The covering factor of high-redshift damped Lyman-α systems

We have used the Very Long Baseline Array to image 18 quasars with foreground damped Lyman-alpha systems (DLAs) at 327, 610 or 1420 MHz, to measure the covering factor f of each DLA at or near its redshifted H I 21 cm line frequency. Including six systems from the literature, we find that none of 24 DLAs at 0.09 < z < 3.45 has an exceptionally low covering factor, with f similar to 0.45-1 for the 14 DLAs at z > 1.5 and f similar to 0.41-1 for the 10 systems at z < 1, and consistent covering factor distributions in the two subsamples. The observed paucity of detections of HI 21 cm absorption in high-z DLAs thus cannot be explained by low covering factors and is instead likely to arise due to a larger fraction of warm HI in these absorbers.