Recent Postings from Galaxies

Multitracing Anisotropic Non-Gaussianity with Galaxy Shapes

Correlations between intrinsic galaxy shapes on large-scales arise due to the effect of the tidal field of the large-scale structure. Anisotropic primordial non-Gaussianity induces a distinct scale-dependent imprint in these tidal alignments on large scales. Motivated by the observational finding that the alignment strength of luminous red galaxies depends on how galaxy shapes are measured, we study the use of two different shape estimators as a multi-tracer probe of intrinsic alignments. We show, by means of a Fisher analysis, that this technique promises a significant improvement on anisotropic non-Gaussianity constraints over a single-tracer method. For future weak lensing surveys, the uncertainty in the anisotropic non-Gaussianity parameter, $A_2$, is forecast to be $\sigma(A_2)\approx 50$, $\sim 40\%$ smaller than currently available constraints from the bispectrum of the Cosmic Microwave Background. This corresponds to an improvement of a factor of $4-5$ over the uncertainty from a single-tracer analysis.

Universal MOND relation between the baryonic and `dynamical' central surface densities of disc galaxies

I derive a new MOND relation for pure-disc galaxies: The `dynamical' central surface density, $\Sigma^0_D$, deduced from the measured velocities, is a universal function of only the true, `baryonic' central surface density, $\Sigma^0_B$: $\Sigma^0_D=\Sigma_M \mathcal{S}(\Sigma^0_B/\Sigma_M)$, where $\Sigma_M\equiv a_0/2\pi G$ is the MOND surface density constant. This surprising result is shown to hold in both existing, nonrelativistic MOND theories (the nonlinear Poisson formulation, and QUMOND). $\mathcal{S}(y)$ is derived, giving in the two limits: $\Sigma^0_D=\Sigma^0_B$ for very high arguments, and $\Sigma^0_D=(4\Sigma_M\Sigma^0_B)^{1/2}$ for $\Sigma^0_B/\Sigma_M\ll 1$. This study was prompted by the recent finding of a correlation between related attributes in a large sample of disc galaxies by Lelli et al. (2016). The MOND relation is shown to agree very well with these results.

Universal MOND relation between the baryonic and `dynamical' central surface densities of disc galaxies [Cross-Listing]

I derive a new MOND relation for pure-disc galaxies: The `dynamical' central surface density, $\Sigma^0_D$, deduced from the measured velocities, is a universal function of only the true, `baryonic' central surface density, $\Sigma^0_B$: $\Sigma^0_D=\Sigma_M \mathcal{S}(\Sigma^0_B/\Sigma_M)$, where $\Sigma_M\equiv a_0/2\pi G$ is the MOND surface density constant. This surprising result is shown to hold in both existing, nonrelativistic MOND theories (the nonlinear Poisson formulation, and QUMOND). $\mathcal{S}(y)$ is derived, giving in the two limits: $\Sigma^0_D=\Sigma^0_B$ for very high arguments, and $\Sigma^0_D=(4\Sigma_M\Sigma^0_B)^{1/2}$ for $\Sigma^0_B/\Sigma_M\ll 1$. This study was prompted by the recent finding of a correlation between related attributes in a large sample of disc galaxies by Lelli et al. (2016). The MOND relation is shown to agree very well with these results.

Absolute kinematics of radio-source components in the complete S5 polar cap sample. IV. Proper motions of the radio cores over a decade and spectral properties

We have carried out a high-precision astrometric analysis of two very-long-baseline-interferometry (VLBI) epochs of observation of the 13 extragalactic radio sources in the complete S5 polar cap sample. The VLBI epochs span a time baseline of 10 years and enable us to achieve precisions in the proper motions of the source cores up to a few micro-arcseconds per year. The observations were performed at 14.4 GHz and 43.1 GHz, and enable us to estimate the frequency core-shifts in a subset of sources, for which the spectral-index distributions can be computed. We study the source-position stability by analysing the changes in the relative positions of fiducial source points (the jet cores) over a decade. We find motions of 0.1-0.9 mas among close-by sources between the two epochs, which imply drifts in the jet cores of approximately a few tens of micro-as per year. These results have implications for the standard Active Galactic Nucleus (AGN) jet model (where the core locations are supposed to be stable in time). For one of our sources, 0615+820, the morphological and spectral properties in year 2010, as well as the relative astrometry between years 2000 and 2010, suggest the possibility of either a strong parsec-scale interaction of the AGN jet with the ISM, a gravitational lens with ~1 mas diameter, or a resolved massive binary black hole.

Hall current effects in mean-field dynamo theory

The role of the Hall term on large scale dynamo action is investigated by means of the First Order Smoothing Approximation. It is shown that the standard $\alpha$ coefficient is altered, and is zero when a specific double Beltrami state is attained, in contrast to the Alfv\'enic state for MHD dynamos. The $\beta$ coefficient is no longer positive definite, and thereby enables dynamo action even if $\alpha$-quenching were to operate. The similarities and differences with the (magnetic) shear-current effect are pointed out, and a mechanism that may be potentially responsible for $\beta < 0$ is advanced. The results are compared against previous studies, and their astrophysical relevance is also highlighted.

`Zwicky's Nonet': a compact merging ensemble of nine galaxies and 4C 35.06, a peculiar radio galaxy with dancing radio jets

We report the results of our radio, optical and infrared studies of a peculiar radio source 4C 35.06, an extended radio-loud AGN found at the center of galaxy cluster Abell 407 (z=0.047). The central region also hosts a remarkably tight ensemble of nine galaxies within an ~1' (53 kpc) region, surrounded by a very faint, diffuse stellar halo. This remarkable system (named here the 'Zwicky's Nonet') provides a unique and compelling evidence for an ongoing formation of a giant cD galaxy at the cluster center. Multifrequency deep radio observations carried out with Giant Meterwave Radio Telescope at 610, 235 and 150 MHz reveal a system of 400 kpc scale helically twisted and kinked radio jets and outer relic lobes associated with 4C 35.06 in 'Zwicky's Nonet'. The outer extremities of jets show extremely steep spectrum (spectral index -1.7 to -2.5) relic or fossil radio plasma with a relatively short spectral age of few times 10^6 - 10^7 years. Such ultra steep spectrum relic radio plasma lobes without definitive hot-spots are very rare and they provide an interesting opportunity towards understanding the life-cycle of relativistic jets and physics of black hole mergers. We discuss our observations of this complex radio galaxy in the context of growth of its central black hole, triggering of its AGN activity and jet oscillations; presumably all caused by mergers in this dense galactic system. A slow conical precession of the jet axis due to gravitational perturbation effects between interacting black holes is invoked to understand the peculiar jet morphology. The observed close resemblance of the morphology of 4C 35.06 with the precessing relativistic jets of the galactic microquasar SS 433 is noted, and scale-invariance of the disk-jet coupling processes in two systems over a large black hole mass range is discussed.

YSO jets in the Galactic Plane from UWISH2: III - Jets and Outflows in Cassiopeia and Auriga

We present the analysis of 35.5 square degrees of images in the 1-0S(1) line of H2 from the UK Widefield Infrared Survey for H2 (UWISH2) towards Cassiopeia and Auriga. We have identified 98 Molecular Hydrogen emission-line Objects (MHOs) driven by Young Stellar Objects, 60% of which are bipolar outflows and all are new discoveries. We estimate that the UWISH2 extended emission object catalogue contains fewer than % false positives and is complete at the 95% level for jets and outflows brighter than the UWISH2 detection limit. We identified reliable driving source candidates for three quarters of the detected outflows, 40% of which are associated with groups and clusters of stars. The driving source candidates are 20% protostars, the remainder are CTTSs. We also identified 15 new star cluster candidates near MHOs in the survey area. We find that the typical outflow identified in the sample has the following characteristics: the position angles are randomly orientated; bipolar outflows are straight within a few degrees; the two lobes are slightly asymmetrical in length and brightness; the length and brightness of the lobes are not correlated; typical time gaps between major ejections of material are 1-3kyr, hence FU-Ori or EX-Ori eruptions are most likely not the cause of these, but we suggest MNors as a possible source. Furthermore, we find that outflow lobe length distributions are statistically different from the widely used total length distributions. There are a larger than expected number of bright outflows indicating that the flux distribution does not follow a power law.

A robust derivation of the tight relationship of radio core dominance to inclination angle in high redshift 3CRR sources

It is believed that, in radio-loud active galactic nuclei (AGN), the core radio flux density can be normalized to the flux density of the extended lobe emission to infer the orientation of a radio source. However very little is known about the reliability and precision of this method, and we are unaware of any robust conversion recipe to infer the inclination from the core dominance. Investigating whether or not the radio core dominance parameter R separates the quasars from the radio-galaxies in the $z \ge$~1 3CRR catalog, we found excellent agreement of R with optical type, infrared flux ratios and optical polarization. This indicates that probably both R and optical classification are very good orientation indicators, and the unified model is strongly predictive for these objects. The relative number densities indicate half-opening angles close to 60$^\circ$, as expected from large surveys. The separations of optical types according to radio core dominance as well as NIR/MIR ratios, which are essentially perfect, means that there can be only a small dispersion of torus half-opening angles. Also, even though torus dust is thought to be clumpy, there is an almost zero probability to see a type-1 source at high inclination. Finally, using only the Copernican Principle, i,e, the assumption that solid angle is filled uniformly with source axis orientations, we estimated a semi-empirical relation between core dominance and AGN inclination. This makes it possible to use R to infer the inclination of a source to an accuracy of $\sim$ 10 degrees or less, at least for this type of object.

Magnetic field disorder and Faraday effects on the polarization of extragalactic radio sources

We present a polarization catalog of 533 extragalactic radio sources with 2.3 GHz total intensity above 420 mJy from the S-band Polarization All Sky Survey, S-PASS, with corresponding 1.4 GHz polarization information from the NRAO VLA Sky Survey, NVSS. We studied selection effects and found that fractional polarization, $\pi$, of radio objects at both wavelengths depends on the spectral index, source magnetic field disorder, source size and depolarization. The relationship between depolarization, spectrum and size shows that depolarization occurs primarily in the source vicinity. The median $\pi_{2.3}$ of resolved objects in NVSS is approximately two times larger than that of unresolved sources. Sources with little depolarization are $\sim2$ times more polarized than both highly depolarized and re-polarized sources. This indicates that intrinsic magnetic field disorder is the dominant mechanism responsible for the observed low fractional polarization of radio sources at high frequencies. We predict that number counts from polarization surveys will be similar at 1.4 GHz and at 2.3 GHz, for fixed sensitivity, although $\sim$10% of all sources may be currently missing because of strong depolarization. Objects with $\pi_{1.4}\approx \pi_{2.3} \ge 4\%$ typically have simple Faraday structures, so are most useful for background samples. Almost half of flat spectrum ($\alpha \ge -0.5$) and $\sim$25% of steep spectrum objects are re-polarized. Steep spectrum, depolarized sources show a weak negative correlation of depolarization with redshift in the range 0 $<$ z $<$ 2.3. Previous non-detections of redshift evolution are likely due the inclusion of re-polarized sources as well.

The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo

The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays, extend $\approx$10 kpc vertically from the Galactic center, and formed from either nuclear star formation or accretion activity on Sgr A*. Simulations predict a range of shock strengths as the bubbles expand into the surrounding hot gas halo distribution ($T_{halo} \approx 2 \times 10^6$ K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between $10^6$ and $10^8$ K, with potential X-ray signatures. In this work, we constrain the bubbles' thermal gas structure by modeling the OVII and OVIII emission line strengths from archival XMM-Newton and Suzaku data. Our emission model includes a hot thermal volume-filled bubble component cospatial with the gamma-ray region, and a shell of compressed material. We find that a bubble/shell model with $n \approx 1 \times 10^{-3}$ cm$^{-3}$ and with log($T$) $\approx$ 6.60-6.70 is consistent with the observed line intensities. In the framework of a continuous Galactic outflow, we infer a bubble expansion rate, age, and energy injection rate of $490_{-77}^{+230}$ km s$^{-1}$, $4.3_{-1.4}^{+0.8}$ Myr, and $2.3_{-0.9}^{+5.1} \times 10^{42}$ erg s$^{-1}$. These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.

Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies

We present a statistical method for measuring the average HI spin temperature in distant galaxies using the expected detection yields from future wide-field 21-cm absorption surveys. As a demonstrative case study we consider a simulated survey with the Australian Square Kilometre Array Pathfinder for intervening HI absorbers at intermediate cosmological redshifts between $z = 0.4$ and 1. If such a survey yielded $\sim 1000$ absorbers we would infer a harmonic-mean spin temperature of $\overline{T}_\mathrm{spin} \sim 100$K for the population of Damped Lyman-$\alpha$ absorbers at these redshifts, indicating that more than $50$ percent of the neutral gas in these systems is in a cold neutral medium (CNM). Conversely, a lower yield of only 100 detections would imply $\overline{T}_\mathrm{spin} \sim 1000$K and a correspondingly lower CNM fraction. We propose that this method can be used to provide independent verification of the spin temperature evolution reported in recent 21-cm surveys of known DLAs at high redshift and for measuring the spin temperature at intermediate redshifts below $z \approx 1.7$, where the Lyman-$\alpha$ line is inaccessible using ground-based observatories. Increasingly more sensitive and larger surveys with the Square Kilometre Array should provide stronger statistical constraints on the average spin temperature. However, these will ultimately be limited by the accuracy to which we can determine the HI column density frequency distribution, the covering factor and the redshift distribution of the background radio source population.

The source structure of 0642+449 detected from the CONT14 observations

The CONT14 campaign with state-of-the-art VLBI data has observed the source 0642+449 with about one thousand observables each day during a continuous observing period of fifteen days, providing tens of thousands of closure delays---the sum of the delays around a closed loop of baselines. The closure delay is independent of the instrumental and propagation delays and provides valuable additional information about the source structure. We demonstrate the use of this new "observable" for the determination of the structure in the radio source 0642+449. This source, as one of the defining sources in the second realization of the International Celestial Reference Frame (ICRF2), is found to have two point-like components with a relative position offset of -426 microarcseconds in right ascension and -66 microarcseconds in declination. The two components are almost equally bright with a flux-density ratio of 0.92. The standard deviation of closure delays for source 0642+449 was reduced from 139 ps to 90 ps by using this two-component model. Closure delays larger than one nanosecond are found to be related to the source structure, demonstrating that structure effects for a source with this simple structure could be up to tens of nanoseconds. The method described in this paper does not rely on a priori source structure information, such as knowledge of source structure determined from direct (Fourier) imaging of the same observations or observations at other epochs. We anticipate our study to be a starting point for more effective determination of the structure effect in VLBI observations.

The Correlation Between Halo Mass and Stellar Mass for the Most Massive Galaxies in the Universe

We present measurements of the clustering of galaxies as a function of their stellar mass in the Baryon Oscillation Spectroscopic Survey. We compare the clustering of samples using 12 different methods for estimating stellar mass, isolating the method that has the smallest scatter at fixed halo mass. In this test, the stellar mass estimate with the smallest errors yields the highest amplitude of clustering at fixed number density. We find that the PCA stellar masses of Chen etal (2012) clearly have the tightest correlation with halo mass. The PCA masses use the full galaxy spectrum, differentiating them from other estimates that only use optical photometric information. Using the PCA masses, we measure the large-scale bias as a function of Mgal for galaxies with logMgal>=11.4, correcting for incompleteness at the low-mass end of our measurements. Using the abundance-matching ansatz to connect dark matter halo mass to stellar mass, we construct theoretical models of b(Mgal) that match the same stellar mass function but have different amounts of scatter in stellar mass at fixed halo mass, sigma_logM. Using this approach, we find sigma_logM=0.18^{+0.01}_{-0.02}. This value includes both intrinsic scatter as well as random errors in the stellar masses. To partially remove the latter, we use repeated spectra to estimate statistical errors on the stellar masses, yielding an upper limit to the intrinsic scatter of 0.16 dex.

Merger Hydrodynamics of the Luminous Cluster RXJ1347.5-1145

We present an analysis of the complex gas hydrodynamics in the X-ray luminous galaxy cluster RXJ1347.5-1145 caught in the act of merging with a subcluster to its southeast using a combined $186$ ks Chandra exposure, $2.5$ times greater than previous analyses. The primary cluster hosts a sloshing cold front spiral traced by four surface brightness edges $5.^{\prime \prime}85^{+0.04}_{-0.03}$ west, $7.^{\prime \prime}10^{+0.07}_{-0.03}$ southeast, $11.^{\prime \prime}5^{+1.3}_{-1.2}$ east, and $16.^{\prime \prime}7^{+0.3}_{-0.5}$ northeast from the primary central dominant galaxy, suggesting the merger is in the plane of the sky. We measure temperature and density ratios across these edges, confirming they are sloshing cold fronts. We observe the eastern edge of the subcluster infall shock, confirming the observed subcluster is traveling from the southwest to the northeast in a clockwise orbit. We measure a shock density contrast of $1.38^{+0.16}_{-0.15}$ and infer a Mach number $1.25\pm0.08$ and a shock velocity of $2810^{+210}_{-240}$ km s$^{-1}$. Temperature and entropy maps show cool, low entropy gas trailing the subcluster in a southwestern tail, consistent with core shredding. Simulations suggest a perturber in the plane of the sky on a clockwise orbit would produce a sloshing spiral winding counterclockwise, opposite to that observed. The most compelling solution to this discrepancy is that the observed southeastern subcluster is on its first passage, shock heating gas during its clockwise infall, while the main cluster's clockwise cold front spiral formed from earlier encounters with a second perturber orbiting counterclockwise.

The tidal filament of NGC 4660

NGC 4660, in the Virgo cluster, is a well-studied elliptical galaxy which has a strong disk component (D/T about 0.2-0.3). The central regions including the disk component have stellar populations with ages about 12-13 Gyr from SAURON studies. However we report the discovery of a long narrow tidal filament associated with the galaxy in deep co-added Schmidt plate images and deep CCD frames, implying that the galaxy has undergone a tidal interaction and merger within the last few Gyr. The relative narrowness of the filament implies a wet merger with at least one spiral galaxy involved, but the current state of the system has little evidence for this. However a 2-component photometric fit using GALFIT shows much bluer B-V colours for the disk component than for the elliptical component, which may represent a residual trace of enhanced star formation in the disk caused by the interaction 1-2 Gyr ago. There are brighter concentrations within the filament which resemble Tidal Dwarf Galaxies, although they are at least 40 times fainter. These may represent faint, evolved versions of these galaxies. A previously detected stripped satellite galaxy south of the nucleus is seen in our residual image and may imply that the filament is a tidal stream produced by perigalactic passages of this satellite.

Supernova Remnant Evolution: from explosion to dissipation

Here is considered the full evolution of a spherical supernova remnant. We start by calculating the early time ejecta dominated stage and continue through the different phases of interaction with the circumstellar medium, and end with the dissipation and merger phase. The physical connection between the phases reveals new results. One is that the blast wave radius during the adiabatic phase is significantly smaller than it would be, if one does notaccount for the blast wave interaction with the ejecta.

Inference of Unresolved Point Sources At High Galactic Latitudes Using Probabilistic Catalogs [Cross-Listing]

We construct a Bayesian framework to perform inference of dim or overlapping point sources. The method involves probabilistic cataloging, where samples are taken from the posterior probability distribution of catalogs consistent with an observed photon count map. By implementing across-model jumps between point source models of different dimensionality, we collect a representative ensemble of catalogs consistent with the image. In order to validate our method we sample random catalogs of the gamma-ray sky in the direction of the North Galactic Pole (NGP) by binning the data in energy and PSF (Point Spread Function) classes. Using three energy bins between $0.3 - 1$, $1 - 3$ and $3 - 10$ GeV, we identify $270\substack{+30 -10}$ point sources inside a $40^\circ \times 40^\circ$ region around the NGP above our point-source inclusion limit of $3 \times 10^{-11}$/cm$^2$/s/sr/GeV at the $1-3$ GeV energy bin. Most of these point sources are time-variable blazars. Modeling the flux distribution as a single power law, we infer the slope to be $-1.92\substack{+0.07 -0.05}$ and estimate the contribution of point sources (resolved and unresolved) to the total emission as $18\substack{+2 -2}$\%. Further analyses that rely on the ensemble of sample catalogs instead of only the most likely catalog, can perform reliable marginalization over uncertainties in the number as well as spatial and spectral properties of the point sources. This marginalization allows a robust test of whether the apparently isotropic emission in an image is due to unresolved point sources or of truly diffuse origin. With the increase in the availability of computational resources in the near future, probabilistic cataloging can potentially be applied to full sky datasets or optical images and replace the standard data reduction pipelines for crowded fields.

The many faces of LINER-like galaxies: a WISE view

We use the SDSS and WISE surveys to investigate the real nature of galaxies defined as LINERs in the BPT diagram. After establishing a mid-infrared colour W2-W3 = 2.5 as the optimal separator between galaxies with and without star formation, we investigate the loci of different galaxy classes in the W_{Ha} versus W2-W3 space. We find that: (1) A large fraction of LINER-like galaxies are emission-line retired galaxies, i.e galaxies which have stopped forming stars and are powered by hot low-mass evolved stars (HOLMES). Their W2-W3 colours show no sign of star formation and their Ha equivalent widths, W_{Ha}, are consistent with ionization by their old stellar populations. (2) Another important fraction have W2-W3 indicative of star formation. This includes objects located in the supposedly `pure AGN' zone of the BPT diagram. (3) A smaller fraction of LINER-like galaxies have no trace of star formation from W2-W3 and a high W_{Ha}, pointing to the presence of an AGN. (4) Finally, a few LINERs tagged as retired by their W_{Ha} but with W2-W3 values indicative of star formation are late-type galaxies whose SDSS spectra cover only the old `retired' bulge. This reinforces the view that LINER-like galaxies are a mixed bag of objects involving different physical phenomena and observational effects thrusted into the same locus of the BPT diagram.

The redshift evolution of massive galaxy clusters in the MACSIS simulations

We present the MAssive ClusterS and Intercluster Structures (MACSIS) project, a suite of 390 clusters simulated with baryonic physics that yields realistic massive galaxy clusters capable of matching a wide range of observed properties. MACSIS extends the recent BAHAMAS simulation to higher masses, enabling robust predictions for the redshift evolution of cluster properties and an assessment of the effect of selecting only the hottest systems. We study the observable-total mass and X-ray luminosity-temperature scaling relations across the complete observed cluster mass range, finding the slope of the scaling relations and the evolution of their normalization with redshift to depart significantly from self-similar predictions. This is driven by the impact of AGN feedback, the presence of non-thermal pressure support and biased X-ray temperatures. For a sample of hot clusters with core-excised temperatures $k_{\rm{B}}T\geq5\,\rm{keV}$ the normalization and slope of the observable-mass relations and their evolution are significantly closer to self-similar. The exception is the temperature-mass relation, for which the increased importance of non-thermal pressure support and biased X-ray temperatures leads to a greater departure from self-similarity in the hottest systems. We also demonstrate that this affects the slope and evolution of the normalization in the luminosity-temperature relation. The median hot gas profiles also show good agreement with observational data at $z=0$ and $z=1$, with their evolution again departing significantly from the self-similar prediction. However, selecting a hot sample of clusters yields profiles that evolve significantly closer to the self-similar prediction.

Polarized near-infrared light of the Dusty S-cluster Object (DSO/G2) at the Galactic Center

We investigate an infrared-excess source called G2 or Dusty S-cluster Object (DSO) moving on a highly eccentric orbit around the Galaxy's central black hole, Sgr A*. We use, for the first time, near-infrared polarimetric imaging data to determine the nature and the properties of the DSO, and obtain an improved K_s-band identification of this source in median polarimetry images of different observing years. The source starts to deviate from the stellar confusion in 2008, and it does not show any flux density variability over the years we analyzed it. We measure the polarization degree and angle of the DSO between 2008 and 2012 and conclude, based on the significance analysis on polarization parameters, that it is an intrinsically polarized source (>20%) with a varying polarization angle as it approaches Sgr A* position. DSO shows a near-infrared excess of K_s-L' > 3 that remains compact close to the pericenter of its orbit. Its observed parameters and the significant polarization obtained in this work show that the DSO might be a dust-enshrouded young star, forming a bow shock as it approaches the super massive black hole. The significantly high measured polarization degree indicates that it has a non-spherical geometry and it can be modelled as a combination of a bow shock with a bipolar wind of the star. We use a 3D radiative transfer model that can reproduce the observed properties of the source such as the total flux density and the polarization degree. We obtain that the change of the polarization angle can be due to an intrinsic change in the source structure. Accretion disc precession of the young star in the gravitational field of the black hole can lead to the change of the bipolar outflow and therefore the polarization angle variation. It might also be the result of the source interaction with the ambient medium.

Infrared Interferometry and AGNs: Parsec-scale Disks and Dusty Outflows

The "torus" is the central element of the most popular theory unifying various classes of AGNs, but it is usually described as "putative" because it has not been imaged yet. Since it is too small to be resolved with single-dish telescopes, one can only make indirect assumptions about its structure using models. Using infrared interferometry, however, we were able to resolve the circum-nuclear dust distributions for several nearby AGNs and achieved constraints on some further two dozen sources. We discovered circum-nuclear dust on parsec scales in all sources and, in two nearby sources, were able to dissect this dust into two distinct components. The compact component, a very thin disk, appears to be connected to the maser disk and the extended one, which is responsible for most of the mid-IR flux, is oriented perpendicularly to the circum-nuclear gas disks. What may come as a surprise when having in mind the standard unification cartoon actually connects well to observations on larger scales. Optically thin dust in the polar region, perhaps driven by a disk wind, could solve both the scale height problem of the torus and explain the missing anisotropy in the mid-IR - X-ray relation.

The Quiescent Intracluster Medium in the Core of the Perseus Cluster

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.

Partition functions 1: Improved partition functions and thermodynamic quantities for normal, equilibrium, and ortho and para molecular hydrogen [Cross-Listing]

Aims. In this work we rigorously show the shortcomings of various simplifications that are used to calculate the total internal partition function. These shortcomings can lead to errors of up to 40 percent or more in the estimated partition function. These errors carry on to calculations of thermodynamic quantities. Therefore a more complicated approach has to be taken. Methods. Seven possible simplifications of various complexity are described, together with advantages and disadvantages of direct summation of experimental values. These were compared to what we consider the most accurate and most complete treatment (case 8). Dunham coefficients were determined from experimental and theoretical energy levels of a number of electronically excited states of H$_2$ . Both equilibrium and normal hydrogen was taken into consideration. Results. Various shortcomings in existing calculations are demonstrated, and the reasons for them are explained. New partition functions for equilibrium, normal, and ortho and para hydrogen are calculated and thermodynamic quantities are reported for the temperature range 1 - 20000 K. Our results are compared to previous estimates in the literature. The calculations are not limited to the ground electronic state, but include all bound and quasi-bound levels of excited electronic states. Dunham coefficients of these states of H$_2$ are also reported. Conclusions. For most of the relevant astrophysical cases it is strongly advised to avoid using simplifications, such as a harmonic oscillator and rigid rotor or ad hoc summation limits of the eigenstates to estimate accurate partition functions and to be particularly careful when using polynomial fits to the computed values. Reported internal partition functions and thermodynamic quantities in the present work are shown to be more accurate than previously available data.

A survey of luminous high-redshift quasars with SDSS and WISE II. the bright end of the quasar luminosity function at z ~ 5

This is the second paper in a series on a new luminous z ~ 5 quasar survey using optical and near-infrared colors. Here we present a new determination of the bright end of the quasar luminosity function (QLF) at z ~ 5. Combined our 45 new quasars with previously known quasars that satisfy our selections, we construct the largest uniform luminous z ~ 5 quasar sample to date, with 99 quasars in the range 4.7 <= z < 5.4 and -29 < M1450 <= -26.8, within the Sloan Digital Sky Survey (SDSS) footprint. We use a modified 1/Va method including flux limit correction to derive a binned QLF, and we model the parametric QLF using maximum likelihood estimation. With the faint-end slope of the QLF fixed as alpha = -2.03 from previous deeper samples, the best fit of our QLF gives a flatter bright end slope beta = -3.58+/-0.24 and a fainter break magnitude M*1450 = -26.98+/-0.23 than previous studies at similar redshift. Combined with previous work at lower and higher redshifts, our result is consistent with a luminosity evolution and density evolution (LEDE) model. Using the best fit QLF, the contribution of quasars to the ionizing background at z ~ 5 is found to be 18% - 45% with a clumping factor C of 2 - 5. Our sample suggests an evolution of radio loud fraction with optical luminosity but no obvious evolution with redshift.

Far-infrared and dust properties of present-day galaxies in the EAGLE simulations

The EAGLE cosmological simulations reproduce the observed galaxy stellar mass function and many galaxy properties. In this work, we study the dust-related properties of present-day EAGLE galaxies through mock observations in the far-infrared and submm wavelength ranges obtained with the 3D dust radiative transfer code SKIRT. To prepare an EAGLE galaxy for radiative transfer processing, we derive a diffuse dust distribution from the gas particles and we re-sample the star-forming gas particles and the youngest star particles into star-forming regions that are assigned dedicated emission templates. We select a set of redshift-zero EAGLE galaxies that matches the K-band luminosity distribution of the galaxies in the Herschel Reference Survey (HRS), a volume-limited sample of about 300 normal galaxies in the Local Universe. We find overall agreement of the EAGLE dust scaling relations with those observed in the HRS, such as the dust-to-stellar mass ratio versus stellar mass and versus NUV-r colour relations. A discrepancy in the f_250/f_350 versus f_350/f_500 submm colour-colour relation implies that part of the simulated dust is insufficiently heated, likely because of limitations in our sub-grid model for star-forming regions. We also investigate the effect of adjusting the metal-to-dust ratio and the covering factor of the photodissociation regions surrounding the star-forming cores. We are able to constrain the important dust-related parameters in our method, informing the calculation of dust attenuation for EAGLE galaxies in the UV and optical domain.

Molecular outflows in starburst nuclei

Recent observations have detected molecular outflows in a few nearby starburst nuclei. We discuss the physical processes at work in such an environment in order to outline a scenario that can explain the observed parameters of the phenomenon, such as the molecular mass, speed and size of the outflows. We show that outflows triggered by OB associations, with $N_{OB}\ge 10^5$ (corresponding to a star formation rate (SFR)$\ge 1$ M$_{\odot}$ yr$^{-1}$ in the nuclear region), in a stratified disk with mid-plane density $n_0\sim 200\hbox{--}1000$ cm$^{-3}$ and scale height $z_0\ge 200 (n_0/10^2 \, {\rm cm}^{-3})^{-3/5}$ pc, can form molecules in a cool dense and expanding shell. The associated molecular mass is $\ge 10^7$ M$_\odot$ at a distance of a few hundred pc, with a speed of several tens of km s$^{-1}$. We show that a SFR surface density of $10 \le \Sigma_{SFR} \le 50$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$ favours the production of molecular outflows, consistent with observed values.

A spectroscopic study of blue supergiant stars in the Sculptor galaxy NGC 55: chemical evolution and distance

Low resolution (4.5 to 5 Angstroem) spectra of 58 blue supergiant stars distributed over the disk of the Magellanic spiral galaxy NGC 55 in the Sculptor group are analyzed by means of non-LTE techniques to determine stellar temperatures, gravities and metallicities (from iron peak and alpha-elements). A metallicity gradient of -0.22 +/- 0.06$ dex/R_25 is detected. The central metallicity on a logarithmic scale relative to the Sun is [Z] = -0.37 +\- 0.03. A chemical evolution model using the observed distribution of stellar and interstellar medium gas mass column densities reproduces the observed metallicity distribution well and reveals a recent history of strong galactic mass accretion and wind outflows with accretion and mass-loss rates of the order of the star formation rate. There is an indication of spatial inhomogeneity in metallicity. In addition, the relatively high central metallicity of the disk confirms that two extra-planar metal poor HII regions detected in previous work 1.13 to 2.22 kpc above the galactic plane are ionized by massive stars formed in-situ outside the disk. For a sub-sample of supergiants, for which Hubble Space Telescope photometry is available, the flux-weighted gravity--luminosity relationship is used to determine a distance modulus of 26.85 +\- 0.10 mag.

Study of velocity centroids based on the theory of fluctuations in position-position-velocity space

We study possibility of obtaining velocity spectra by studying turbulence in an optically thick medium using velocity centroids (VCs).We find that the regime of universal, i.e. independent of underlying turbulence statistics, fluctuations discovered originally within the velocity channel analysis (VCA) carries over to the statistics of VCs. In other words, for large absorptions the VC lose their ability to reflect the spectra of turbulence. Combining our present study with the earlier studies of centroids in Esquivel & Lazarian, we conclude that centroids are applicable for studies subsonic/transsonic turbulence for the range of scales that is limited by the absorption effects. We also consider VCs based on absorption lines and define the range of their applicability. We address the problem of analytical description of spectra and anisotropies of fluctuations that are available through studies using VC. We obtain spectra and anisotropy of VC fluctuations arising from Alfv\'en, slow and fast modes that constitute the compressible MHD cascade to address the issue of anisotropy of VC statistics, and show how the VC anisotropy can be used to find the media magnetization as well as to identify and separate contributions from Alfv\'en, slow and fast modes. Our study demonstrates that VCs are complementary to the tools provided by the VCA. In order to study turbulent volume for which the resolution of single dish telescopes is insufficient, we demonstrate how the studies of anisotropy can be performed using interferometers.

The complex star cluster system of NGC 1316 (Fornax A)

This paper presents Gemini-$gri'$ high quality photometry for cluster candidates in the field of NGC 1316 (Fornax A) as part of a study that also includes GMOS spectroscopy. A preliminary discussion of the photometric data indicates the presence of four stellar cluster populations with distinctive features in terms of age, chemical abundance and spatial distribution. Two of them seem to be the usually old (metal poor and metal rich) populations typically found in elliptical galaxies. In turn, an intermediate-age (5 Gyr) globular cluster population is the dominant component of the sample (as reported by previous papers). We also find a younger cluster population with a tentative age of $\approx$ 1 Gyr.

Extracting Information from AGN Variability

AGN exhibit rapid, high amplitude stochastic flux variability across the entire electromagnetic spectrum on timescales ranging from hours to years. The cause of this variability is poorly understood. We present a new method for using variability to (1) measure the time-scales on which flux perturbations evolve and (2) characterize the driving flux perturbations. We model the observed light curve of an AGN as a linear differential equation driven by stochastic impulses. Physically, the impulses could be local `hot-spots' in the accretion disk---the linear differential equation then governs how the hot spots evolve and dissipate. The impulse-response function of the accretion disk material is given by the Green's function of the linear differential equation. The timescales on which the hot-spots radiate energy is characterized by the powerspectrum of the driving stochastic impulses. We analyze the light curve of the \Kepler AGN Zw 229-15 and find that the observed variability behavior can be modeled as a damped harmonic oscillator perturbed by a colored noise process. The model powerspectrum turns over on time-scale $385$~d. On shorter time-scales, the log-powerspectrum slope varies between $2$ and $4$, explaining the behavior noted by previous studies. We recover and identify both the $5.6$~d and $67$~d timescales reported by previous work. These timescales represent the time-scale on which flux perturbations grow, and the time-scale on which flux perturbations decay back to the steady-state flux level respectively. We make the software package used to study light curves using our method, \textsc{k\={a}l\={i}}, available to the community.

Kinematics and Structure of Clumps in Broad-line Regions in Active Galactic Nuclei

We use the Jeans equations for an ensemble of collisionless particles to describe the distribution of broad-line region (BLR) cloud in three classes: (A) non disc (B) disc-wind (c) pure disc structure. We propose that clumpy structures in the brightest quasars belong to class A, fainter quasars and brighter Seyferts belong to class B, and dimmer Seyfert galaxies and all low-luminosity AGNs (LLAGNs) belong to class C. We derive the virial factor, $f$, for disc-like structures and find a negative correlation between the inclination angle, $\theta_{0}$, and $f$. We find similar behaviour for $f$ as a function of the FWHM and $\sigma_{z}$, the $z$ component of velocity dispersion. For different values of $\theta_{0}$ we find that $ 1.0 \lesssim f \lesssim 9.0 $ in type1 AGNs and $ 0.5 \lesssim f \lesssim 1.0 $ in type2 AGNs. Moreover we have $ 0.5 \lesssim f \lesssim 6.5 $ for different values of FWHM and $ 1.4 \lesssim f \lesssim 1.8 $ for different values of $ \sigma_{z} $. We also find that $ f $ is relatively insensitive to the variations of bolometric luminosity and column density of each cloud and the range of variation of $ f $ is in order of 0.01. Considering wide range of $ f $ we see the use of average virial factor $ \langle f \rangle $ is not very safe. Therefore we propose AGN community to divide a sample into a few subsamples based on the value of $ \theta_{0}$ and FWHM of members and calculate $ \langle f \rangle $ for each group separately to reduce uncertainty in black hole mass estimation.

A Common Origin for Globular Clusters and Ultra-faint Dwarfs in Simulations of the First Galaxies

In this paper, the first in a series on galaxy formation before reionization, we focus on understanding what determines the size and morphology of stellar objects in the first low mass galaxies, using parsec- scale cosmological simulations performed with an adaptive mesh hydrodynamics code. Although the dense gas in which stars are formed tends to have a disk structure, stars are found in spheroids with little rotation. Halos with masses between 10^6 M_sun and 5x10^8 M_sun form stars stochastically, with stellar masses in the range 10^4 M_sun to 2x10^6 M_sun. Nearly independent of stellar mass, we observe a large range of half-light radii for the stars, from a few parsecs to a few hundred parsecs and surface brightnesses and mass-to-light ratios ranging from those typical of globular clusters to ultra-faint dwarfs. In our simulations, stars form in dense stellar clusters with high gas-to-star conversion efficiencies and rather uniform metallicities. A fraction of these clusters remain bound after the gas is removed by feedback, but others are destroyed, and their stars, which typically have velocity dispersions of 20 to 40 km/s, expand until they become bound by the dark matter halo. We thus speculate that the stars in ultra-faint dwarf galaxies may show kinematic and chemical signatures consistent with their origin in a few distinct stellar clusters. On the other hand, some globular clusters may form at the center of primordial dwarf galaxies and may contain dark matter, perhaps detectable in the outer parts.

X-ray Twinkles and Pop III Stars

Pop III stars are typically massive stars of primordial composition forming at the centers of the first collapsed dark matter structures. Here we estimate the optimal X-ray emission in the early universe for promoting the formation of Pop III stars. This is important in determining the number of dwarf galaxies formed before reionization and their fossils in the local universe, as well as the number of intermediate-mass seed black holes. A mean X-ray emission per source above the optimal level reduces the number of Pop III stars because of the increased Jeans mass of the intergalactic medium (IGM), while a lower emission suppresses the formation rate of H2 preventing or delaying star formation in dark matter minihalos above the Jeans mass. The build up of the H2 dissociating background is slower than the X-ray background due to the shielding effect of resonant hydrogen Lyman lines. Hence, the nearly unavoidable X-ray emission from supernova remnants of Pop III stars is sufficient to boost their number to few tens per comoving Mpc^3 by redshift z~15. We find that there is a critical X-ray to UV energy ratio emitted per source that produces a universe where the number of Pop III stars is largest: 400 per comoving- Mpc^3. This critical ratio is very close to the one provided by 20-40 M_sun Pop III stars exploding as hypernovae. High mass X-ray binaries in dwarf galaxies are far less effective at increasing the number of Pop III stars than normal supernova remnants, we thus conclude that supernovae drove the formation of Pop III stars.

Interpreting the evolution of galaxy colours from $z = 8$ to $z = 5$ [Replacement]

We attempt to interpret existing data on the evolution of the UV luminosity function and UV colours, $\beta$, of galaxies at $5 \leq z \leq 8$, to improve our understanding of their dust content and ISM properties. To this aim, we post-process the results of a cosmological hydrodynamical simulation with a chemical evolution model, which includes dust formation by supernovae and intermediate mass stars, dust destruction in supernova shocks, and grain growth by accretion of gas-phase elements in dense gas. We find that observations require a steep, Small Magellanic Cloud-like extinction curve and a clumpy dust distribution, where stellar populations younger than 15 Myr are still embedded in their dusty natal clouds. Investigating the scatter in the colour distribution and stellar mass, we find that the observed trends can be explained by the presence of two populations: younger, less massive galaxies where dust enrichment is mainly due to stellar sources, and massive, more chemically evolved ones, where efficient grain growth provides the dominant contribution to the total dust mass. Computing the IR-excess - UV color relation we find that all but the dustiest model galaxies follow a relation shallower than the Meurer et al. (1999) one, usually adopted to correct the observed UV luminosities of high-$z$ galaxies for the effects of dust extinction. As a result, their total star formation rates might have been over-estimated. Our study illustrates the importance to incorporate a proper treatment of dust in simulations of high-$z$ galaxies, and that massive, dusty, UV-faint galaxies might have already appeared at $z \lesssim 7$.

The evolution of high-z galaxy colours: Interpreting the evolution of galaxy colours from $z = 8$ to $z = 5$

We attempt to interpret existing data on the evolution of the UV luminosity function and UV colours, $\beta$, of galaxies at $5 \leq z \leq 8$, to improve our understanding of their dust content and ISM properties. To this aim, we post-process the results of a cosmological hydrodynamical simulation with a chemical evolution model, which includes dust formation by supernovae and intermediate mass stars, dust destruction in supernova shocks, and grain growth by accretion of gas-phase elements in dense gas. We find that observations require a steep, Small Magellanic Cloud-like extinction curve and a clumpy dust distribution, where stellar populations younger than 15 Myr are still embedded in their dusty natal clouds. Investigating the scatter in the colour distribution and stellar mass, we find that the observed trends can be explained by the presence of two populations: younger, less massive galaxies where dust enrichment is mainly due to stellar sources, and massive, more chemically evolved ones, where efficient grain growth provides the dominant contribution to the total dust mass. Computing the IR-excess - UV color relation we find that all but the dustiest model galaxies follow a relation shallower than the Meurer et al. (1999) one, usually adopted to correct the observed UV luminosities of high-$z$ galaxies for the effects of dust extinction. As a result, their total star formation rates might have been over-estimated. Our study illustrates the importance to incorporate a proper treatment of dust in simulations of high-$z$ galaxies, and that massive, dusty, UV-faint galaxies might have already appeared at $z \lesssim 7$.

A semi-analytic dynamical friction model for cored galaxies

We present a dynamical friction model based on Chandrasekhar's formula that reproduces the fast inspiral and stalling experienced by satellites orbiting galaxies with a large constant density core. We show that the fast inspiral phase does not owe to resonance. Rather, it owes to the background velocity distribution function for the constant density cores being dissimilar from the usually-assumed Maxwellian distribution. Using the correct background velocity distribution function and the semi-analytic model from Petts et al. (2015), we are able to correctly reproduce the infall rate in both cored and cusped potentials. However, in the case of large cores, our model is no longer able to correctly capture core-stalling. We show that this stalling owes to the tidal radius of the satellite approaching the size of the core. By switching off dynamical friction when rt(r) = r (where rt is the tidal radius at the satellite's position) we arrive at a model which reproduces the N-body results remarkably well. Since the tidal radius can be very large for constant density background distributions, our model recovers the result that stalling can occur for Ms/Menc << 1, where Ms and Menc are the mass of the satellite and the enclosed galaxy mass, respectively. Finally, we include the contribution to dynamical friction that comes from stars moving faster than the satellite. This next-to-leading order effect becomes the dominant driver of inspiral near the core region, prior to stalling.

Evolution of cosmic star formation in the SCUBA-2 Cosmology Legacy Survey

We present a new exploration of the cosmic star-formation history and dust obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest 450 and 850$\mu$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the AEGIS, COSMOS and UDS fields, together with 100-250$\mu$m imaging from Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach below the confusion limit, using multi-wavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star-formation rate (SFR) and stellar mass up to $z\sim5$, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-$\beta$ relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in $M_\ast>10^{10}M_\odot$ galaxies at $0.5<z<6$. This is dominated by obscured star formation by a factor of $>10$. One third of this is accounted for by 450$\mu$m-detected sources, while one fifth is attributed to UV-luminous sources (brighter than $L^\ast_{UV}$), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at $z\lesssim3$, and from UV-only data at $z\sim5$. The cosmic star-formation history undergoes a transition at $z\sim3-4$, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.

Bursty star formation feedback and cooling outflows

We study how outflows of gas launched from a central galaxy undergoing repeated starbursts propagate through the circumgalactic medium (CGM), using the simulation code RAMSES. We assume that the outflow from the disk can be modelled as a rapidly moving bubble of hot gas at $\mathrm{\sim1\;kpc}$ above disk, then ask what happens as it moves out further into the halo around the galaxy on $\mathrm{\sim 100\;kpc}$ scales. To do this we run 60 two-dimensional simulations scanning over parameters of the outflow. Each of these is repeated with and without radiative cooling, assuming a primordial gas composition to give a lower bound on the importance of cooling. In a large fraction of radiative-cooling cases we are able to form rapidly outflowing cool gas from in situ cooling of the flow. We show that the amount of cool gas formed depends strongly on the 'burstiness' of energy injection; sharper, stronger bursts typically lead to a larger fraction of cool gas forming in the outflow. The abundance ratio of ions in the CGM may therefore change in response to the detailed historical pattern of star formation. For instance, outflows generated by star formation with short, intense bursts contain up to 60 per cent of their gas mass at temperatures $<5 \times 10^4\,\mathrm{K}$; for near-continuous star formation the figure is $\lesssim$ 5 per cent. Further study of cosmological simulations, and of idealised simulations with e.g., metal-cooling, magnetic fields and/or thermal conduction, will help to understand the precise signature of bursty outflows on observed ion abundances.

The Scaling of Stellar Mass and Central Stellar Velocity Dispersion for Quiescent galaxies at z < 0.7

We examine the relation between stellar mass and central stellar velocity dispersion-the M-sigma relation-for massive quiescent galaxies at z<0.7. We measure the local relation from the Sloan Digital Sky Survey and the intermediate redshift relation from the Smithsonian Hectospec Lensing Survey. Both samples are highly complete (>85%) and we consistently measure the stellar mass and velocity dispersion for the two samples. The M-sigma relation and its scatter are independent of redshift with sigma ~ M^0.3 for M > 10^10.3 solar masses. The measured slope of the M-sigma relation is the same as the scaling between the total halo mass and the dark matter halo velocity dispersion obtained by N-body simulations. This consistency suggests that massive quiescent galaxies are virialized systems where the central dark matter concentration is either a constant or negligible fraction of the stellar mass. The relation between the total galaxy mass (stellar + dark matter) and the central stellar velocity dispersion is consistent with the observed relation between the total mass of a galaxy cluster and the velocity dispersion of the cluster members. This result suggests that the central stellar velocity dispersion is directly proportional to the velocity dispersion of the dark matter halo. Thus the central stellar velocity dispersion is a fundamental, directly observable property of galaxies that may robustly connect galaxies to dark matter halos in N-body simulations. To interpret the results further in the context of Lambda-CDM, it would be useful to analyze the relationship between the velocity dispersion of stellar particles and the velocity dispersion characterizing their dark matter halos in high-resolution cosmological hydrodynamic simulations.

Self-Consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations

The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is thus a key quantity for modeling cosmic reionization. The vast majority of cosmological hydrodynamical simulations implement the UVB via a set of spatially uniform photoionization and photoheating rates derived from UVB synthesis models. We show that simulations using canonical UVB rates reionize, and perhaps more importantly, spuriously heat the IGM, much earlier z ~ 15 than they should. This problem arises because at z > 6, where observational constraints are non-existent, the UVB amplitude is far too high. We introduce a new methodology to remedy this issue, and generate self-consistent photoionization and photoheating rates to model any chosen reionization history. Following this approach, we run a suite of hydrodynamical simulations of different reionization scenarios, and explore the impact of the timing of reionization and its concomitant heat injection on the the thermal state of the IGM. We present a comprehensive study of the pressure smoothing scale of IGM gas, illustrating its dependence on the details of both hydrogen and helium reionization, and argue that it plays a fundamental role in interpreting Lyman-alpha forest statistics and the thermal evolution of the IGM. The premature IGM heating we have uncovered implies previous work has likely dramatically overestimated the impact of photoionization feedback on galaxy formation, which sets the minimum halo mass able to form stars at high redshifts. We make our new UVB photoionization and photoheating rates publicly available for use in future simulations.

Spectral Clustering for Optical Confirmation and Redshift Estimation of X-ray Selected Galaxy Cluster Candidates in the SDSS Stripe 82

We develop a galaxy cluster finding algorithm based on spectral clustering technique to identify optical counterparts and estimate optical redshifts for X-ray selected cluster candidates. As an application, we run our algorithm on a sample of X-ray cluster candidates selected from the third XMM-Newton serendipitous source catalog (3XMM-DR5) that are located in the Stripe 82 of the Sloan Digital Sky Survey (SDSS). Our method works on galaxies described in the color-magnitude feature space. We begin by examining 45 galaxy clusters with published spectroscopic redshifts in the range of 0.1 to 0.8 with a median of 0.36. As a result, we are able to identify their optical counterparts and estimate their photometric redshifts, which have a typical accuracy of 0.025 and agree with the published ones. Then, we investigate another 40 X-ray cluster candidates (from the same cluster survey) with no redshift information in the literature and found that 12 candidates are considered as galaxy clusters in the redshift range from 0.29 to 0.76 with a median of 0.57. These systems are newly discovered clusters in X-rays and optical data. Among them 7 clusters have spectroscopic redshifts for at least one member galaxy.

21-year timing of the black-widow pulsar J2051-0827

Timing results for the black-widow pulsar J2051-0827 are presented, using a 21-year dataset from four European Pulsar Timing Array telescopes and the Parkes radio telescope. This dataset, which is the longest published to date for a black-widow system, allows for an improved analysis that addresses previously unknown biases. While secular variations, as identified in previous analyses, are recovered, short-term variations are detected for the first time. Concurrently, a significant decrease of approx. 2.5x10-3 cm-3 pc in the dispersion measure associated with PSR J2051-0827 is measured for the first time and improvements are also made to estimates of the proper motion. Finally, PSR J2051-0827 is shown to have entered a relatively stable state suggesting the possibility of its eventual inclusion in pulsar timing arrays.

Inverse scattering problem in turbulent magnetic fluctuations [Cross-Listing]

We apply a particular form of the inverse scattering theory to turbulent magnetic fluctuations in a plasma. In the present note we develop the theory, formulate the magnetic fluctuation problem in terms of its electrodynamic turbulent response function, and reduce it to the solution of a special form of the famous Gel$'$fand-Levitan-Marchenko equation of quantum mechanical scattering theory.

A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. II. Mass accretion in the Pre-Main Sequence Population

We present a detailed analysis of the pre-main-sequence (PMS) population of the young star cluster Westerlund~2 (Wd2), the central ionizing cluster of the HII region RCW 49, using data from a high resolution multi-band survey with the Hubble Space Telescope. The data were acquired with the Advanced Camera for Surveys in the F555W, F814W, and F658N filters and with the Wide Field Camera 3 in the F125W, F160W, and F128N filters. We find a mean age of the region of 1.04+-0.72 Myr. The combination of dereddened F555W and F814W photometry in combination with F658N photometry allows us to study and identify stars with H_alpha excess emission. With a careful selection of 240 bona-fide PMS H_alpha excess emitters we were able to determine their H_alpha luminosity, which has a mean value L(H_alpha)=1.67 x 10^{-31} erg s^{-1}. Using the PARSEC 1.2S isochrones to obtain the stellar parameters of the PMS stars we determined a mean mass accretion rate \dot M_acc=4.43 x 10^{-8} M_sun yr^{-1} per star. A careful analysis of the spatial dependence of the mass-accretion rate suggests that this rate is ~25% lower in center of the two density peaks of Wd2 in close proximity to the luminous OB stars, compared to the Wd2 average. This rate is higher with increasing distance from the OB stars, indicating that the PMS accretion disks are being rapidly destroyed by the far-ultra-violet radiation emitted by the OB population.

Why do some cores remain starless ?

Physical conditions that could render a core starless(in the local Universe) is the subject of investigation in this work. To this end we studied the evolution of four starless cores, B68, L694-2, L1517B, L1689, and L1521F, a VeLLO. The density profile of a typical core extracted from an earlier simulation developed to study core-formation in a molecular cloud was used for the purpose. We demonstrate - (i) cores contracted in quasistatic manner over a timescale on the order of $\sim 10^{5}$ years. Those that remained starless did briefly acquire a centrally concentrated density configuration that mimicked the density profile of a unstable Bonnor Ebert sphere before rebounding, (ii) three of our test cores viz. L694-2, L1689-SMM16 and L1521F remained starless despite becoming thermally super-critical. On the contrary B68 and L1517B remained sub-critical; L1521F collapsed to become a VeLLO only when gas-cooling was enhanced by increasing the size of dust-grains. This result is robust, for other cores viz. B68, L694-2, L1517B and L1689 that previously remained starless could also be similarly induced to collapse. Our principle conclusions are : (a) acquiring the thermally super-critical state does not ensure that a core will necessarily become protostellar, (b) potentially star-forming cores (VeLLO L1521F here), could be experiencing coagulation of dust-grains that must enhance the gas-dust coupling and in turn lower the gas temperature, thereby assisting collapse. This hypothesis appears to have some observational support, and (c) depending on its dynamic state at any given epoch, a core could appear to be pressure-confined, gravitationally/virially bound, suggesting that gravitational/virial boundedness of a core is insufficient to ensure it will form stars, though it is crucial for gas in a contracting core to cool efficiently so it can collapse further to become protostellar.

Radio continuum and X-ray emission from the most extreme FIR-excess galaxy NGC 1377: An extremely obscured AGN revealed

Galaxies which strongly deviate from the radio-far IR correlation are of great importance for studies of galaxy evolution as they may be tracing early, short-lived stages of starbursts and active galactic nuclei (AGNs). The most extreme FIR-excess galaxy NGC1377 has long been interpreted as a young dusty starburst, but millimeter observations of CO lines revealed a powerful collimated molecular outflow which cannot be explained by star formation alone. We present new radio observations at 1.5 and 10 GHz obtained with the Jansky Very Large Array (JVLA) and Chandra X-ray observations towards NGC1377. The observations are compared to synthetic starburst models to constrain the properties of the central energy source. We obtained the first detection of the cm radio continuum and X-ray emission in NGC1377. We find that the radio emission is distributed in two components, one on the nucleus and another offset by 4$"$.5 to the South-West. We confirm the extreme FIR-excess of the galaxy, with a $q_\mathrm{FIR}\simeq$4.2, which deviates by more than 7-$\sigma$ from the radio-FIR correlation. Soft X-ray emission is detected on the off-nucleus component. From the radio emission we estimate for a young ($<10$ Myr) starburst a star formation rate SFR$<$0.1 M$_\odot$ yr$^{-1}$. Such a SFR is not sufficient to power the observed IR luminosity and to drive the CO outflow. We find that a young starburst cannot reproduce all the observed properties of the nucleus of NGC1377. We suggest that the galaxy may be harboring a radio-quiet, obscured AGN of 10$^6$M$_\odot$, accreting at near-Eddington rates. We speculate that the off-nucleus component may be tracing an hot-spot in the AGN jet.

Deep observations of the Super-CLASS super-cluster at 325 MHz with the GMRT: the low-frequency source catalogue

We present the results of 325 MHz GMRT observations of a super-cluster field, known to contain five Abell clusters at redshift $z \sim 0.2$. We achieve a nominal sensitivity of $34\,\mu$Jy beam$^{-1}$ toward the phase centre. We compile a catalogue of 3257 sources with flux densities in the range $183\,\mu\rm{Jy}\,-\,1.5\,\rm{Jy}$ within the entire $\sim 6.5$ square degree field of view. Subsequently, we use available survey data at other frequencies to derive the spectral index distribution for a sub-sample of these sources, recovering two distinct populations -- a dominant population which exhibit spectral index trends typical of steep-spectrum synchrotron emission, and a smaller population of sources with typically flat or rising spectra. We identify a number of sources with ultra-steep spectra or rising spectra for further analysis, finding two candidate high-redshift radio galaxies and three gigahertz-peaked-spectrum radio sources. Finally, we derive the Euclidean-normalised differential source counts using the catalogue compiled in this work, for sources with flux densities in excess of $223 \, \mu$Jy. Our differential source counts are consistent with both previous observations at this frequency and models of the low-frequency source population. These represent the deepest source counts yet derived at 325 MHz. Our source counts exhibit the well-known flattening at mJy flux densities, consistent with an emerging population of star-forming galaxies; we also find marginal evidence of a downturn at flux densities below $308 \, \mu$Jy, a feature so far only seen at 1.4 GHz.

Enhanced Star Formation of Less Massive Galaxies in a Proto-Cluster at z=2.5

We investigate a correlation between star-formation rate (SFR) and stellar mass for Halpha emission line galaxies (HAEs) in one of the richest proto-clusters ever known at z~2.5, USS 1558-003 proto-cluster. This study is based on a 9.7-hour narrow-band imaging data with MOIRCS on the Subaru telescope. We are able to construct a sample, in combination with additional H-band data taken with WFC3 on Hubble Space Telescope (HST), of 100 HAEs reaching the dust-corrected SFRs down to 3 Msun/yr and the stellar masses down to $10^{8.0}$ Msun. We find that while the star-forming galaxies with >$10^{9.3}$ Msun are located on the universal SFR-mass main sequence irrespective of the environment, less massive star-forming galaxies with <$10^{9.3}$ Msun show a significant upward scatter from the main sequence in this proto-cluster. This suggests that some less massive galaxies are in a starburst phase, although we do not know yet if this is due to environmental effects.

Testing the two planes of satellites in the Centaurus Group

The existence of satellite galaxy planes poses a major challenge for the standard picture of structure formation with non-baryonic dark matter. Recently Tully et al. (2015) reported the discovery of two almost parallel planes in the nearby Cen A group using mostly high-mass galaxies (M$_B$ $<$ -10 mag) in their analysis. Our team detected a large number of new group member candidates in the Cen A group (M\"uller et al. 2016). This dwarf galaxy sample combined with other recent results from the literature enables us to test the galaxy distribution in the direction of the Cen A group and to determine the statistical significance of the geometric alignment. Taking advantage of the fact that the two galaxy planes lie almost edge-on along the line of sight, the newly found 13 group members by Crnojevic et al. (2014, 2016) and our 16 new Cen A group candidates (M\"uller et al. 2016) can be assigned relative to the two planes. We use various statistical methods to test whether the distribution of galaxies follows a single normal distribution or shows evidence of bimodality as it has been reported earlier. We confirm that the data used for the Tully et al. (2015) study support the picture of a bimodal structure. However, when the new galaxy samples are included, the gap between the two galaxy planes is closing and the significance level of the bimodality is reduced. Instead, the plane that contains Cen A becomes more prominent. We found evidence that the galaxy system around Cen A is made up of only one plane of satellites. This plane is almost orthogonal to the dust plane of Cen A. Accurate distances to the new dwarf galaxies will be required to measure the precise 3D distribution of the galaxies around Cen A.

Galaxy groups in the 2MASS Redshift Survey [Replacement]

A galaxy group catalog is constructed from the 2MASS Redshift Survey (2MRS) with the use of a halo-based group finder. The halo mass associated with a group is estimated using a `GAP' method based on the luminosity of the central galaxy and its gap with other member galaxies. Tests using mock samples shows that this method is reliable, particularly for poor systems containing only a few members. On average 80% of all the groups have completeness >0.8, and about 65% of the groups have zero contamination. Halo masses are estimated with a typical uncertainty $\sim 0.35\,{\rm dex}$. The application of the group finder to the 2MRS gives 29,904 groups from a total of 43,246 galaxies at $z \leq 0.08$, with 5,286 groups having two or more members. Some basic properties of this group catalog is presented, and comparisons are made with other groups catalogs in overlap regions. With a depth to $z\sim 0.08$ and uniformly covering about 91% of the whole sky, this group catalog provides a useful data base to study galaxies in the local cosmic web, and to reconstruct the mass distribution in the local Universe.


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