Daily Image

Click here or on the picture for a full size image.

Probing the gas content of radio galaxies through HI absorption stacking

Submitter: Katinka Gereb
Description: Accretion onto the central black hole of galaxies is thought to be connected with the presence and kinematical properties of the gas. In radio-loud active galactic nuclei (AGN), neutral hydrogen (HI) absorption studies provide a key tool to trace the cold gas component that may play a role in these processes. From previous studies it appears that compact radio sources, which are likely to be young AGN, are embedded in an HI-rich medium, and this may be connected with the triggering of AGN activity.

We carried out a systemic search for HI absorption in a relatively large sample of 101 radio AGN. Despite our shallow, 4-hour observations with the Westerbork Synthesis Radio Telescope (WSRT), we obtain a direct detection rate of 30%, similar to deeper studies. The detection rate does not depend on the apparent flux of a source, suggesting that HI absorption studies of even fainter radio sources will bring a large number of detections.

For the first time, we carried out a spectral stacking analysis of HI absorption. We find a dichotomy in the presence of HI in the sense that even when a large number of spectra are averaged, galaxies that do not show HI absorption in their individual spectra remain undetected (Fig. 1). The dichotomy suggests that in many galaxies, HI must be in a flattened structure so that orientation effects play a role. However, orientation effects alone cannot fully explain the dichotomy, suggesting that some galaxies must be depleted of cold gas.

Compact sources show higher detection rate, optical depth and Full Width at Half Maximum (FWHM) than extended sources (Fig. 2), strongly suggesting that different gas conditions exist in these two types of radio sources. It seems that nuclear activity in most of the young AGN is connected with the presence of unsettled gas.

These results have been presented in a A&A paper now in press by Gereb, K; Morganti, R; Oosterloo, T; http://arxiv.org/abs/1407.1799
Copyright: Katinka Gereb
  Follow us on Twitter
Please feel free to submit an image using the Submit page.