Prevalence of neutral gas in centres of merging galaxies

Abstract

We present Giant Metrewave Radio Telescope and Very Large Array observations\nof HI 21-cm absorption in ten z<=0.2 galaxy mergers that host strong radio\nsources. Seven of these mergers show strong absorption [with N(HI) ~10^{21-22}\ncm^{-2}, for spin temperature of 100 K and unit covering factor], leading to a\ntotal detection rate of 83 +/- 17% in low-z radio-loud galaxy mergers. This is\n~3-4 times higher than that found for intrinsic HI 21-cm absorption in low-z\nradio sources in general. The fraction of intrinsic absorbers that are\nassociated with mergers increases with increasing N(HI) threshold, i.e. ~40%\nand 100% of the absorbers with N(HI) >10^{21} cm^{-2} and >10^{22} cm^{-2}\narise from mergers, respectively. The distribution of N(HI) among the mergers\nis significantly different from that found in isolated systems, with mergers\ngiving rise to six times stronger absorption on average. The fraction of\nredshifted absorption components (with respect to the systemic velocity of the\nradio source obtained from optical emission lines) among mergers is found to be\nhigher by two to three times compared to that found for non-interacting\nsystems. Follow-up spatially-resolved multi-wavelength spectroscopy is\nessential to understand the exact connection between the presence of\ncircumnuclear neutral gas and the AGN activity in these mergers.\n