Posts Tagged ism

Recent Postings from ism

Modelling Dust Evolution in Galaxies with a Multiphase, Inhomogeneous ISM

We develop a model of dust evolution in a multiphase, inhomogeneous ISM including dust growth and destruction processes. The physical conditions for grain evolution are taken from hydrodynamical simulations of giant molecular clouds in a Milky Way-like spiral galaxy. We improve the treatment of dust growth by accretion in the ISM to investigate the role of the temperature-dependent sticking coefficient and ion-grain interactions. From detailed observational data on the gas-phase Si abundances [Si/H]_{gas} measured in the local Galaxy, we derive a relation between the average [Si/H]_{gas} and the local gas density n(H) which we use as a critical constraint for the models. This relation requires a sticking coefficient that decreases with the gas temperature. The synthetic relation constructed from the spatial dust distribution reproduces the slope of -0.5 of the observed relation in cold clouds. This slope is steeper than that for the warm medium and is explained by the dust growth. We find that it occurs for all adopted values of the minimum grain size a_{min} from 1 to 5nm. For the classical cut-off of a_{min}=5 nm, the ion-grain interactions result in longer growth timescales and higher [Si/H]_{gas} than the observed values. For a_{min} below 3 nm, the ion-grain interactions enhance the growth rates, steepen the slope of [Si/H]_{gas}-n(H) relation and provide a better match to observations. The rates of dust re-formation in the ISM by far exceed the rates of dust production by stellar sources as expected from simple evolution models. After the cycle of matter in and out of dust reaches a steady state, the dust growth balances the destruction operating on similar timescales of 350 Myr.

XMM-Newton Large Program on SN1006 - II: Thermal Emission

Based on the XMM-Newton large program on SN1006 and our newly developed spatially resolved spectroscopy tools (Paper~I), we study the thermal emission from ISM and ejecta of SN1006 by analyzing the spectra extracted from 583 tessellated regions dominated by thermal emission. With some key improvements in spectral analysis as compared to Paper~I, we obtain much better spectral fitting results with less residuals. The spatial distributions of the thermal and ionization states of the ISM and ejecta show different features, which are consistent with a scenario that the ISM (ejecta) is heated and ionized by the forward (reverse) shock propagating outward (inward). Different elements have different spatial distributions and origins, with Ne mostly from the ISM, Si and S from the ejecta, and O and Mg from both ISM and ejecta. Fe L-shell lines are only detected in a small shell-like region SE to the center of SN1006, indicating that most of the Fe-rich ejecta has not yet or just recently been reached by the reverse shock. The overall ejecta abundance patterns for most of the heavy elements, except for Fe and sometimes S, are consistent with typical Type~Ia SN products. The NW half of the SNR interior probably represents a region with turbulently mixed ISM and ejecta, so has enhanced emission from O, Mg, Si, S, lower ejecta temperature, and a large diversity of ionization age. In addition to the asymmetric ISM distribution, an asymmetric explosion of the progenitor star is also needed to explain the asymmetric ejecta distribution.

Interstellar medium structure and the slope of the radio $\Sigma-D$ relation of supernova remnants

We analyze the influence of fractal structure of the interstellar matter (ISM) density on the parameter values for the radio surface brightness to diameter ($\Sigma-D$) relation for supernovae remnants (SNRs). We model a dense ISM as a molecular cloud with fractal density structure. SNRs are modelled as spheres of different radius scattered in the modelled ISM. The surface brightness of the SNRs is calculated from the simple relation $\Sigma \propto \rho^{0.5}D^{-3.5}$ and also from the parametrized more general form $\Sigma \propto \rho^{\eta}D^{-\beta_0}$. Our results demonstrate that empirical $\Sigma-D$ slopes that are steeper than the ones derived from theory, might be partly explained with the fractal structure of the ambient medium into which SNRs expand. The slope of the $\Sigma-D$ relation steepens if the density of the regions where SNRs are formed is higher. The simple geometrical effects combined with the fractal structure of the ISM can contribute to a steeper empirical $\Sigma-D$ slopes, especially for older remnants, and this is more pronounced if $\Sigma$ has a stronger dependence on ambient density.

Modelling dust processing and the evolution of grain sizes in the ISM using the method of moments

Interstellar dust grains do not have a single well-defined origin. Stars are demonstrably dust producers, but also efficient destroyers of cosmic dust. Dust destruction in the ISM is believed to be the result of SN shocks hitting the ambient ISM gas (and dust) and lead to an increased rate of ion sputtering, which reduces the dust mass. Grains located in cold molecular clouds can on the other hand grow by condensation, thus providing a replenishment mechanism or even a dominant channel of dust formation. In dense environments grains may coagulate and form large composite grains and aggregates and if grains collide with large enough energies they may be shattered, forming a range of smaller debris grains. The present paper presents a statistical modelling approach using the method of moments, which is computationally very inexpensive and may therefore be an attractive option when combining dust processing with, e.g., detailed simulations of interstellar gas dynamics. A solar-neighbourhood-like toy model of interstellar dust evolution is presented as an example.

Radiative Transfer Model of Dust Attenuation Curves in Clumpy, Galactic Environments

The attenuation of starlight by dust in galactic environments is investigated through models of radiative transfer in a spherical, clumpy ISM. Extinction properties for MW, LMC, and SMC dust types are considered. It is illustrated that the attenuation curves are primarily determined by the wavelength dependence of absorption rather than by the underlying extinction (absorption+scattering) curve. Attenuation curves consistent with the "Calzetti attenuation curve" are found by assuming the silicate-carbonaceous dust model for the MW, but with the 2175A absorption bump suppressed or absent. The discrepancy between our results and previous work that claimed the SMC-type dust to be the most likely origin of the Calzetti curve is ascribed to the difference in adopted albedos; this study uses the theoretically calculated albedos whereas the previous ones adopted empirically derived albedos from observations of reflection nebulae. It is also found that the model attenuation curves calculated with the MW dust are well represented by a modified Calzetti curve with a varying slope and UV bump strength. The strong correlation between the slope and UV bump strength, with steeper curves having stronger bumps, as found in star-forming galaxies at 0.5<z<2.0, is well reproduced by our models if the abundance of the UV bump carriers or PAHs is assumed to be 30% or 40% of that of the MW-dust. The trend is explained by radiative transfer effects which lead to shallower attenuation curves with weaker UV bumps as the ISM is more clumpy and dustier. We also argue that at least some of the IUE local starburst galaxies may have a UV bump feature in their attenuation curves, albeit much weaker than that of the MW extinction curve.

A detailed framework to incorporate dust in hydrodynamical simulations

Dust plays a key role in the evolution of the ISM and its correct modelling in numerical simulations is therefore fundamental. We present a new and self-consistent model that treats grain thermal coupling with the gas, radiation balance, and surface chemistry for molecular hydrogen. This method can be applied to any dust distribution with an arbitrary number of grain types without affecting the overall computational cost. In this paper we describe in detail the physics and the algorithm behind our approach, and in order to test the methodology, we present some examples of astrophysical interest, namely (i) a one-zone collapse with complete gas chemistry and thermochemical processes, (ii) a 3D model of a low-metallicity collapse of a minihalo starting from cosmological initial conditions, and (iii) a turbulent molecular cloud with H-C-O chemistry (277 reactions), together with self-consistent cooling and heating solved on the fly. Although these examples employ the publicly available code KROME, our approach can be easily integrated into any computational framework.

Relativistic jet feedback in high-redshift galaxies I: Dynamics

We present the results of three dimensional relativistic hydrodynamic simulations of interaction of AGN jets with a dense turbulent two-phase interstellar medium, which would be typical of high redshift galaxies. We describe the effect of the jet on the evolution of the density of the turbulent ISM. The jet driven energy bubble affects the gas to distances up to several kiloparsecs from the injection region. The shocks resulting from such interactions create a multi-phase ISM and radial outflows. One of the striking result of this work is that low power jets (P_jet < 10^{43} erg/s) although less efficient in accelerating clouds, are trapped in the ISM for a longer time and hence affect the ISM over a larger volume. Jets of higher power drill through with relative ease. Although the relativistic jets launch strong outflows, there is little net mass ejection to very large distances, supporting a galactic fountain scenario for local feedback.

The Contribution of Chemical Abundances in Nova Ejecta to the Interstellar Medium

According to the nova model from \citet{Yaron2005} and \citet{Jose1998} and using Monte Carlo simulation method, we investigate the contribution of chemical abundances in nova ejecta to the interstellar medium (ISM) of the Galaxy. We find that the ejected mass by classical novae (CNe) is about $2.7\times10^{-3}$ $ \rm M_\odot\ {\rm yr^{-1}}$. In the nova ejecta, the isotopic ratios of C, N and O, that is, $^{13}$C/$^{12}$C, $^{15}$N/$^{14}$N and $^{17}$O/$^{16}$O, are higher about one order of magnitude than those in red giants. We estimate that about 10$\%$, 5$\%$ and 20$\%$ of $^{13}$C, $^{15}$N and $^{17}$O in the ISM of the Galaxy come from nova ejecta, respectively. However, the chemical abundances of C, N and O calculated by our model can not cover all of observational values. This means that there is still a long way to go for understanding novae.

Chemical Complementarity between the Gas Phase of the Interstellar Medium and the Rocky Material of Our Planetary System

We compare the elemental depletions in the gas phase of the interstellar medium (ISM) with the elemental depletions in the rocky material of our Solar System. Our analysis finds a high degree of chemical complementarity: elements depleted in the gas phase of the ISM are enriched in the rocky material of our Solar System, and vice versa. This chemical complementarity reveals the generic connections between interstellar dust and rocky planetary material. We use an inheritance model to explain the formation of primordial grains in the solar nebula. The primary dust grains inherited from the ISM, in combination with the secondary ones condensed from the solar nebula, constitute the primordial rocky material of our planetary system, from which terrestrial planets are formed through the effects of the progressive accretion and sublimation. The semi-major-axis-dependence of the chemical composition of rocky planetary material is also observed by comparing elemental depletions in the Earth, CI chondrites and other types of carbonaceous chondrites.

Supernova Remnants in the Local Group I: A model for the radio luminosity function and visibility times of supernova remnants

Supernova remnants (SNRs) in Local Group galaxies offer unique insights into the origin of different types of supernovae. In order to take full advantage of these insights, one must understand the intrinsic and environmental diversity of SNRs in the context of their host galaxies. We introduce a semi-analytic model that reproduces the statistical properties of a radio continuum-selected SNR population, taking into account the detection limits of radio surveys, the range of SN kinetic energies, the measured ISM and stellar mass distribution in the host galaxy from multi-wavelength images and the current understanding of electron acceleration and field amplification in SNR shocks from first-principle kinetic simulations. Applying our model to the SNR population in M33, we reproduce the SNR radio luminosity function with a median SN rate of $\sim 3.1 \times 10^{-3}$ per year and an electron acceleration efficiency, $\epsilon_{\rm{e}} \sim 4.2 \times 10^{-3}$. We predict that the radio visibility times of $\sim 70\%$ of M33 SNRs will be determined by their Sedov-Taylor lifetimes, and correlated with the measured ISM column density, $N_H$ ($t_{\rm{vis}} \propto N_H^{-a}$, with $a \sim 0.33$) while the remaining will have visibility times determined by the detection limit of the radio survey. These observational constraints on the visibility time of SNRs will allow us to use SNR catalogs as `SN surveys' to calculate SN rates and delay time distributions in the Local Group.

Changing physical conditions in star-forming galaxies between redshifts 0 < z < 4: [OIII]/Hb evolution

We investigate the redshift evolution of the [OIII]/Hb nebular emission line ratio for a sample of galaxies spanning the redshift range 0 < z < 4. We compare the observed evolution to a set of theoretical models which account for the independent evolution of chemical abundance, ionization parameter and interstellar-medium (ISM) pressure in star-forming galaxies with redshift. Accounting for selection effects in the combined datasets, we show that the evolution to higher [OIII]/Hb ratios with redshift is a real physical effect which is best accounted for by a model in which the ionization parameter is elevated from the average values typical of local star-forming galaxies, with a possible simultaneous increase in the ISM pressure. We rule out the possibility that the observed [OIII]/Hb evolution is purely due to metallicity evolution. We discuss the implications of these results for using local empirical metallicity calibrations to measure metallicities at high redshift, and briefly discuss possible theoretical implications of our results.

Understanding inverse metallicity gradients in galactic discs as a consequence of inside-out formation

The early stages of a galaxy's evolution leave an imprint on its metallicity distribution. We discuss the origins and evolution of radial metallicity gradients in discs of spiral galaxies using an analytical chemical evolution model. We explain how radial metallicity gradients in stellar populations are determined by three factors: the radial metallicity profile of the star-forming ISM, radial changes in the star-formation history (in particular inside-out formation), and radial mixing of stars. Under reasonable assumptions, inside-out formation steepens the negative ISM metallicity gradient, but contributes positively to the stellar metallicity gradient, up to inverting the metallicity profile to a positive d[Fe/H]/dR. This reconciles steep negative d[Fe/H]/dR in some high redshift galaxies to generally flatter gradients in local observations. We discuss the evidence for inverse radial metallicity gradients (positive d[X/H]/dR) at high redshifts and the inverse relationship between azimuthal velocity and the metallicity (positive dV_{\phi}/d[Fe/H]) of stars for the Milky Way's thick disc. We show that the former can be achieved by high central gas-loss rates and re-distribution processes, e.g. re-accretion of enriched material in conjunction with inside-out formation, and near-disc galactic fountaining. For the Milky Way thick disc, we show that the positive dV_{\phi}/d[Fe/H] correlation points to inside-out formation on a comparable timescale to SNIa enrichment. We argue that the original ISM metallicity gradient could be inferred with better data from the high-metallicity tail of the alpha enhanced population. Including inside-out formation in our models changes the local vertical metallicity gradient by about -0.2dex/kpc, in line with local measurements.

Iron: A Key Element for Understanding the Origin and Evolution of Interstellar Dust

The origin and depletion of iron differ from all other abundant refractory elements that make up the composition of the interstellar dust. Iron is primarily synthesized in Type Ia supernovae (SNe Ia) and in core collapse supernovae (CCSN), and is present in the outflows from AGB stars. Only the latter two are observed to be sources of interstellar dust, since searches for dust in SN~Ia have provided strong evidence for the absence of any significant mass of dust in their ejecta. Consequently, more than 65% of the iron is injected into the ISM in gaseous form. Yet, ultraviolet and X-ray observations along many lines of sight in the ISM show that iron is severely depleted in the gas phase compared to expected solar abundances. The missing iron, comprising about 90% of the total, is believed to be locked up in interstellar dust. This suggests that most of the missing iron must have precipitated from the ISM gas by cold accretion onto preexisting silicate, carbon, or composite grains. Iron is thus the only element that requires most of its growth to occur outside the traditional stellar condensation sources. This is a robust statement that does not depend on our evolving understanding of the dust destruction efficiency in the ISM. Reconciling the physical, optical, and chemical properties of such composite grains with their many observational manifestations is a major challenge for understanding the nature and origin of interstellar dust.

[C II] and [N II] from dense ionized regions in the Galaxy

The interstellar medium (ISM) consists of highly ionized and neutral atomic, as well as molecular, components. Knowledge of their distribution is important for tracing the structure and lifecycle of the ISM. Here we determine the properties of the highly ionized and neutral weakly ionized gas in the Galaxy traced by the fine-structure lines of ionized nitrogen, [N II], and ionized carbon, [C II]. To analyze the ionized ISM we utilize [C II] 158 micron and [N II] 205 micron lines taken with the high spectral resolution Heterodyne Instrument in the Far-Infrared (HIFI) on the Herschel Space Observatory along ten lines of sight towards the inner Galaxy. [N II] emission can be used to estimate the contribution of the highly ionized gas to the [C II] emission and separate the highly ionized and weakly ionized neutral gas. We find that [N II] has strong emission in distinct spectral features along all lines of sight associated with strong [C II] emission. The [N II] arises from moderate density extended HII regions or ionized boundary layers of clouds. Comparison of the [N II] and [C II] spectra in 31 separate kinematic features shows that many of the [C II] spectra are affected by absorption from low excitation gas associated with molecular clouds, sometimes strongly so. The apparent fraction of the [C II] associated with the [N II] gas is unrealistically large in many cases, most likely due to the reduction of [C II] by absorption. In a few cases the foreground absorption can be modeled to determine the true source intensity. In these sources we find that the foreground absorbing gas layer has C$^+$ column densities of order 10$^{18}$ cm$^{-2}$. [C II] emission arising from strong sources of [N II] emission is frequently absorbed by low excitation foreground gas complicating the interpretation of the properties of the ionized and neutral gas components that give rise to [C II] emission.

Comparing [CII], HI, and CO dynamics of nearby galaxies

The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M_dyn of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 $\mu$m line of atomic carbon [CII] may be the only way to derive M_dyn. As the distribution and kinematics of the ISM tracer affects the determination of M_dyn, it is important to quantify the relative distributions of HI, CO and [CII]. HI and CO are well-characterised observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS and KINGFISH surveys. We find that within R_25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M_dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M_dyn based on [CII] data, is that of the well-characterised CO distribution. This length scale is similar to that of the optical disk.

Spatially resolved integral field spectroscopy of the ionized gas in IZw18

We present a detailed 2D study of the ionized ISM of IZw18 using new PMAS-IFU optical observations. IZw18 is a high-ionization galaxy which is among the most metal-poor starbursts in the local Universe. This makes IZw18 a local benchmark for understanding the properties most closely resembling those prevailing at distant starbursts. Our IFU-aperture (~ 1.4 kpc x 1.4 kpc) samples the entire IZw18 main body and an extended region of its ionized gas. Maps of relevant emission lines and emission line ratios show that higher-excitation gas is preferentially located close to the NW knot and thereabouts. We detect a Wolf-Rayet feature near the NW knot. We derive spatially resolved and integrated physical-chemical properties for the ionized gas in IZw18. We find no dependence between the metallicity-indicator R23 and the ionization parameter (as traced by [OIII]/[OII]) across IZw18. Over ~ 0.30 kpc^2, using the [OIII]4363 line, we compute Te[OIII] values (~ 15000 - 25000 K), and oxygen abundances are derived from the direct determinations of Te[OIII]. More than 70% of the higher-Te[OIII] (> 22000 K) spaxels are HeII4686-emitting spaxels too. From a statistical analysis, we study the presence of variations in the ISM physical-chemical properties. A galaxy-wide homogeneity, across hundreds of parsecs, is seen in O/H. Based on spaxel-by-spaxel measurements, the error-weighted mean of 12 + log(O/H) = 7.11 +/- 0.01 is taken as the representative O/H for IZw18. Aperture effects on the derivation of O/H are discussed. Using our IFU data we obtain, for the first time, the IZw18 integrated spectrum.

On the Deuterium-to-Hydrogen Ratio of the Interstellar Medium

Observations show that the global deuterium-to-hydrogen ratio (D/H) in the local interstellar medium (ISM) is about 90% of the primordial ratio predicted by big bang nucleosynthesis. The high (D/H)$_{ISM}$ implies that only a small fraction of interstellar gas has been processed through stars, which destroy any deuterium they are born with. Using analytic arguments for one-zone chemical evolution models that include accretion and outflow, I show that the deuterium abundance is tightly coupled to the abundance of core collapse supernova (CCSN) elements such as oxygen. These models predict that the ratio of (D/H)$_{ISM}$ to the primordial abundance is $\approx 1/(1+r Z_O/m_O)$, where r is the recycling fraction, $Z_O$ is the ISM oxygen mass fraction, and $m_O$ is the population averaged CCSN yield of oxygen. Using values $r=0.4$ and $m_O=0.015$ appropriate to a Kroupa (2001) initial mass function and recent CCSN yield calculations, solar oxygen abundance corresponds to an ISM (D/H) that is 87\% of the primordial value, consistent with observations. This approximation is accurate for a wide range of parameter values, and physical arguments suggest that it should remain accurate for more complex chemical evolution models, making the deuterium abundance a robust prediction of almost any model that reproduces the observed ISM metallicity. The good agreement with the upper range of observed (D/H)$_{ISM}$ values supports the long-standing suggestion that sightline-to-sightline variations of deuterium are a consequence of dust depletion, rather than a low global (D/H)$_{ISM}$ enhanced by localized accretion of primordial composition gas. This agreement limits deviations from conventional yield and recycling values, and it implies that Galactic outflows eject ISM hydrogen as efficiently as they eject CCSN metals.

A supernova feedback implementation for the astrophysical simulation software Arepo [Replacement]

Supernova (SN) explosions play an important role in the development of galactic structures. The energy and momentum imparted on the interstellar medium (ISM) in so-called "supernova feedback" drives turbulence, heats the gas, enriches it with heavy elements, can lead to the formation of new stars or even suppress star formation by disrupting stellar nurseries. In the numerical simulation at the sub-galactic level, not including the energy and momentum of supernovas in the physical description of the problem can also lead to several problems that might partially be resolved by including a description of supernovas. In this thesis such an implementation is attempted for the combined numerical hydrodynamics and N-body simulation software Arepo (Springel, 2010) for the high density gas in the ISM only. This allows supernova driven turbulence in boxes of 400pc cubed to be studied. In a stochastic process a large amount of thermal energy is imparted on a number of neighbouring cells, mimicking the effect of a supernova explosions. We test this approach by modelling the explosion of a single supernova in a uniform density medium and comparing the evolution of the resulting supernova remnant to the theoretically-predicted behaviour. We also run a simulation with our feedback code and a fixed supernova rate derived from the Kennicutt-Schmidt relation (Kennicutt, 1998) for a duration of about 20 Myrs. We describe our method in detail in this text and discuss the properties of our implementation. vii

Unveiling The Physics of Star Formation and Feedback in Galaxies

Recent studies show the importance of feedback in the evolution of the star formation rate in the Universe. However, the nature and physics of the feedback are still pressing questions. Radio continuum observations can provide unique dust-unbiased tracers of massive star formation and of the interstellar medium (ISM) and hence are ideal to address the regulation of star formation in galaxies. Our multi-frequency and multi-resolution radio surveys in nearby galaxies enable us to trace various phases of star formation and dissect the thermal and nonthermal ISM in galaxies. Mapping the cosmic ray electron energy index and magnetic field strength, we have found observational evidence that massive star formation significantly affects the energy balance in the ISM through the injection and acceleration of cosmic rays and the amplification of magnetic fields. How the next generation of stars could form in such a magnetized and turbulent ISM will be addressed in our 'EVLA cloud-scale survey of the local group galaxy M33' and in forthcoming surveys with the SKA.

The Impact of a Supernova Remnant on Fast Radio Bursts [Replacement]

Fast radio bursts (FRBs) are millisecond bursts of radio radiation whose progenitors so far remain mysterious. Nevertheless, the timescales and energetics of the events have lead to many theories associating FRBs with young neutron stars. Motivated by this, I explore the interaction of FRBs with young supernova remnants (SNRs), and I discuss the potential observational consequences and constraints of such a scenario. As the SN ejecta plows into the interstellar medium (ISM), a reverse shock is generated that passes back through the material and ionizes it. This leads to a dispersion measure (DM) associated with the SNR as well as a time derivative for DM. Times when DM is high are generally overshadowed by free-free absorption, which, depending on the mass of the ejecta and the density of the ISM, may be probed at frequencies of $400\,{\rm MHz}$ to $1.4\,{\rm GHz}$ on timescales of $\sim100-500\,{\rm yrs}$ after the SN. Magnetic fields generated at the reverse shock may be high enough to explain Faraday rotation that has been measured for one FRB. If FRBs are powered by the spin energy of a young NS (rather than magnetic energy), the NS must have a magnetic field $\lesssim10^{11}-10^{12}\,{\rm G}$ to ensure that it does not spin down too quickly while the SNR is still optically thick at radio frequencies. In the future, once there are distance measurements to FRBs and their energetics are better understood, the spin of the NS can also be constrained.

Supernova-blast waves in wind-blown bubbles, turbulent, and power-law ambient media

Supernova (SN) blast waves inject energy and momentum into the interstellar medium (ISM), control its turbulent multiphase structure and the launching of galactic outflows. Accurate modelling of the blast wave evolution is therefore essential for ISM and galaxy formation simulations. We present an efficient method to compute the input of momentum, thermal energy, and the velocity distribution of the shock-accelerated gas for ambient media with uniform (and with stellar wind blown bubbles), power-law, and turbulent density distributions. Assuming solar metallicity cooling, the blast wave evolution is followed to the beginning of the momentum conserving snowplough phase. The model recovers previous results for uniform ambient media. The momentum injection in wind-blown bubbles depend on the swept-up mass and the efficiency of cooling, when the blast wave hits the wind shell. For power-law density distributions with $n(r) \sim$ $r^{-2}$ (for $n(r) > n_{_{\rm floor}}$) the amount of momentum injection is solely regulated by the background density $n_{_{\rm floor}}$ and compares to $n_{_{\rm uni}}$ = $n_{_{\rm floor}}$. However, in turbulent ambient media with log-normal density distributions the momentum input can increase by a factor of 2 (compared to the homogeneous case) for high Mach numbers. The average momentum boost can be approximated as $p_{_{\rm turb}}/\mathrm{p_{_{0}}}\ =23.07\, \left(\frac{n_{_{0,\rm turb}}}{1\,{\rm cm}^{-3}}\right)^{-0.12} + 0.82 (\ln(1+b^{2}\mathcal{M}^{2}))^{1.49}\left(\frac{n_{_{0,\rm turb}}}{1\,{\rm cm}^{-3}}\right)^{-1.6}$. The velocity distributions are broad as gas can be accelerated to high velocities in low-density channels. The model values agree with results from recent, computationally expensive, three-dimensional simulations of SN explosions in turbulent media.

First Detection of $^3$He$^+$ in the Planetary Nebula IC$\,$418

The $^3$He isotope is important to many fields of astrophysics, including stellar evolution, chemical evolution, and cosmology. The isotope is produced in low-mass stars which evolve through the planetary nebula (PN) phase. $^3$He abundances in PNe can help test models of the chemical evolution of the Galaxy. We present the detection of the $^3$He$^+$ emission line using the single dish Deep Space Station 63, towards the PN IC$\,$418. We derived a $^3$He/H abundance in the range 1.74$\pm$0.8$\times$10$^{-3}$ to 5.8$\pm$1.7$\times$10$^{-3}$, depending on whether part of the line arises in an outer ionized halo. The lower value for $^3$He/H ratio approaches values predicted by stellar models which include thermohaline mixing, but requires that large amounts of $^3$He are produced inside low-mass stars which enrich the interstellar medium (ISM). However, this over-predicts the $^3$He abundance in HII regions, the ISM, and proto-solar grains, which is known to be of the order of 10$^{-5}$. This discrepancy questions our understanding of the evolution of the $^3$He, from circumstellar environments to the ISM.

HST Imaging of Dust Structures and Stars in the Ram Pressure Stripped Virgo Spirals NGC 4402 and NGC 4522: Stripped from the Outside In with Dense Cloud Decoupling

We describe and constrain the origins of ISM structures likely created by ongoing ICM ram pressure stripping in two Virgo Cluster spirals, NGC 4522 and NGC 4402, using HST BVI images of dust extinction and stars, as well as supplementary HI, Halpha, and radio continuum images. This is the highest-resolution study to date of the physical processes that occur during an ICM-ISM ram pressure stripping interaction, ram pressure stripping's effects on the multi-phase, multi-density ISM, and the formation and evolution of ram-pressure-stripped tails. In dust extinction, we view the leading side of NGC 4402 and the trailing side of NGC 4522; we see distinct types of features in both galaxies. NGC 4522 has experienced stronger, more recent pressure and has the jellyfish morphology characteristic of some ram pressure stripped galaxies. Its stripped tail extends up from the disk plane in continuous upturns of dust and stars curving ~2 kpc above the disk plane. A kinematically and morphologically distinct extraplanar arm of young, blue stars and ISM extends above a mostly-stripped portion of the disk, and between it and the disk plane are decoupled dust clouds. NGC 4402 contains long dust ridges, suggesting that large parts of the ISM are being pushed out at once. Both galaxies contain long ridges of polarized radio continuum emission indicating the presence of large-scale ordered magnetic fields. We propose that magnetic fields could bind together gas of different densities, causing nearby gas of different densities to be stripped at the same rate and creating the large, coherent dust ridges and upturns. A number of factors that play roles in determining what types of structures form as a result of ram pressure: ram pressure strength and history, the location within the galaxy relative to the leading side, and pre-existing substructure in the ISM that may be bound together by magnetic fields.

Evolution of Molecular and Atomic Gas Phases in the Milky Way

We analyze radial and azimuthal variations of the phase balance between the molecular and atomic ISM in the Milky Way. In particular, the azimuthal variations -- between spiral arm and interarm regions -- are analyzed without any explicit definition of spiral arm locations. We show that the molecular gas mass fraction, i.e., fmol=H2/ (HI+H2) in mass, varies predominantly in the radial direction: starting from ~100% at the center, remaining ~>50% (~>60%) to R~6kpc, and decreasing to ~10-20% (~50%) at R=8.5 kpc when averaged over the whole disk thickness (in the mid plane). Azimuthal, arm-interarm variations are secondary: only ~20%, in the globally molecule-dominated inner MW, but becoming larger, ~40-50%, in the atom-dominated outskirts. This suggests that in the inner MW, the gas stays highly molecular (fmol>50%) as it goes from an interarm region, into a spiral arm, and back into the next interarm region. Stellar feedback does not dissociate molecules much, and the coagulation and fragmentation of molecular clouds dominate the evolution of the ISM at these radii. The trend differs in the outskirts, where the gas phase is globally atomic (fmol<50%). The HI and H2 phases cycle through spiral arm passage there. These different regimes of ISM evolution are also seen in external galaxies (e.g., LMC, M33, and M51). We explain the radial gradient of fmol by a simple flow continuity model. The effects of spiral arms on this analysis are illustrated in Appendix.

ISM gas studies towards the TeV PWN HESS J1825-137 and northern region

HESS J1825-137 is a pulsar wind nebula (PWN) whose TeV emission extends across ~1 deg. Its large asymmetric shape indicates that its progenitor supernova interacted with a molecular cloud located in the north of the PWN as detected by previous CO Galactic survey (e.g Lemiere, Terrier & Djannati-Ata\"i 2006). Here we provide a detailed picture of the ISM towards the region north of HESS J1825-137, with the analysis of the dense molecular gas from our 7mm and 12mm Mopra survey and the more diffuse molecular gas from the Nanten CO(1-0) and GRS $^{13}$CO(1-0) surveys. Our focus is the possible association between HESS J1825-137 and the unidentified TeV source to the north, HESS J1826-130. We report several dense molecular regions whose kinematic distance matched the dispersion measured distance of the pulsar. Among them, the dense molecular gas located at (RA, Dec)=(18.421h,-13.282$^{\circ}$) shows enhanced turbulence and we suggest that the velocity structure in this region may be explained by a cloud-cloud collision scenario. Furthermore, the presence of a H$\alpha$ rim may be the first evidence of the progenitor SNR of the pulsar PSR J1826-1334 as the distance between the H$\alpha$ rim and the TeV source matched with the predicted SNR radius R$_{\text{SNR}}$~120 pc. From our ISM study, we identify a few plausible origins of the HESS J1826-130 emission, including the progenitor SNR of PSR J1826-1334 and the PWN G018.5-0.4 powered by PSR J1826-1256. A deeper TeV study however, is required to fully identify the origin of this mysterious TeV source.

The (impossible?) formation of acetaldehyde on the grain surfaces: insights from quantum chemical calculations

Complex Organic Molecules (COMs) have been detected in the interstellar medium (ISM). However, it is not clear whether their synthesis occurs on the icy surfaces of interstellar grains or via a series of gas-phase reactions. As a test case of the COMs synthesis in the ISM, we present new quantum chemical calculations on the formation of acetaldehyde (CH3CHO) from the coupling of the HCO and CH3 radicals, both in gas phase and on water ice surfaces. The binding energies of HCO and CH3 on the amorphous water ice were also computed (2333 and 734 K, respectively). Results indicate that, in gas phase, the products could be either CH3CHO, CH4 + CO, or CH3OCH, depending on the relative orientation of the two radicals. However, on the amorphous water ice, only the CH4 + CO product is possible due to the geometrical constraints imposed by the water ice surface. Therefore, acetaldehyde cannot be synthesized by the CH3 + HCO coupling on the icy grains. We discuss the implications of these results and other cases, such as ethylene glycol and dimethyl ether, in which similar situations can occur, suggesting that formation of these molecules on the grain surfaces might be unlikely.

Constraining Gamma-Ray Emission from Luminous Infrared Galaxies with Fermi-LAT; Tentative Detection of Arp 220

Star-forming galaxies produce gamma-rays primarily via pion production, resulting from inelastic collisions between cosmic ray protons and the interstellar medium (ISM). The dense ISM and high star formation rates of luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) imply that they should be strong gamma-ray emitters, but so far only NGC 1068 has been detected. Theoretical models for their emission depend on the unknown fraction of cosmic ray protons that escape these galaxies before interacting. We analyze Fermi Large Area Telescope data for 82 of the brightest IRAS LIRGs and ULIRGs on the sky. We examine each system individually and carry out a stacking analysis to constrain their gamma-ray fluxes. We report the detection of the nearest ULIRG Arp 220 (3.44sigma above background), with a gamma-ray flux (0.8-100 GeV) of 3.2e-10 phot cm^-2 s^-1 (1.5e42 ergs s^-1 at 77 Mpc). We also derive upper limits for the stacked LIRGs and ULIRGs. The gamma-ray luminosity of Arp 220 and the stacked upper limits agree with calorimetric predictions for dense star-forming galaxies. With the detection of Arp 220, we extend the gamma-ray--IR luminosity correlation to the high luminosity regime with log{L_(0.1-100 GeV)} = 1.27log{L_(8-1000)} + 26.6 as well as the gamma-ray--radio continuum luminosity correlation with log{L_(0.1-100 GeV)} = 1.21log{L_(1.4 GHz)} + 13.4. The current survey of Fermi-LAT is on the verge of detecting more LIRGs/ULIRGs in the local universe, and we expect even more detections with deeper Fermi-LAT observations or the next generation of gamma-ray detectors.

Constraining Gamma-Ray Emission from Luminous Infrared Galaxies with Fermi-LAT; Tentative Detection of Arp 220 [Replacement]

Star-forming galaxies produce gamma-rays primarily via pion production, resulting from inelastic collisions between cosmic ray protons and the interstellar medium (ISM). The dense ISM and high star formation rates of luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) imply that they should be strong gamma-ray emitters, but so far only two LIRGs have been detected. Theoretical models for their emission depend on the unknown fraction of cosmic ray protons that escape these galaxies before interacting. We analyze Fermi-LAT data for 82 of the brightest IRAS LIRGs and ULIRGs. We examine each system individually and carry out a stacking analysis to constrain their gamma-ray fluxes. We report the detection of the nearest ULIRG Arp 220 (~4.6sigma). We observe a gamma-ray flux (0.8--100 GeV) of 2.4e-10 phot cm^-2 s^-1 with photon index of 2.23 (8.2e10^41 ergs s^-1 at 77 Mpc) We also derive upper limits for the stacked LIRGs and ULIRGs. The gamma-ray luminosity of Arp~220 and the stacked upper limits agree with calorimetric predictions for dense star-forming galaxies. With the detection of Arp 220, we extend the gamma-ray--IR luminosity correlation to the high luminosity regime with log(L_[0.1-100 GeV]) = 1.25log(L_[8-1000]) + 26.7 as well as the gamma-ray--radio continuum luminosity correlation with log(L_[0.1-100 GeV]) = 1.22log(L_[1.4 GHz]) + 13.3. The current survey of Fermi-LAT is on the verge of detecting more LIRGs/ULIRGs in the local universe, and we expect even more detections with deeper Fermi-LAT observations or the next generation of gamma-ray detectors.

Spinning dust emission from ultrasmall silicates: emissivity and polarization spectrum

Anomalous microwave emission (AME) is an important Galactic foreground of Cosmic Microwave Background (CMB) radiation. It is believed that the AME arises from rotational emission by spinning polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). In this paper, we assume that a population of ultrasmall silicate grains may exist in the ISM, and quantify rotational emissivity from these tiny particles and its polarization spectrum. We found that spinning silicate nanoparticles can produce strong rotational emission when those small grains follow a log-normal size distribution. The polarization fraction of spinning dust emission from tiny silicates increases with decreasing the dipole moment per atom ($\beta$) and can reach $P\sim 20\%$ for $\beta\sim 0.1$D at grain temperature of 60 K. We identify a parameter space $(\beta,Y_{Si})$ for silicate nanoparticles in which its rotational emission can adequately reproduce both the observed AME and the polarization of the AME, without violating the observational constraints by the ultraviolet extinction and polarization of starlight. Our results reveal that rotational emission from spinning silicate may be an important source of the AME.

Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM

Two-thirds of long duration gamma-ray bursts (GRBs) show soft X-ray absorption in excess of the Milky Way. The column densities of metals inferred from UV and optical spectra differ from those derived from soft X-ray spectra, at times by an order of magnitude, with the latter being higher. The origin of the soft X-ray absorption excess observed in GRB X-ray afterglow spectra remains a heavily debated issue, which has resulted in numerous investigations on the effect of hot material both internal and external to the GRB host galaxy on our X-ray afterglow observations. Nevertheless, all models proposed so far have either only been able to account for a subset of our observations (i.e. at z > 2), or they have required fairly extreme conditions to be present within the absorbing material. In this paper, we investigate the absorption of the GRB afterglow by a collisionally ionised and turbulent interstellar medium (ISM). We find that a dense (3 per cubic centimeters) collisionally ionised ISM could produce UV/optical and soft X-ray absorbing column densities that differ by a factor of 10, however the UV/optical and soft X-ray absorbing column densities for such sightlines and are 2-3 orders of magnitude lower in comparison to the GRB afterglow spectra. For those GRBs with a larger soft X-ray excess of up to an order of magnitude, the contribution in absorption from a turbulent ISM as considered here would ease the required conditions of additional absorbing components, such as the GRB circumburst medium and intergalactic medium.

On the observability of bow shocks of Galactic runaway OB stars

Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow shock nebulae that can be observed. We present two-dimensional, axisymmetric hydrodynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3. Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyze them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass (~40 Mo) runaway stars yield H$\alpha$ fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at Ha by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 Mo. Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 Mo.

Magnetic fields on a wide range of scales in star-forming galaxies [Replacement]

A key ingredient in the evolution of galaxies is the star formation cycle. Recent progress in the study of magnetic fields is revealing the close connection between star formation and its effect on the small-scale structure in the magnetized interstellar medium (ISM). In this contribution we describe how the modern generation of radio telescopes is being used to probe the physics of the ISM through sensitive multiwavelength surveys of gas and magnetic fields, from the inner star forming disk and outward into the galaxy outskirts where large-scale magnetic fields may also play a key role. We highlight unique pioneering efforts towards performing and scientifically exploiting large-scale surveys of the type that the SKA will undertake routinely. Looking to the future, we describe plans for using the Square Kilometre Array (SKA) and its pathfinders to gain important new insights into the cosmic history of galaxy evolution.

Magnetic fields on a wide range of scales in star-forming galaxies

A key ingredient in the evolution of galaxies is the star formation cycle. Recent progress in the study of magnetic fields is revealing the close connection between star formation and its effect on the small-scale structure in the magnetized interstellar medium (ISM). In this contribution we describe how the modern generation of radio telescopes is being used to probe the physics of the ISM through sensitive multiwavelength surveys of gas and magnetic fields, from the inner star forming disk and outward into the galaxy outskirts where large-scale magnetic fields may also play a key role. We highlight unique pioneering efforts towards performing and scientifically exploiting large-scale surveys of the type that the SKA will undertake routinely. Looking to the future, we describe plans for using the Square Kilometre Array (SKA) and its pathfinders to gain important new insights into the cosmic history of galaxy evolution.

How Does Metallicity Affect the Gas and Dust Properties of Galaxies?

Comparison of the ISM properties of a wide range of metal-poor galaxies with normal metal-rich galaxies reveals striking differences. We find that the combination of the low dust abundance and the active star formation results in a very porous ISM filled with hard photons, heating the dust in dwarf galaxies to overall higher temperatures than their metal-rich counterparts. This results in photodissociation of molecular clouds to greater depths, leaving relatively large PDR envelopes and difficult-to-detect CO cores. From detailed modeling of the low-metallicity ISM, we find significant fractions of CO-dark H2 - a reservoir of molecular gas not traced by CO, but present in the [CII] and [CI]-emitting envelopes. Self-consistent analyses of the neutral and ionized gas diagnostics along with the dust SED is the necessary way forward in uncovering the multiphase structure of galaxies

A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor

More complete knowledge of galaxy evolution requires understanding the process of star formation and interaction between the interstellar radiation field and the interstellar medium in galactic environments traversing a wide range of physical parameter space. Here we focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the early universe, results in less ultra-violet shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photo-dissociation regions where the chemistry and thermal balance are regulated by far-ultraviolet photons (6 eV< h\nu <13.6 eV). We present Herschel observations of far-infrared fine-structure lines obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and the structure of the gas by modeling it with the Meudon PDR code. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to 3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three dimensional structure of the gas. (Abridged)

A merger in the dusty, $z=7.5$ galaxy A1689-zD1?

The gravitationally-lensed galaxy A1689-zD1 is one of the most distant spectroscopically confirmed sources ($z=7.5$). It is the earliest known galaxy where the interstellar medium (ISM) has been detected; dust emission was detected with the Atacama Large Millimetre Array (ALMA). A1689-zD1 is also unusual among high-redshift dust emitters as it is a sub-L* galaxy and is therefore a good prospect for the detection of gaseous ISM in a more typical galaxy at this redshift. We observed A1689-zD1 with ALMA in bands 6 and 7 and with the Green Bank Telescope (GBT) in band Q. We map the mm thermal dust emission, confirming the large dust emission found before, and finding two spatial components with sizes about 0.4-1.7 kpc (lensing-corrected). The rough spatial morphology is similar to what is observed in the near-infrared with HST and points to a perturbed dynamical state, perhaps indicative of a major merger or a disc in early formation. The ALMA photometry is used to constrain the far-infrared spectral energy distribution, yielding a moderate dust temperature ($T_{\rm dust} \sim 35$ K for $\beta = 1.65$). We do not detect the CO(3-2) line in the GBT data with a 95% upper limit of 0.3mJy observed. We find a slight excess emission in ALMA band 6 at 220.9 GHz. If this excess is real, it is likely due to emission from the [CII] 158.8 $\mu$m line at $z_{\rm [CII]} = 7.603$. The stringent upper limits on the [CII] and CO(3-2) line luminosities suggest a high ISM gas density in A1689-zD1.

Neutral gas outflows in nearby [U]LIRGs via optical NaD feature

We studied the properties of the neutral gas in a sample of 38 local [U]LIRGs, which mainly covers the less explored LIRG luminosity range. This study is based on the analysis of the spatially integrated and spatially resolved spectra of the NaD feature obtained with the VIMOS/VLT-IFS. Analyzing spatially integrated spectra, we find that the contribution of stars to the observed NaD-EWs is generally small (<35%), and therefore this feature is dominated by ISM absorption. After subtracting the stellar contribution, we find that the pure-ISM integrated spectra generally show blueshifted NaD profiles, indicating neutral gas outflow velocities V in the range 65-260 km/s. Excluding galaxies with AGNs, V shows a dependency with the SFR of the type V \propto SFR^0.15 (in rather good agreement with previous results). The spatially resolved analysis was performed for 40 galaxies, 22 of which have neutral gas velocity fields dominated by noncircular motions with signatures of cone-like winds. However, a modest number of targets (11/40) show disk rotation. We found that the wind masses are in the range (0.4-7.5)x10^8 Msun, reaching up to 3% of the dynamical mass of the host. The mass rates are typically only 0.2-0.4 times the SFR indicating that, the mass loss is too small to slow down the SF significantly. In the majority of cases, the velocity of the outflowing gas is not sufficient to escape the host potential well and, therefore, most of the gas rains back into the disk. On average V/v(esc) is higher in less massive galaxies, confirming that the galaxy mass has a primary role in shaping the recycling of gas and metals. The comparison between the wind power and kinetic power of the starburst associated with SNe indicates that the starburst could drive the outflows in nearly all the [U]LIRGs galaxies, as the wind power is generally lower than 20% of the kinetic power supplied by the starburst.

VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes

Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium (ISM), yet the branching ratio between ionization and fragmentation is poorly studied. This is crucial for the stability and charge state of PAHs in the ISM in different environments, affecting in turn the chemistry, the energy balance, and the contribution of PAHs to the extinction and emission curves. We studied the interaction of PAH cations with VUV photons in the 7-20 eV range from the synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization for a set of eight PAH cations ranging in size from 14 to 24 carbon atoms, with different structures. At photon energies below ~13.6 eV fragmentation dominates for the smaller species, while for larger species ionization is immediately competitive after the second ionization potential (IP). At higher photon energies, all species behave similarly, the ionization yield gradually increases, leveling off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to mainly affect the fragmentation cross section, but not the ionization cross section. We also measured the second IP for all species and the third IP for two of them, all are in good agreement with theoretical ones confirming that PAH cations can be further ionized in the diffuse ISM. Determining actual PAH dication abundances in the ISM will require detailed modeling. Our measured photo-ionization yields for several PAH cations provide a necessary ingredient for such models.

Transport and mixing of r-process elements in neutron star binary merger blast waves

The r-process nuclei are robustly synthesized in the material ejected during a neutron star binary merger (NSBM), as tidal torques transport angular momentum and energy through the outer Lagrange point in the form of a vast tidal tail. If NSBM are indeed solely responsible for the solar system r- process abundances, a galaxy like our own would require to host a few NSBM per million years, with each event ejecting, on average, about 5x10^{-2} M_sun of r-process material. Because the ejecta velocities in the tidal tail are significantly larger than in ordinary supernovae, NSBM deposit a comparable amount of energy into the interstellar medium (ISM). In contrast to extensive efforts studying spherical models for supernova remnant evolution, calculations quantifying the impact of NSBM ejecta in the ISM have been lacking. To better understand their evolution in a cosmological context, we perform a suite of three-dimensional hydrodynamic simulations with optically-thin radiative cooling of isolated NSBM ejecta expanding in environments with conditions adopted from Milky Way-like galaxy simulations. Although the remnant morphology is highly complex at early times, the subsequent radiative evolution that results from thermal instability in atomic gas is remarkably similar to that of a standard supernova blast wave. This implies that sub-resolution supernova feedback models can be used in galaxy-scale simulations that are unable to resolve the key evolutionary phases of NSBM blast waves. Among other quantities, we examine the radius, time, mass and kinetic energy content of the NSBM remnant at shell formation as well as the momentum injected to the ISM. We find that the shell formation epoch is attained when the swept-up mass is about 10^3 M_sun, at this point the mass fraction of r-process material is drastically enhanced up to two orders of magnitude in relation to a solar metallicity ISM.

Effects of environmental gas compression on the multiphase ISM and star formation

The cluster environment can affect galaxy evolution in different ways: via ram pressure stripping or by gravitational perturbations caused by galactic encounters. New IRAM 30m HERA CO(2-1) data of NGC 4501 and NGC 4567/68 are presented. We find an increase in the molecular fraction where the ISM is compressed. The gas is close to self-gravitation in compressed regions. This leads to an increase in gas pressure and a decrease in the ratio between the molecular fraction and total ISM pressure. The overall Kennicutt Schmidt relation based on a pixel-by-pixel analysis at ~1.5 kpc resolution is not significantly modified by compression. However, we detected continuous regions of low molecular star formation efficiencies in the compressed parts of the galactic gas disks. The data suggest that a relation between the molecular star formation efficiency SFE_H2 and gas self-gravitation exists. Both systems show spatial variations in the star formation efficiency with respect to the molecular gas that can be related to environmental compression of the ISM. An analytical model was used to investigate the dependence of SFE_H2 on self-gravitation. The model correctly reproduces the correlations between R_mol/P_tot, SFE_H2, and Toomre Q if different global turbulent velocity dispersions are assumed for the three galaxies. We found that variations in the N_H_2/I_CO conversion factor can mask most of the correlation between SFE_H2 and the Q parameter. Dynamical simulations were used to compare the effects of ram pressure and tidal ISM compression. We conclude that a gravitationally induced ISM compression has the same consequences as ram pressure compression: (i) an increasing gas surface density, (ii) an increasing molecular fraction, and (iii) a decreasing R_mol/P_tot in the compressed region due to the presence of nearly self-gravitating gas. The response of SFE_H2 to compression is more complex.

Absorption at 11 microns in the interstellar medium and embedded sources: evidence for crystalline silicates

An absorption feature is occasionally reported around 11 ?microns in astronomical spectra, including those of forming stars. Candidate carriers include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide, crystalline silicates or even carbonates. All are known constituents of cosmic dust in one or more types of environments, though not necessarily together. In this paper we present new ground-based 8-13 ?micron spectra of one evolved star, several embedded young stellar objects (YSOs) and a background source lying behind a large column of the interstellar medium (ISM) toward the Galactic Centre. Our observations, obtained at a spectral resolution of ?approximately 100, are compared with previous lower resolution data, as well as data obtained with the Infrared Space Observatory (ISO) on these and other targets. By presenting a subset of a larger sample our aim is to establish the reality of the feature and subsequently speculate on its carrier. All evidence points toward crystalline silicate. For instance, the 11 ?micron band profile is well matched with the emissivity of crystalline olivine. Furthermore, the apparent association of the absorption feature with a sharp polarisation signature in the spectrum of two previously reported cases suggests a carrier with a relatively high band strength compared to amorphous silicates. If true, this would either set back the evolutionary stage in which silicates are crystallised, either to the embedded phase or even before within the ISM, or else the silicates ejected from the outflows of evolved stars retain some of their crystalline identity during their long residence in the ISM.

Dicke's Superradiance in Astrophysics. I -- The 21 cm Line

We have applied the concept of superradiance introduced by Dicke in 1954 to the ISM by extending the corresponding analysis to the magnetic dipole interaction characterizing the atomic hydrogen 21 cm line. Although it is unlikely that superradiance could take place in thermally relaxed regions, in situations where the conditions necessary for superradiance are met (i.e., close atomic spacing, high velocity coherence, population inversion, and long dephasing time-scales compared to those related to coherent behavior), our results suggest that relatively low levels of population inversion over short astronomical length-scales (e.g., as compared to those required for maser amplification) can lead to the cooperative behavior required for superradiance in the ISM. Given the results of our analysis, we expect the observational properties of superradiance to be characterized by the emission of high intensity, spatially compact, burst-like features potentially taking place over short periods ranging from minutes to days.

Scintillation arcs in low-frequency observations of the timing-array millisecond pulsar J0437-4715

Low-frequency observations of pulsars provide a powerful means for probing the microstructure in the turbulent interstellar medium (ISM). Here we report on high-resolution dynamic spectral analysis of our observations of the timing-array millisecond pulsar J0437-4715 with the Murchison Widefield Array (MWA), enabled by our recently commissioned tied-array beam processing pipeline for voltage data recorded from the high time resolution mode of the MWA. A secondary spectral analysis reveals faint parabolic arcs, akin to those seen in high-frequency observations of pulsars with the Green Bank and Arecibo telescopes. Data from Parkes observations at a higher frequency of 732 MHz reveal a similar parabolic feature, with a curvature that scales approximately as the square of the observing wavelength ($\lambda^2$) to the MWA's frequency of 192 MHz. Our analysis suggests that scattering toward PSR J0437-4715 predominantly arises from a compact region about 115 pc from the Earth, which matches well with the expected location of the edge of the Local Bubble that envelopes the local Solar neighborhood. As well as demonstrating new and improved pulsar science capabilities of the MWA, our analysis underscores the potential of low-frequency pulsar observations for gaining valuable insights into the local ISM and for characterizing the ISM toward timing-array pulsars.

Isoscalar monopole and dipole excitations of cluster states and giant resonances in $^{12}$C

The isoscalar monopole(ISM) and dipole(ISD) excitations in $^{12}$C are theoretically investigated with the shifted antisymmetrized molecular dynamics(AMD) plus $3\alpha$-cluster generator coordinate method(GCM). The small amplitude vibration modes are described by coherent one-particle one-hole excitations expressed by small shift of single-nucleon Gaussian wave functions within the AMD framework, whereas the large amplitude cluster modes are incorporated by superposing $3\alpha$-cluster wave functions in the GCM. The coupling of the excitations in the intrinsic frame with the rotation and parity transformation is taken into account microscopically by the angular-momentum and parity projections. The present calculation describes the ISM and ISD excitations in a wide energy region covering cluster modes in the low-energy region and the giant resonances in the high-energy region, though the quantitative description of the high-energy part is not satisfactory. The low-energy ISM and ISD strengths of the cluster modes are enhanced by the radial motion of $\alpha$ clusters, and they split into a couple of states because of the angular motion of $\alpha$ clusters. The low-energy ISM strengths exhaust 26% of the EWSR, which is consistent with the experimental data for the $^{12}$C($0^+_2$;7.65 MeV) and $^{12}$C($0^+_3$;10.3 MeV) measured by $(e,e')$, $(\alpha,\alpha')$, and ($^6$Li,$^6$Li$'$) scatterings. In the calculated low-energy ISD strengths, two $1^-$ states (the $1^-_1$ and $1^-_2$) with the significant strengths are obtained in $E=10-15$ MeV. It is indicated that the ISD excitations can be a good probe to experimentally search for new cluster states such as the $^{12}$C($1^-_2$) obtained in the present calculation.

Isoscalar monopole and dipole excitations of cluster states and giant resonances in $^{12}$C

The isoscalar monopole(ISM) and dipole(ISD) excitations in $^{12}$C are theoretically investigated with the shifted antisymmetrized molecular dynamics(AMD) plus $3\alpha$-cluster generator coordinate method(GCM). The small amplitude vibration modes are described by coherent one-particle one-hole excitations expressed by small shift of single-nucleon Gaussian wave functions within the AMD framework, whereas the large amplitude cluster modes are incorporated by superposing $3\alpha$-cluster wave functions in the GCM. The coupling of the excitations in the intrinsic frame with the rotation and parity transformation is taken into account microscopically by the angular-momentum and parity projections. The present calculation describes the ISM and ISD excitations in a wide energy region covering cluster modes in the low-energy region and the giant resonances in the high-energy region, though the quantitative description of the high-energy part is not satisfactory. The low-energy ISM and ISD strengths of the cluster modes are enhanced by the radial motion of $\alpha$ clusters, and they split into a couple of states because of the angular motion of $\alpha$ clusters. The low-energy ISM strengths exhaust 26% of the EWSR, which is consistent with the experimental data for the $^{12}$C($0^+_2$;7.65 MeV) and $^{12}$C($0^+_3$;10.3 MeV) measured by $(e,e')$, $(\alpha,\alpha')$, and ($^6$Li,$^6$Li$'$) scatterings. In the calculated low-energy ISD strengths, two $1^-$ states (the $1^-_1$ and $1^-_2$) with the significant strengths are obtained in $E=10-15$ MeV. It is indicated that the ISD excitations can be a good probe to experimentally search for new cluster states such as the $^{12}$C($1^-_2$) obtained in the present calculation.

Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity? An Observational Test [Replacement]

Galaxies' interstellar media (ISM) are observed to be supersonically-turbulent, but the ultimate power source that drives turbulent motion remains uncertain. The two dominant models are that the turbulence is driven by star formation feedback and/or that it is produced by gravitational instability in the gas. Here we show that, while both models predict that the galaxies' ISM velocity dispersions will be positively correlated with their star formation rates, the forms of the correlation predicted by these two models are subtly but measurably different. A feedback-driven origin for the turbulence predicts a velocity dispersion that rises more sharply with star formation rate, and that does not depend on the gas fraction (i.e. $\dot{M}_* \propto \sigma^2$), while a gravity-driven model yields a shallower rise and a strong dependence on gas fraction (i.e. $\dot{M}_* \propto f_g^2 \sigma$). We compare the models to a collection of data on local and high-redshift galaxies culled from the literature, and show that the correlation expected for gravity-driven turbulence is a better match to the observations than a feedback-driven model. This suggests that gravity is the ultimate source of ISM turbulence, at least in the rapidly-star-forming, high velocity dispersion galaxies for which our test is most effective. We conclude by discussing the limitations of the present data set, and the prospects for future measurements to enable a more definitive test of the two models.

Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity? An Observational Test

Galaxies' interstellar media (ISM) are observed to be supersonically-turbulent, but the ultimate power source that drives turbulent motion remains uncertain. The two dominant models are that the turbulence is driven by star formation feedback and/or that it is produced by gravitational instability in the gas. Here we show that, while both models predict that the galaxies' ISM velocity dispersions will be positively correlated with their star formation rates, the forms of the correlation predicted by these two models are subtly but measurably different. A feedback-driven origin for the turbulence predicts a velocity dispersion that rises more sharply with star formation rate, and that does not depend on the gas fraction (i.e. $\dot{M}_* \propto \sigma^2$), while a gravity-driven model yields a shallower rise and a strong dependence on gas fraction(i.e. $\dot{M}_* \propto f_g^2 \sigma$). We compare the models to a collection of data on local and high-redshift galaxies culled from the literature, and show that the correlation expected for gravity-driven turbulence is a better match to the observations than a feedback-driven model. This suggests that gravity is the ultimate source of ISM turbulence, at least in the rapidly-star-forming, high velocity dispersion galaxies for which our test is most effective. We conclude by discussing the limitations of the present data set, and the prospects for future measurements to enable a more definitive test of the two models.

Type Ia Supernovae and their Environment: Theory and Applications to SN 2014J

We present theoretical semi-analytic models for the interaction of stellar winds with the interstellar medium (ISM) or prior mass loss implemented in our code SPICE (Supernovae Progenitor Interaction Calculator for parameterized Environments, available on request), assuming spherical symmetry and power-law ambient density profiles and using the Pi-theorem. This allows us to test a wide variety of configurations, their functional dependencies, and to find classes of solutions for given observations. Here, we study Type Ia (SN~Ia) surroundings of single and double degenerate systems, and their observational signatures. Winds may originate from the progenitor prior to the white dwarf (WD) stage, the WD, a donor star, or an accretion disk (AD). For M_Ch explosions,the AD wind dominates and produces a low-density void several light years across surrounded by a dense shell. The bubble explains the lack of observed interaction in late time SN light curves for, at least, several years. The shell produces narrow ISM lines Doppler shifted by 10-100 km/s, and equivalent widths of approximately 100 mA and 1 mA in case of ambient environments with constant density and produced by prior mass loss, respectively. For SN 2014J, both mergers and M_Ch mass explosions have been suggested based on radio and narrow lines. As a consistent and most likely solution, we find an AD wind running into an environment produced by the RG wind of the progenitor during the pre-WD stage, and a short delay, 0.013 to 1.4 Myr, between the WD formation and the explosion. Our framework may be applied more generally to stellar winds and star-formation feedback in large scale galactic evolution simulations.

Type Ia Supernovae and their Environment: Theory and Applications to SN 2014J [Replacement]

We present theoretical semi-analytic models for the interaction of stellar winds with the interstellar medium (ISM) or prior mass loss implemented in our code SPICE (Supernovae Progenitor Interaction Calculator for parameterized Environments, available on request), assuming spherical symmetry and power-law ambient density profiles and using the Pi-theorem. This allows us to test a wide variety of configurations, their functional dependencies, and to find classes of solutions for given observations. Here, we study Type Ia (SN~Ia) surroundings of single and double degenerate systems, and their observational signatures. Winds may originate from the progenitor prior to the white dwarf (WD) stage, the WD, a donor star, or an accretion disk (AD). For M_Ch explosions,the AD wind dominates and produces a low-density void several light years across surrounded by a dense shell. The bubble explains the lack of observed interaction in late time SN light curves for, at least, several years. The shell produces narrow ISM lines Doppler shifted by 10-100 km/s, and equivalent widths of approximately 100 mA and 1 mA in case of ambient environments with constant density and produced by prior mass loss, respectively. For SN 2014J, both mergers and M_Ch mass explosions have been suggested based on radio and narrow lines. As a consistent and most likely solution, we find an AD wind running into an environment produced by the RG wind of the progenitor during the pre-WD stage, and a short delay, 0.013 to 1.4 Myr, between the WD formation and the explosion. Our framework may be applied more generally to stellar winds and star-formation feedback in large scale galactic evolution simulations.

Velocity resolved [CII] spectroscopy of the center and the BCLMP302 region of M33 (HerM33es)

We aim to understand the contribution of the ionized, atomic and molecular phases of the ISM to the [CII] emission from clouds near the dynamical center and the BCLMP302 HII region in the north of the nearby galaxy M33 at a spatial resolution of 50pc. We combine high resolution [CII] spectra taken with the HIFI spectrometer onboard the Herschel satellite with [CII] Herschel-PACS maps and ground-based observations of CO(2-1) and HI. All data are at a common spatial resolution of 50pc. Typically, the [CII] lines have widths intermediate between the narrower CO(2-1) and broader HI line profiles. We decomposed the [CII] spectra in terms of contribution from molecular and atomic gas detected in CO(2-1) and HI, respectively. We find that the relative contribution of molecular and atomic gas traced by CO(2-1) and HI varies depends mostly on the local physical conditions and geometry. We estimate that 11-60% and 5-34% of the [CII] intensities in the center and in BCLMP302, respectively, arise at velocities showing no CO(2-1) or HI emission and could arise in CO-dark molecular gas. The deduced strong variation in the [CII] emission not associated with CO and HI cannot be explained in terms of differences in A_v, far-ultraviolet radiation field, and metallicity between the two studied regions. Hence the relative amounts of diffuse (CO-dark) and dense molecular gas possibly vary on spatial scales smaller than 50pc. Based on the emission measure observed at radio wavelengths we estimate the contribution of ionized gas at a few positions to lie between 10-25%. The correlations between the intensities of tracers corresponding to the same velocity range as [CII], differ from the correlation derived from PACS data. The results in this paper emphasize the need for velocity-resolved observations to discern the contribution of different components of the ISM to [CII] emission. (abridged)

 

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