Parsec-scale structures and diffuse bands in a translucent interstellar
  medium at z 0.079
  We present a detailed study of the QSO-galaxy pair [SDSS J163956.35+112758.7
(zq = 0.993) and SDSS J163956.38+112802.1 (zg = 0.079)] based on observations
carried out using the Giant Meterwave Radio Telescope (GMRT), the Very Large
Baseline Array (VLBA), the Sloan Digital Sky Survey (SDSS) and the ESO New
Technology Telescope (NTT). We show that the interstellar medium of the galaxy
probed by the QSO line of sight has near-solar metallicity (12+log(O/H) =
8.47+/-0.25) and dust extinction (E(B-V) 0.83+/-0.11) typical of what is
usually seen in translucent clouds. We report the detection of absorption in
the \lambda 6284 diffuse interstellar band (DIB) with a rest equivalent width
of 1.45+/-0.20\AA. Our GMRT spectrum shows a strong 21-cm absorption at the
redshift of the galaxy with an integrated optical depth of 15.70+/-0.13 km/s.
Follow-up VLBA observations show that the background radio source is resolved
into three components with a maximum projected separation of 89 pc at the
redshift of the galaxy. One of these components is too weak to provide useful
HI 21-cm absorption information. The integrated HI optical depth towards the
other two components are higher than that measured in our GMRT spectrum and
differ by a factor 2. By comparing the GMRT and VLBA spectra we show the
presence of structures in the 21-cm optical depth on parsec scales. We discuss
the implications of such structures for the spin-temperature measurements in
high-z damped Lyman-alpha systems. The analysis presented here suggests that
this QSO-galaxy pair is an ideal target for studying the DIBs and molecular
species using future observations in optical and radio wavebands.
