Small-scale Properties of Atomic Gas in Extended Disks of Galaxies
  We present high-resolution HI 21 cm observations with the Karl G. Jansky Very
Large Array (VLA) for three HI rich galaxies in absorption against radio
quasars. Our sample contains six sightlines with impact parameters from 2.6 to
32.4 kpc. We detected a narrow HI absorber of FWHM 1.1 km/s at 444.5 km/s
towards J122106.854+454852.16 probing the dwarf galaxy UCG 7408 at an impact
parameter of 2.8 kpc. The absorption feature was barely resolved and its width
corresponds to a maximum kinetic temperature, $\rm T_k \approx 26~K$. We
estimate a limiting peak optical depth of 1.37 and a column density of $\rm
6\times 10^{19}~cm^{-2}$. The physical extent of the absorber is $\rm
0.04~kpc^2$ and covers $\sim$25-30\% of the background source. A comparison
between the emission and absorption strengths suggests the cold-to-total HI
column density in the absorber is ~30%. Folding in the covering fraction, the
cold-to-total HI mass is ~10%. This suggest that condensation of warm HI ($\rm
T_s\sim 1000~K$) to cold phase ($\rm T_s < 100~K$) is suppressed in UGC 7408.
The unusually low temperature of the HI absorber also indicates inefficiency in
condensation of atomic gas into molecular gas. The suppression in condensation
is likely to be the result of low-metal content in this galaxy. The same
process might explain the low efficiency of star formation in dwarf galaxies
despite their huge of gas reservoirs.
  We also report the non-detection of HI in absorption in five other
sightlines. This indicates that either the cold gas distribution is highly
patchy or the gas is much warmer ($\rm T_s~>1000~K$) towards these sightlines.
