Prevalence of neutral gas in centres of merging galaxies

We present Giant Metrewave Radio Telescope and Very Large Array observations of HI 21-cm absorption in 10 z = 0.2 galaxy mergers that host strong radio sources. Seven of these mergers show strong absorption [with N(H I) ~ 1021-22 cm-2, for spin temperature of 100 K and unit covering factor]. Including literature sources, this leads to a total detection rate of 83±17 per cent in low-z radio-loud galaxy mergers. This is~3-4 times higher than that found for intrinsic HI 21-cm absorption in low-z radio sources in general. The fraction of intrinsic absorbers that are associated with mergers increases with increasing N(H I) threshold, i.e. ~40 per cent and 100 per cent of the absorbers with N(H I) > 1021 cm-2 and > 1022 cm-2 arise from mergers, respectively. The distribution of N(H I) among the mergers is significantly different from that found in isolated systems, with mergers giving rise to six times stronger absorption on average. The fraction of redshifted absorption components (with respect to the systemic velocity of the radio source obtained from optical emission lines) among mergers is found to be higher by two to three times compared to that found for non-interacting systems. Follow-up spatially resolved multiwavelength spectroscopy is essential to understand the exact connection between the presence of circumnuclear neutral gas and the active galactic nucleus activity in these mergers.