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