$ years). We therefore also use a simulated sample of sources, to estimate the diffuse emission we should expect in an optimistic case assuming that the X-ray luminosity of our Galaxy is on average similar to that of other galaxies. In the calculations we also take into account the enhancement of the total scattering cross-section due to coherence effects in the elastic scattering from multi-electron atoms and molecules. This scattered emission can be distinguished from the contribution of low X-ray luminosity stars by the presence of narrow fluorescent K-$\alpha$ lines of Fe, Si, and other abundant elements present in the ISM and by directly resolving the contribution of low X-ray luminosity stars. We find that within $1^\circ$ latitude of the Galactic plane the scattered emission contributes on average $10-30\%$ of the GRXE flux in the case of known sources and over $50\%$ in the case of simulated sources. In the latter case, the scattered component is found to even dominate the stellar emission in certain parts of the Galactic plane. X-rays with energies $\gtrsim 1$ keV from XBs should also penetrate deep inside the HI and molecular clouds, where they are absorbed and heat the ISM. We find that this heating rate dominates the heating by cosmic rays (assuming a solar neighborhood energy density) in a considerable part of the Galaxy. [abridged]

Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?

Many evolved stars travel through space at supersonic velocities, which leads to the formation of bow shocks ahead of the star where the stellar wind collides with the interstellar medium (ISM). Herschel observations of the bow shock of $\alpha$-Orionis show that the shock is almost free of instabilities, despite being, at least in theory, subject to both Kelvin-Helmholtz and Rayleigh-Taylor instabilities. A possible explanation for the lack of instabilities lies in the presence of an interstellar magnetic field. We wish to investigate whether the magnetic field of the interstellar medium (ISM) in the Orion arm can inhibit the growth of instabilities in the bow shock of $\alpha$-Orionis. We used the code MPI-AMRVAC to make magneto-hydrodynamic simulations of a circumstellar bow shock, using the wind parameters derived for $\alpha$-Orionis and interstellar magnetic field strengths of $B\,=\,1.4,\, 3.0$, and $5.0\, \mu$G, which fall within the boundaries of the observed magnetic field strength in the Orion arm of the Milky Way. Our results show that even a relatively weak magnetic field in the interstellar medium can suppress the growth of Rayleigh-Taylor and Kelvin-Helmholtz instabilities, which occur along the contact discontinuity between the shocked wind and the shocked ISM. The presence of even a weak magnetic field in the ISM effectively inhibits the growth of instabilities in the bow shock. This may explain the absence of such instabilities in the Herschel observations of $\alpha$-Orionis.

Prospects of Turbulence Studies in High-Energy Density Laser-Generated Plasma: Numerical Investigations in Two Dimensions [Cross-Listing]

We investigate the possibility of generating and studying turbulence in plasma by means of high-energy density laser-driven experiments. Our focus is to create supersonic, self-magnetized turbulence with characteristics that resemble those found in the interstellar medium (ISM). We consider a target made of a spherical core surrounded by a shell made of denser material. The shell is irradiated by a sequence of laser pulses sending inward-propagating shocks that convert the inner core into plasma and create turbulence. In the context of the evolution of the ISM, the shocks play the role of supernova remnant shocks and the core represents the ionized interstellar medium. We consider the effects of both pre-existing and self-generating magnetic fields and study the evolution of the system by means of two-dimensional numerical simulations. We find that the evolution of the turbulent core is generally, subsonic with rms-Mach number $M_t\approx 0.2$. We observe an isotropic, turbulent velocity field with an inertial range power spectra of $P(k)\propto k^{-2.3}$. We account for the effects of self-magnetization and find that the resulting magnetic field has characteristic strength $\approx 3\times 10^{4}$ G. The corresponding plasma beta is $\approx 1\times 10^{4}$–$1\times 10^{5}$, indicating that the magnetic field does not play an important role in the dynamical evolution of the system. The natural extension of this work is to study the system evolution in three-dimensions, with various laser drive configurations, and targets with shells and cores of different masses. The latter modification may help to increase the turbulent intensity and possibly create transonic turbulence. One of the key challenges is to obtain transonic turbulent conditions in a quasi-steady state environment.

On the Origin of Highly Ionized X-ray Absorbers Detected in the Galactic X-ray Binaries

X-ray observations of the Galactic X-ray binaries (XRB) revealed numerous highly ionized metal absorption lines. However, it is unclear whether such lines are produced by the hot interstellar medium (ISM) or the circumstellar medium (CSM) intrinsic to the binaries. Here we present a Chandra X-ray absorption line study of 28 observations on 12 X-ray binaries, with a focus on the NeIX and FeXVII lines. We report the first detections of these lines in a significant amount of observations. We do not find significant dependence of the line equivalent width on the distance of the XRBs, but we do see weak dependence on the source X-ray luminosity. We also find two out of twelve selected targets show strong temporal variation of the NeIX absorbers. While the line ratio between the two ion species suggests a temperature consistent with the previous predictions of the ISM, comparing with two theoretical models of the ISM shows the observed column densities are significantly higher than predictions. On the other hand, photoionzation by the XRBs provides reasonably good fit to the data. Our findings suggest that a significant fraction of these X-ray absorbers may originate in the hot gas intrinsic to the X-ray binaries, and that the ISM makes small, if not negligible, contribution. We briefly discuss the implications to the study of the Milky Way hot gas content.

The Pressure of the Star Forming ISM in Cosmological Simulations

We examine the pressure of the star-forming interstellar medium (ISM) of Milky-Way sized disk galaxies using fully cosmological SPH+N-body, high resolution simulations. These simulations include explicit treatment of metal-line cooling in addition to dust and self-shielding, $\mathrm{H_{2}}$ based star formation. The 4 simulated halos have masses ranging from a few times $10^{10}$ to nearly $10^{12}$ solar masses. Using a kinematic decomposition of these galaxies into present-day bulge and disk components, we find that the typical pressure of the star-forming ISM in the present-day bulge is higher than that in the present-day disk by an order of magnitude. We also find that pressure of the star-forming ISM at high redshift is on average, higher than ISM pressures at low redshift. This explains the why the bulge forms at higher pressures: the disk assembles at lower redshift, when the ISM is lower pressure and the bulge forms at high redshift, when the ISM is at higher pressure. If ISM pressure and IMF variation are tied together as suggested in studies like \cite{Conroy2012}, these results could indicate a time-dependent IMF in Milky-Way like systems, as well as a different IMF in the bulge and the disk.

Magnetically Aligned HI Fibers and the Rolling Hough Transform [Replacement]

We present observations of a new group of structures in the diffuse Galactic ISM: slender, linear HI features we dub "fibers" that extend for many degrees at high Galactic latitude. To characterize and measure the extent and strength of these fibers, we present the Rolling Hough Transform (RHT), a new machine vision method for parameterizing the coherent linearity of structures in the image plane. With this powerful new tool we show the fibers are oriented along the interstellar magnetic field as probed by starlight polarization. We find that these low column density (N(HI) ~ 5 x 10^18 cm^-2) fiber features are most likely a component of the local cavity wall, about 100 pc away. The HI data we use to demonstrate this alignment at high latitude are from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey and the Parkes Galactic All Sky Survey (GASS). We find better alignment in the higher resolution GALFA-HI data, where the fibers are more visually evident. This trend continues in our investigation of magnetically aligned linear features in the Riegel-Crutcher HI cold cloud, detected in the Southern Galactic Plane Survey (SGPS). We propose an application of the RHT for estimating the field strength in such a cloud, based on the Chandrasekhar-Fermi method. We conclude that data-driven, quantitative studies of ISM morphology can be very powerful predictors of underlying physical quantities.

Magnetized HI Fibers and the Rolling Hough Transform

We present observations of a new group of structures in the diffuse Galactic ISM: slender, linear HI features we dub "fibers" that extend for many degrees at high Galactic latitude. To characterize and measure the extent and strength of these fibers, we present the Rolling Hough Transform (RHT), a new machine vision method for parameterizing the coherent linearity of structures in the image plane. With this powerful new tool we show the fibers are oriented along the interstellar magnetic field as probed by starlight polarization. We find that these low column (N(HI) ~ 5 x 10^18 cm^-2) fiber features are most likely a component of the local cavity wall, about 100 pc away. The HI data we use to demonstrate this alignment at high latitude are from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey and the Parkes Galactic All Sky Survey (GASS). We find better alignment in the higher resolution GALFA-HI data, where the fibers are more visually evident. This trend continues in our investigation of magnetically aligned linear features in the Riegel-Crutcher HI cold cloud, detected in the Southern Galactic Plane Survey (SGPS). We propose an application of the RHT for estimating the field strength in such a cloud, based on the Chandrasekhar-Fermi method. We conclude that data-driven, quantitative studies of ISM morphology can be very powerful predictors of underlying physical quantities.

3D simulations of the early stages of AGN jets: geometry, thermodynamics and backflow

We investigate the interplay between jets from Active Galactic Nuclei (AGNs) and the surrounding InterStellar Medium (ISM) through full 3D, high resolution, Adaptive Mesh Refinement simulations performed with the FLASH code. We follow the jet- ISM system for several Myr in its transition from an early, compact source to an extended one including a large cocoon. During the jet evolution, we identify three major evolutionary stages and we find that, contrary to the prediction of popular theoretical models, none of the simulations shows a self-similar behavior. We also follow the evolution of the energy budget, and find that the fraction of input power deposited into the ISM (the AGN coupling constant) is of order of a few percent during the first few Myr. This is in broad agreement with galaxy formation models employing AGN feedback. However, we find that in these early stages, this energy is deposited only in a small fraction (< 1%) of the total ISM volume. Finally we demonstrate the relevance of backflows arising within the extended cocoon generated by a relativistic AGN jet within the ISM of its host galaxy, previously proposed as a mechanism for self-regulating the gas accretion onto the central object. These backflows tend later to be destabilized by the 3D dynamics, rather than by hydrodynamic (Kelvin- Helmholtz) instabilities. Yet, in the first few hundred thousand years, backflows may create a central accretion region of significant extent, and convey there as much as a few millions of solar masses.

Size distribution of supernova remnants and the interstellar medium: the case of M33

The size distribution of supernova remnants (SNRs) can help to clarify the various aspects of their evolution and interaction with the interstellar medium (ISM). Since the observed samples of SNRs are a collection of objects with very different ages and origin that evolve in different conditions of the ISM, statistical Monte Carlo methods can be used to model their statistical distributions. Based on very general assumptions on the evolution, we have modeled samples of SNRs at various initial and environmental conditions, which were then compared with observed collections of SNRs. In the evolution of SNRs the pressure of the ISM is taken into account, which determines their maximum sizes and lifetimes. When comparing the modeled and observed distributions, it is very important to have homogeneous observational data free from selection effects. We found that a recently published collection of SNRs in M33 (Long et al. 2010, ApJS,187,495) satisfies this requirement if we select the X-ray SNRs with hardness ratios in a limited range of values. An excellent agreement between distributions of this subset of SNRs and the subset of modeled SNRs was reached for a volume filling-factor of the warm phase of the ISM (partly ionized gas with $n_{\rm H}\sim 0.2-0.5~ \rm {cm}^{-3}; T \sim 8000-10000~K $) in M33 of $\sim\ 90%$. The statistical distributions constructed in this way, which reproduce practically all the statistical properties of observed SNRs, allowed us to obtain one of the important parameters of M33: the birthrate is one SNR every $ {140} – {150}$ yr, and the total number of SNRs with a shock Mach number $M_{s} \geq 2$ is larger than $\sim 1000$.

Two-phase model for Black Hole feeding and feedback

We study effects of AGN feedback outflows on multi-phase inter stellar medium (ISM) of the host galaxy. We argue that SMBH growth is dominated by accretion of dense cold clumps and filaments. AGN feedback outflows overtake the cold medium, compress it, and trigger a powerful starburst — a positive AGN feedback. This predicts a statistical correlation between AGN luminosity and star formation rate at high luminosities. Most of the outflow’s kinetic energy escapes from the bulge via low density voids. The cold phase is pushed outward only by the ram pressure (momentum) of the outflow. The combination of the negative and positive forms of AGN feedback leads to an $M-\sigma$ relation similar to the result of King (2003). Due to porosity of cold ISM in the bulge, SMBH influence on the low density medium of the host galaxy is significant even for SMBH well below the $M-\sigma$ mass. The role of SMBH feedback in our model evolves in space and time with the ISM structure. In the early gas rich phase, SMBH accelerates star formation in the bulge. During later gas poor (red-and-dead) phases, SMBH feedback is mostly negative everywhere due to scarcity of the cold ISM.

 

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