Circumnuclear and infalling HI gas in a merging galaxy pair at z=0.123
  Using long-slit optical spectra obtained with the 2-m telescope at IUCAA
Girawali Observatory, we show that the radio source J094221.98+062335.2
(z=0.123) is associated with a galaxy pair undergoing a major merger. Its
companion is a normal star-forming galaxy infalling with a velocity of 185 km/s
at a projected separation of 4.8 kpc. Using the Westerbork Synthesis Radio
Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT) we detect a strong
HI 21-cm absorption at the systemic redshift of the radio galaxy with
N(HI)~9x10^21 cm^{-2} for an assumed spin-temperature of 100 K. Such a strong
HI 21-cm absorption is rare and has been seen only in a few compact radio
sources associated with similar merging galaxy pairs. Milliarcsecond resolution
Very Long Baseline Array (VLBA) observations resolve the radio source into a
compact symmetric object with the hotspot separation of 89 pc. The 21-cm
absorption is detected in the VLBA spectra towards both the radio lobes albeit
with a strong optical depth gradient. We show that the strong 21-cm absorption
is consistent with it being arising from a clumpy circumnuclear disk/torus. We
also detect two weaker absorption lines redshifted with respect to the radio
source in the WSRT/GMRT spectrum. They probably represent cold (i.e. T< 10^4 K)
HI gas falling into the radio source. The presence of high concentration of HI
gas in the circumnuclear regions and signature of infalling cold gas allows us
to conjecture that the young radio source may have been triggered by the gas
infall due to the ongoing merger.
