Using 21cm Absorption in Small Impact Parameter Galaxy-QSO Pairs to Probe Low-Redshift Damped and Sub-Damped Lyman-alpha System

TL;DR

This study uses 21cm absorption observations of a low-redshift galaxy-quasar pair to investigate the properties of cold neutral gas in galaxy disks, revealing unexpectedly low HI column density and insights into galaxy gas depletion.

Contribution

First detection of 21cm absorption in a low-redshift galaxy-quasar pair at small impact parameter, providing detailed gas properties and dynamics of the galaxy's interstellar medium.

Findings

Detected cold, quiescent disk gas with low HI column density.

Absorber size constrained to ~27x14 parsecs, indicating low-density cold clouds.

Galaxy appears to be depleting its HI without replenishment.

Abstract

To search for low-redshift damped Lyman-alpha (DLA) and sub-DLA quasar absorbers, we have conducted a 21cm absorption survey of radio-loud quasars at small impact parameters to foreground galaxies selected from the Sloan Digital Sky Survey (SDSS). Here we present the first results from this survey based on observations of SDSS J104257.58+074850.5 ($z_{QSO}$ = 2.66521), a quasar at an angular separation from a foreground galaxy ($z_{gal}$ = 0.03321) of 2.5" (1.7 kpc in projection). The foreground galaxy is a low-luminosity spiral with on-going star formation (0.004 M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$) and a metallicity of $-0.27 \pm 0.05$ dex. We detect 21cm absorption from the galaxy with the Green Bank Telescope (GBT), the Very Large Array (VLA), and the Very Long Baseline Array (VLBA). The absorption appears to be quiescent disk gas corotating with the galaxy and we do not find any evidence for outflowing cold neutral gas. The width of the main absorption line indicates that the gas is cold, $T_{k} < 283$ K, and the HI column is surprisingly low given the impact parameter of 1.7 kpc; we find that N(HI) $\leq 9.6 \times 10^{19}$ cm$^{-2}$ (GBT) and N(HI) $\leq 1.5 \times 10^{20}$ cm$^{-2}$ (VLBA). VLBA marginally resolves the continuum source and the absorber, and a lower limit of 27.1 $\times$ 13.9 pc is derived for the size of the absorbing cloud. In turn, this indicates a low density for a cold cloud, n(HI) $<$ 3.5 cm$^{-3}$. We hypothesize that this galaxy, which is relatively isolated, is becoming depleted in HI because it is converting its interstellar matter into stars without a replenishing source of gas, and we suggest future observations to probe this and similar galaxies.