Probing the gas content of radio galaxies through HI absorption stacking

TL;DR

This study used stacking of HI absorption in radio AGN to detect cold gas content across various flux levels, revealing a significant detection rate and differences between compact and extended sources, especially young radio AGN.

Contribution

First application of HI absorption stacking to a flux-selected radio AGN sample, revealing gas properties and orientation effects without bias toward brightness.

Findings

29% detection rate of HI absorption across flux levels

Higher detection and broader profiles in compact and young radio AGN

No significant evidence of HI outflows in stacked profiles

Abstract

Using the WSRT, we carried out shallow HI absorption observations of a flux-selected (S > 50 mJy) sample of 93 radio AGN with available SDSS redshifts between 0.02 < z < 0.23. We study the gas properties of radio AGN down to fluxes not systematically explored before using, for the first time, stacking of extragalactic HI absorption. Despite the shallow observations, we obtained a direct detection rate of ~29%, comparable with deeper studies. Detections are found at every flux level, showing that HI absorption detections are not biased toward brighter sources. The stacks of detections and non-detections reveal a clear dichotomy in the presence of HI, with the 27 detections showing an average peak {\tau} = 0.02, while the 66 non-detections remain undetected with an upper limit {\tau} < 0.002. Separating the sample into compact and extended AGN increases the detection rate, {\tau}, and FWHM for the compact sample. The dichotomy for the stacked profiles of detections and non-detections still holds between these two groups. We argue that orientation effects connected to a disk-like distribution of HI can be partly responsible for the dichotomy, however some of our galaxies must be genuinely depleted of cold gas. A fraction of the compact sources are confirmed by previous studies as likely young radio sources. These show an even higher detection rate of 55%. Along with their high integrated optical depth and wider profile, this reinforces the idea that young radio AGN are particularly rich in atomic gas. Part of our motivation is to probe for the presence of HI outflows. However, the stacked profiles do not reveal any significant blueshifted wing. Our results are particularly relevant for future surveys. The lack of bias toward bright sources is encouraging for the search of HI at lower radio fluxes. The results also represent a reference point for search for HI absorption at higher redshifts.