The Magellanic Bridge as a Damped Lyman Alpha System: Physical Properties of Cold Gas Toward PKS 0312-770

We measure the physical properties of a local multicomponent absorption-line system at V-circle dot similar to 200 km s(-1) toward the quasar PKS 0312-770 behind the Magellanic Bridge (MB) using Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) spectroscopy in conjunction with photoionization modeling. At an impact parameter of similar to 10 kpc from the Small Magellanic Cloud (SMC), this sightline provides a unique opportunity to probe the chemical properties and ionization structure in a nearby absorption line system with a column density of log N-H I similar to 20.2, at the transition between damped Ly alpha (DLA) and sub-DLA systems. We find that metallicity of -1.0 < log(Z/Z(circle dot)) < -0.5 and ionization parameter of -6 < logU < -5 for three low-ionization components and logU similar to -2.6 for one high-ionization component. One component at V-circle dot = 207 km s(-1) shows an alpha-element abundance log( Si/H) similar to -5.0, making it similar to 0.2 dex more metal-rich than both SMC HII regions and stars within the MB and the SMC. The N/Si ratio in this component is log( N/Si)= -0.3 +/- 0.1, making it comparable to other N-poor dwarf galaxies and similar to 0.2 dex lower than Hii regions in the SMC. Another component at V-circle dot = 236 km s(-1) shows a similar Si/H ratio but has log(N/Si) = -1.0 +/- 0.2, indicating a nitrogen deficiency comparable to that seen in the most N-poor DLA systems. These differences imply different chemical enrichment histories between components along the same sightline. Our results suggest that if these absorbers are representative some fraction of DLA systems, then (1) DLA systems along single sightlines do not necessarily represent the global properties of the absorbing cloud, and (2) the chemical composition within a given DLA cloud may be inhomogeneous.ArticleThe Astrophysical Journal. 695:1382-1398 (2009)journal articl

Compact High-Velocity Clouds at High Resolution

Six examples of the compact, isolated high-velocity clouds catalogued by Braun & Burton (1999) and identified with a dynamically cold ensemble of primitive objects falling towards the barycenter of the Local Group have been imaged with the Westerbork Synthesis Radio Telescope; an additional ten have been imaged with the Arecibo telescope. The imaging reveals a characteristic core/halo morphology: one or several cores of cool, relatively high-column-density material, are embedded in an extended halo of warmer, lower-density material. Several of the cores show kinematic gradients consistent with rotation; these CHVCs are evidently rotationally supported and dark-matter dominated. The imaging data allows several independent estimates of the distances to these objects, which lie in the range 0.3 to 1.0 Mpc. The CHVC properties resemble what might be expected from very dark dwarf irregular galaxies.Comment: 12 pages, 7 figures, to appear in "The Chemical Evolution of the Milky Way: Stars versus Clusters", eds. F. Matteuchi and F. Giovannelli, Kluwer Academic Publisher

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

We report the discovery of molecular hydrogen absorption in a Galactic high-velocity cloud (HVC) in the direction of the Large Magellanic Cloud. For the same HVC we derive an iron abundance which is half of the solar value. Thus, all evidence points to a Galactic origin for high-velocity cloud complex in front of the LMC.Comment: Published in Nature, this week; 14 pages, 3 figure

Diagnosing galactic feedback with the line broadening in the low redshift Lyα forest

We compare the low-redshift (z ≃ 0.1) Lyα forest from hydrodynamical simulations with data from the Cosmic Origins Spectrograph. We find the tension between the observed number of lines with $\textit{b}$-parameters in the range of 25–45 km s$^{-1}$ and the predictions from simulations that incorporate either vigorous feedback from active galactic nuclei or that exclude feedback altogether. The gas in these simulations is, respectively, either too hot to contribute to the Lyα absorption or too cold to produce the required linewidths. Matching the observed $\textit{b}$-parameter distribution therefore requires feedback processes that thermally or turbulently broaden the absorption features without collisionally (over)ionizing hydrogen. This suggests that the Lyα forest $\textit{b}$-parameter distribution is a valuable diagnostic of galactic feedback in the low-redshift Universe. We furthermore confirm that the low-redshift Lyα forest column density distribution is better reproduced by an ultraviolet background with the H$\textit{I}$ photoionization rate a factor of 1.5–3 higher than predicted by Haardt and Madau.MV is supported by INFN/PD51 Indark, and (with TSK) ERC Grant 257670-cosmoIGM. JSB is supported by a Royal Society URF. MGH and EP acknowledge support from the FP7 ERC Grant Emergence-320596 and the Kavli Foundation. Simulations were performed at the University of Cam- bridge with Darwin-HPCS and COSMOS, operated on behalf of the STFC DiRAC facility (funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1), and on the Curie supercomputer at TGCC through the 8th PRACE call. BPW is supported by NASA grants HST-AR-12842, HST-AR-13893 from STSCI operated by AURA under contract NAS5-26555 and AST-1108913 by NSF

Gaseous Galaxy Halos

Galactic halo gas traces inflowing star formation fuel and feedback from a galaxy's disk and is therefore crucial to our understanding of galaxy evolution. In this review, we summarize the multi-wavelength observational properties and origin models of Galactic and low redshift spiral galaxy halo gas. Galactic halos contain multiphase gas flows that are dominated in mass by the ionized component and extend to large radii. The densest, coldest halo gas observed in neutral hydrogen (HI) is generally closest to the disk (< 20 kpc), and absorption line results indicate warm and warm-hot diffuse halo gas is present throughout a galaxy's halo. The hot halo gas detected is not a significant fraction of a galaxy's baryons. The disk-halo interface is where the multiphase flows are integrated into the star forming disk, and there is evidence for both feedback and fueling at this interface from the temperature and kinematic gradient of the gas and HI structures. The origin and fate of halo gas is considered in the context of cosmological and idealized local simulations. Accretion along cosmic filaments occurs in both a hot (> 10^5.5 K) and cold mode in simulations, with the compressed material close to the disk the coldest and densest, in agreement with observations. There is evidence in halo gas observations for radiative and mechanical feedback mechanisms, including escaping photons from the disk, supernova-driven winds, and a galactic fountain. Satellite accretion also leaves behind abundant halo gas. This satellite gas interacts with the existing halo medium, and much of this gas will become part of the diffuse halo before it can reach the disk. The accretion rate from cold and warm halo gas is generally below a galaxy disk's star formation rate, but gas at the disk-halo interface and stellar feedback may be important additional fuel sources.Comment: 50 pages, 9 figures (1 in 3D, view with a current version of Adobe), to appear in ARA&A, 50, 49

Cold gas accretion in galaxies

Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages. Full-resolution version available at http://www.astron.nl/~oosterlo/accretionRevie

Secular evolution versus hierarchical merging: galaxy evolution along the Hubble sequence, in the field and rich environments

In the current galaxy formation scenarios, two physical phenomena are invoked to build disk galaxies: hierarchical mergers and more quiescent external gas accretion, coming from intergalactic filaments. Although both are thought to play a role, their relative importance is not known precisely. Here we consider the constraints on these scenarios brought by the observation-deduced star formation history on the one hand, and observed dynamics of galaxies on the other hand: the high frequency of bars and spirals, the high frequency of perturbations such as lopsidedness, warps, or polar rings. All these observations are not easily reproduced in simulations without important gas accretion. N-body simulations taking into account the mass exchange between stars and gas through star formation and feedback, can reproduce the data, only if galaxies double their mass in about 10 Gyr through gas accretion. Warped and polar ring systems are good tracers of this accretion, which occurs from cold gas which has not been virialised in the system's potential. The relative importance of these phenomena are compared between the field and rich clusters. The respective role of mergers and gas accretion vary considerably with environment.Comment: 18 pages, 8 figures, review paper to "Penetrating Bars through Masks of Cosmic Dust: the Hubble Tuning Fork Strikes a New Note", Pilanesberg, ed. D. Block et al., Kluwe

DISTRIBUTION AND ORIGIN OF HIGH-VELOCITY CLOUDS .2. STATISTICAL-ANALYSIS OF THE WHOLE-SKY SURVEY

A sensitive, almost complete, whole-sky survey of high-velocity clouds (HVCs) has been made available by Bajaja et al. (1985) and Hulsbosch & Wakker (1988, Paper I). This paper (Paper II in a series on HVCs) is dedicated to the analysis of the statistical properties of these surveys. The main conclusions drawn from the statistics are the following: (1) It is now clear that the fact that HVCs used to be observed only at negative velocities was due to the incomplete and less-sensitive coverage of the southern sky, in spite of earlier statements to the contrary (Oort & Hulsbosch 1978; Hulsbosch 1979). (2) The distributions of flux with galactic longitude and latitude indicate that gas at high velocity concentrates toward the plane and is evenly distributed in longitude. (3) Extrapolating the relation between the sky area covered by high-velocity gas and the detection limit suggests that at a level of congruent-to 2 10(17) cm-2 between 30% and 60% of the sky may be covered with such gas. Such column densities are detectable by means of ultra-violet resonance lines of for instance C II or Si II. However, no high-velocity absorption is detected in a sample of 19 stars more distant than 1.5 kpc (Danly 1989), indicating that in general HVCs are at large distances