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02-12-2014
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The HI absorption Zoo

Submitter: Katinka Gereb
Description: Radio nuclear activity is thought to be connected with the properties of the gas in the circumnuclear region of galaxies. Neutral hydrogen 21-cm (HI) absorption is a sensitive measure of the properties of even small amounts of gas, and it provides one of the key tracers of the interplay between gas and AGN.

Recently, we published a selection of 32 HI absorption lines detected in a survey using the Westerbork Synthesis Radio Telescope, aimed to probe the gas content of radio galaxies through HI absorption stacking (Daily Image of August 20, 2014 ).

The HI absorption spectra show a broad variety of widths and shapes, really an "HI Zoo". We identify three different groups of absorption lines, representing physically different HI structures. Narrow lines are likely to trace regularly rotating HI disks or single gas clouds (see #13, #30). Broader profiles appear to be more asymmetric, possibly tracing unsettled components of HI gas. The broadest lines are associated with gas-rich merging systems (#7, # 22) and are profiles composed by several blended HI components. Furthermore, three profiles (#15, #17, #29) show highly blue-shifted wings, which are the typical signs of outflows driven by interactions between the jets and the ambient medium.

We find a correlation between the power of the radio source and the velocity shift of the HI with respect to the optical systemic velocity. This suggests that more powerful AGN are more efficient in affecting the kinematics of the surrounding gas. This relation appears to be tighter in compact, young sources (shown in red in Fig. 1), where the radio jets are starting their growth. This is also supported by the fact that both stacking and the individual analysis of the HI lines show that in compact sources the profiles are overall wider and more asymmetric.

These results have been presented by Gereb, K; Maccagni, F, M; Morganti, R; Oosterloo, T; in a paper now accepted for publication in A&A: ArXiv:1411.036
Copyright: Katinka Gereb
 
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