The HI absorption "Zoo"

K. Geréb, F. M. Maccagni, R. Morganti, T. A. Oosterloo

Research output: Contribution to journalArticleAcademicpeer-review

70 Citations (Scopus)
159 Downloads (Pure)


We present an analysis of the H I 21 cm absorption in a sample of 101flux-selected radio AGN (S1.4 GHz> 50 mJy) observed withthe Westerbork Synthesis Radio Telescope (WSRT). We detect H Iabsorption in 32 objects (30% of the sample). In a previous paper, weperformed a spectral stacking analysis on the radio sources, while herewe characterize the absorption spectra of the individual detectionsusing the recently presented busy function. The H I absorption spectrashow a broad variety of widths, shapes, and kinematical properties. Thefull width half maximum (FWHM) of the busy function fits of the detectedH I lines lies in the range 32 km s-1 200 km s-1). Westudy the kinematical and radio source properties of each group, withthe goal of identifying different morphological structures of H I.Narrow lines mostly lie at the systemic velocity and are likely producedby regularly rotating H I disks or gas clouds. More H I disks can bepresent among galaxies with lines of intermediate widths; however, the HI in these sources is more unsettled. We study the asymmetry parameterand blueshift/redshift distribution of the lines as a function of theirwidth. We find a trend for which narrow profiles are also symmetric,while broad lines are the most asymmetric. Among the broadest lines,more lines appear blueshifted than redshifted, similarly to what wasfound by previous studies. Interestingly, symmetric broad lines areabsent from the sample. We argue that if a profile is broad, it is alsoasymmetric and shifted relative to the systemic velocity because it istracing unsettled H I gas. In particular, besides three of the broadest(up to FW20 = 825 km s-1) detections, which are associatedwith gas-rich mergers, we find three new cases of profiles withblueshifted broad wings (with FW20 ≳ 500 km s-1) in highradio power AGN. These detections are good candidates for being HIoutflows. Together with the known cases of outflows already included inthe sample (3C 293 and 3C 305), the detection rate of H I outflows is 5%in the total radio AGN sample. Because of the effects of spintemperature and covering factor of the outflowing gas, this fractioncould represent a lower limit. However, if the relatively low detectionrate is confirmed by more detailed observations, it would suggest that,if outflows are a characteristic phenomenon of all radio AGN, they wouldhave a short depletion timescale compared to the lifetime of the radiosource. This would be consistent with results found for some of theoutflows traced by molecular gas. Using stacking techniques, in ourprevious paper we showed that compact radio sources have higher τ,FWHM, and column density than extended sources. In addition, here wefind that blueshifted and broad/asymmetric lines are more often presentamong compact sources. In good agreement with the results of stacking,this suggests that unsettled gas is responsible for the larger stackedFWHM detected in compact sources. Therefore in such sources the H I ismore likely to be unsettled. This may arise as a result of jet-cloudinteractions, as young radio sources clear their way through the richambient gaseous medium.Appendices are available in electronic form at
Original languageEnglish
Article numberA44
Number of pages17
JournalAstronomy and astrophysics
Publication statusPublished - Mar-2015


  • galaxies: active
  • radio lines: galaxies

Cite this