The HI absorption “Zoo”

Abstract

We present an analysis of the HI absorption in a sample of 101 flux-selected radio AGN (S_1.4 GHz > 50 mJy) observed with the Westerbork Synthesis Radio Telescope (WSRT). HI absorption is detected in 32 galaxies, showing a broad variety of widths, shapes and kinematical properties. We characterize the HI spectra of the individual detections using the busy function (Westmeier et al. 2014). With the goal of identifying different morphological structures of HI, we study the kinematical and radio source properties of the detections as function of their width. Narrow lines (FWHM < 100 km/s) mostly lie at the systemic velocity and are likely produced by regularly rotating HI disks or clouds. More HI disks can be present among galaxies with lines of intermediate widths (100 km/s < FWHM < 200 km/s), however the HI in these sources is more unsettled. We study the asymmetry parameter and blueshift/redshift distribution of the lines. We argue that if a profile is broad, it is also asymmetric and shifted relative to the systemic velocity because it is tracing unsettled HI gas. Besides three of the broadest (up to FW20 = 825 km/s) detections, which are associated with gas-rich mergers, we find three new cases of blueshifted broad wings (with FW20 >= 500 km/s). These detections are good candidates for being HI outflows. The detection rate of HI outflows is 5 percent in the total radio AGN sample. This fraction represents a lower limit, however it could suggests that, if outflows are a characteristic phenomenon of all radio sources, they would have a short depletion timescale compared to the lifetime of the AGN. Blueshifted and broad/asymmetric lines are more often present among young, compact sources, suggesting that in these AGN the HI is likely to be unsettled. This may arise due to jet-cloud interactions, as young radio sources clear their way through the rich ambient gaseous medium.