Low-column density gas clumps in the inner and outer halo of the Milky Way

More than 40 years ago the occurrence of several narrow absorption lines in QSO (quasi-stellar object) spectra was recognized for the first time (Bahcall 1966). Soon after their detection it became clear that the absorption lines are related to intervening gaseous structures in the intergalactic medium (IGM). In recent years, substantial instrumental progress has been made to measure circumgalactic gas structures around the Milky Way and other galaxies. Now it is obvious that the gas structures play a fundamental role in the formation and evolution of the Milky Way and other spiral galaxies. The most promising local counterparts of intervening circumgalactic metal-absorbers seen in QSO absorption line data are the so-called low-, intermediate-, and high-velocity clouds (LVCs, IVCs, HVCs, Muller et al. 1963) in the halo of the Milky Way. They represent clouds of neutral atomic hydrogen seen in 21-cm emission at radial velocities inconsistent with a simple model of Galactic disk rotation. While there are a large number of recent absorption studies on the nature of LVCs, IVCs, and HVCs and their role for the evolution of the Milky Way, relatively little effort has been made to investigate their connection to the distribution and nature of intervening metal-absorption systems seen in QSO spectra around other galaxies. In fact, almost all recent absorption studies of IVCs and HVCs were carried out in the FUV to study in detail metal abundances and ionisation conditions of halo clouds using the many available transitions of low and high ions in the ultraviolet regime (e.g., Richter et al. 2001). These studies were designed as follow-up absorption observations of known IVCs and HVCs, thus providing an 21-cm emission-selected data set. However, to statistically compare the absorption characteristics of the extraplanar Galactic halo structures with the properties of intervening metal-absorption systems towards QSOs we use an absorption-selected data set of IVCs and HVCs. Since in the UV band there is currently only a very limited number ( This work discusses low-column density extraplanar gaseous structures detected in optical CaII and NaI absorption towards quasars. In total 177 lines of sight were observed, providing one of the largest samples today for analyses of IVC and HVC gas which is most likely located in the inner and outer halo of the Milky Way. The study allows us to directly compare the observed absorption column-density distribution of gas in the Milky Way halo with the overall column-density distribution of intervening absorbers towards QSOs. Moreover, the analysis enables us to identify the neutral and ionised absorption structures at low gas column densities and small angular extent that remain unseen in the large 21-cm IVC and HVC all-sky surveys, but that possibly have a considerable absorption cross section (see Richter et al. 2005). We supplement our absorption-line data with new HI 21-cm observations to investigate the relation between intermediate- and high-velocity CaII absorption features and halo 21-cm emission. We discuss the first results of our analysis of the physical and statistical properties of the detected absorption and emission features. Additionally, follow-up HI high-resolution synthesis observations of four of the absorbers are presented, unveiling low-column density small-scale structures embedded in the absorbers environment. The large sample of detected clouds enables for the first time a systematic statistical analysis of the properties of the low-column density cloud population in the Galactic halo. With these observations we demonstrate that the Milky Way halo contains a large number of low-column density neutral gas structures that give rise to intermediate- and high-velocity CaII and NaI absorption. In some cases, the CaII and NaI absorption lines are associated with known intermediate- and high-velocity clouds, but in other cases the observed absorption has no 21-cm counterpart. The observed CaII column density distribution is similar to the distribution found for intervening MgII systems that trace the gaseous environment of other galaxies at low and high redshift. The follow-up observations with radio synthesis telescopes uncovers several cold and compact (sub-pc scale) clumps. For the future, we want to answer the question whether these low-column density small-scale structures are common in the extraplanar environment of the Milky Way and how they influence the evolution of our Galaxy. Therefore, we are planing to obtain and analyse new HI 21-cm data with high sensitivity and high spectral resolution as well as optical and UV absorption line data. This will allow us to enlarge our current absorption-selected sample and to improve the statistical analysis of the distribution and physical parameters of the absorbers to place the gaseous environment of the Milky Way into a cosmological context

Physical properties of two compact high-velocity clouds possibly associated with the Leading Arm of the Magellanic System

We observed two compact high-velocity clouds HVC 291+26+195 and HVC 297+09+253 to analyse their structure, dynamics, and physical parameters. In both cases there is evidence for an association with the Leading Arm of the Magellanic Clouds. The goal of our study is to learn more about the origin of the two CHVCs and to use them as probes for the structure and evolution of the Leading Arm. We have used the Parkes 64 m radio telescope and the Australia Telescope Compact Array (ATCA) to study the two CHVCs in the 21 cm line emission of neutral hydrogen. We present a method to estimate the distance of the two CHVCs. The investigation of the line profiles of HVC 297+09+253 reveals the presence of two line components in the spectra which can be identified with a cold and a warm gas phase. In addition, we find a distinct head-tail structure in combination with a radial velocity gradient along the tail, suggesting a ram-pressure interaction of this cloud with an ambient medium. HVC 291+26+195 has only a cold gas phase and no head-tail structure. The ATCA data show several cold, compact clumps in both clouds which, in the case of HVC 297+09+253, are embedded in the warm, diffuse envelope. All these clumps have very narrow HI lines with typical line widths between 2 and 4 km/s FWHM, yielding an upper limit for the kinetic temperature of the gas of T_max = 300 K. We obtain distance estimates for both CHVCs of the order of 10 to 60 kpc, providing additional evidence for an association of the clouds with the Leading Arm.Comment: 11 pages, 6 Postscript figures, 2 tables; accepted for publication in Astronomy & Astrophysic

Structure and Kinematics of the Nearby Dwarf Galaxy UGCA 105

Owing to their shallow stellar potential, dwarf galaxies possess thick gas disks, which makes them good candidates for studies of the galactic vertical kinematical structure. We present 21 cm line observations of the isolated nearby dwarf irregular galaxy UGCA 105, taken with the Westerbork Synthesis Radio Telescope (WSRT), and analyse the geometry of its neutral hydrogen (HI) disk and its kinematics. The galaxy shows a fragmented HI distribution. It is more extended than the optical disk, and hence allows one to determine its kinematics out to very large galacto-centric distances. The HI kinematics and morphology are well-ordered and symmetric for an irregular galaxy. The HI is sufficiently extended to observe a substantial amount of differential rotation. Moreover, UGCA 105 shows strong signatures for the presence of a kinematically anomalous gas component. Performing tilted-ring modelling by use of the least-squares fitting routine TiRiFiC, we found that the HI disk of UGCA 105 has a moderately warped and diffuse outermost part. Probing a wide range of parameter combinations, we succeeded in modelling the data cube as a disk with a strong vertical gradient in rotation velocity ($\approx -60\,\rm km\,s^{-1}\,kpc^{-1}$), as well as vertically increasing inwards motion ($\approx -70\,\rm km\,s^{-1}\,kpc^{-1}$) within the radius of the stellar disk. The inferred radial gas inflow amounts to $0.06\,\rm M_\odot \rm yr^{-1}$, which is similar to the star formation rate of the galaxy. The observed kinematics are hence compatible with direct or indirect accretion from the intergalactic medium, an extreme backflow of material that has formerly been expelled from the disk, or a combination of both.Comment: 15 pages, 12 figures, accepted for publication in Astronomy & Astrophysic

A population of weak metal-line absorbers surrounding the Milky Way

We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of bright QSOs obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in OI and SiII together with mildly saturated CII absorption at high radial velocities (|v_LSR|=100-320 km/s). The measured OI column densities are small, implying that these structures represent Lyman-Limit Systems and sub-Lyman-Limit System with HI column densities < 3x10^18 cm^-2, thus below the detection limits of current 21cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently are not directly associated with any of the large high-column density HVC complexes, but rather represent isolated, partly neutral gas clumps embedded in a more tenuous, ionized gaseous medium situated in the halo or nearby intergalactic environment of the Galaxy. We speculate that this absorber population represents the local analog of weak MgII systems that are commonly observed in the circumgalactic environment of low- and high-redshift galaxies.Comment: 45 pages, 7 figures, accepted for publication in Ap

The Westerbork HALOGAS Survey: Data Release 1

The Westerbork HALOGAS Survey: Data Release 1 (version 2) The Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey comprises deep neutral hydrogen (HI) 21-cm emission line observations of 24 nearby galaxies, along with ancillary data products such as deep optical imaging, radio continuum images, and single-dish HI data. This first data release (HALOGAS-DR1) provides the following HI data products in FITS format, for each of the 24 galaxies: High resolution (~15") data products Data cube: &lt;galaxy_name&gt;-HR-cube.fits Moment-0 (integrated intensity): &lt;galaxy_name&gt;-HR_mom0m.fits Column density: &lt;galaxy_name&gt;-HR_coldens.fits Moment-1 (average velocity): &lt;galaxy_name&gt;-HR_mom1m.fits Low resolution (~30") data products Data cube: &lt;galaxy_name&gt;-LR-cube.fits Moment-0 (integrated intensity): &lt;galaxy_name&gt;-LR_mom0m.fits Column density: &lt;galaxy_name&gt;-LR_coldens.fits Moment-1 (average velocity): &lt;galaxy_name&gt;-LR_mom1m.fits Moment maps were generated with the SoFiA source finding package. Changes from version 1 A substantial error was identified with the velocity axis of the HI cubes for NGC 4448, leading to a large bulk offset from the true systemic velocity of the galaxy. Version 2 corrects this error in the cubes and the corresponding moment-1 (average velocity) images for NGC 4448. The data products for all other galaxies are unmodified from version 1. Acknowledgements If you make use of these data products in any publication or presentation, we kindly ask you to cite the following paper(s): Primary HALOGAS reference: Heald et al. (2011) Additional reference for NGC 891: Oosterloo et al. (2007) Additional reference for NGC 2403: Fraternali et al. (2002) and to include the following acknowledgement: This research made use of data from WSRT HALOGAS-DR1. The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Institute for Radio Astronomy) with support from the Netherlands Foundation for Scientific Research NWO