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

Abstract

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. <br /> 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. <br /> 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 (<50) of high-quality spectra available such a statistical comparison can be done best in the optical regime, where a large number of high-quality spectra of low- and high-redshift QSOs are available.<br /> 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.<br /> 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. <br /> 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.