HI absorption survey casts light on feedback in radio galaxies
20 February 2017
Studies are ongoing to determine the effects that Active Galactic Nuclei (AGN) feedback has on the host galaxy, whether star-formation is being promoted or suppressed, and what role it plays in galaxy evolution. Insight into this impact of AGN activity on star-forming material comes from studying the neutral gas kinematics close to the AGN. A good tracer for these kinematics is the neutral hydrogen (HI) 21-cm transition in absorption from gas located in front of a radio source (along our line of sight). Higher detection rates of HI absorption are found in compact radio galaxies – which are thought to be the youngest or most recently triggered AGN – than for extended sources. This method allows us to investigate how AGN feedback directly interacts with the clumpy medium in the host galaxy.
In a recent paper, CAASTRO PhD student Marcin Glowacki (University of Sydney) and colleagues presented the results of such a HI absorption survey against 66 nearby compact radio sources with the Australia Telescope Compact Array. In total, seven detections of HI absorption were made, five of these new, giving a detection rate of 11%. Two of the detections were made possible by Bayesian line-finding algorithms developed by CAASTRO Affiliate Dr James Allison (CSIRO).
The majority of their detections (71%) indicated disturbed gas kinematics. Analysing the mid-infrared colours and optical spectra of the surveyed sources, the researchers found that their features were consistent with early-type and low-excitation radio galaxies. These sources are thought to be devoid of a gas-rich disk and to have inefficient AGN accretion processes, suggesting that AGN activity and feedback are disrupting the star-forming material. In contrast, absorption features attributed to gas in galactic disks were found in late-type and high-excitation radio galaxies that are believed to have efficient AGN accretion processes.
The team further identified absorbed X-ray sources, visible as a dip in the lower energy end of archival X-ray spectra, as tracers of HI content in the host galaxy of the AGN.
The sample in their paper extends previous HI surveys in compact radio galaxies to lower radio luminosities. The findings provide a basis for future HI surveys that will explore the higher redshift Universe, such as the First Large Absorption Survey in HI (FLASH) with the Australian Square Kilometre Array Pathfinder (ASKAP).