Abstract

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 Mg ii 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 Mg ii absorbers is intriguing. Further, we find that if 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 H2 (another tracer of cold gas) absorption measurements, we report two new H2 detections and find that in 8/13 cases neither 21-cm nor H2 is detected. This confirms that the H i gas in z > 2 DLAs is predominantly warm. Interestingly, there are two cases where 21-cm absorption is not detected despite the presence of H2 with evidence for the presence of cold gas. This can be explained if H2 components seen in DLA are compact (≤15 pc) and contain ≤10% of the total N(H i). 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.