We present a detailed study of the QSO-galaxy pair [ SDSS J163956.35+112758.7 ( z _ { q } = 0.993 ) and SDSS J163956.38+112802.1 ( z _ { g } = 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 \pm 0.25 ) and dust extinction ( E ( B-V ) \sim 0.83 \pm 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 \pm 0.20Å .
Our GMRT spectrum shows a strong 21-cm absorption at the redshift of the galaxy with an integrated optical depth of 15.70 \pm 0.13 km s ^ { -1 } .
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 21-cm H i information .
The integrated H i 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 .