Galaxies in HI 21-cm absorption at z<3.5
  We present recent results from our searches of 21-cm absorption using GBT, GMRT, VLBA and WSRT to trace the evolution of cold gas in galaxies. Using ~130 sight lines with 21-cm absorption measurements, we find that within the measurement uncertainty, the 21-cm detection rate in strong MgII systems is constant over 0.5<z<1.5. Since stellar feedback processes are expected to diminish the filling factor of CNM over 0.5<z<1, this lack of evolution in the 21-cm detection rate in MgII absorbers is intriguing. Further, we find that if the majority of 21-cm absorbers arise from DLAs then the cross-section of 21-cm absorbing gas i.e. cold neutral medium amongst DLAs has increased from z=3.5 to z=0.5. In a sample of 13 z>2 DLAs with both 21-cm and H$_2$ (another tracer of cold gas) absorption measurements, we report two new H$_2$ detections and find that in 8/13 cases neither 21-cm nor H$_2$ is detected. This confirms that the HI gas in z>2 DLAs is predominantly warm. Interestingly, there are two cases where 21-cm absorption is not detected despite the presence of H$_2$ with evidence for the presence of cold gas. This can be explained if H$_2$ components seen in DLA are compact (<15 pc) and contain <10% of the total N(HI). We briefly discuss results from our ongoing survey to identify 21-cm absorbers at low-z to establish connection between 21-cm absorbers and galaxies, and constrain the extent of absorbing gas. 
N. Gupta (IUCAA), R. Srianand (IUCAA), P. Petitjean (IAP), P.   Noterdaeme (IAP) and E. Momjian (NRAO)
astro-ph.GA astro-ph.CO