The covering factor of high redshift damped Lyman-$\alpha$ systems

TL;DR

This study measures the covering factors of high-redshift damped Lyman-alpha systems using VLBA imaging and finds that low detection rates of HI 21cm absorption are likely due to warmer HI, not low covering factors.

Contribution

It provides the first comprehensive measurement of covering factors for a sizable sample of high-redshift DLAs, challenging previous assumptions about their role in 21cm absorption detection.

Findings

Covering factors range from 0.41 to 1 across the sample.

No exceptionally low covering factors observed in high-z DLAs.

Low 21cm detection rates are attributed to warmer HI, not covering factors.

Abstract

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 HI 21cm 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 $f \sim 0.45 - 1$ for the 14 DLAs at $z > 1.5$, $f \sim 0.41 - 1$ for the 10 systems at $z < 1$, and consistent covering factor distributions in the two sub-samples. The observed paucity of detections of HI 21cm 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.