Probing the gas content of radio galaxies through H I absorption stacking

Abstract

Using the Westerbork Synthesis Radio Telescope, we carried out shallow H i absorption observations of a flux-selected (S 1.4 GHz > 50 mJy) sample of 93 radio active galactic nuclei (AGN), which have available SDSS (Sloan Digital Sky Survey) redshifts between 0.02 < z < 0.23. Our main goal is to study the gas properties of radio sources down to S 1.4 GHz flux densities not systematically explored before using, for the first time, stacking of absorption spectra of extragalactic H i. Despite the shallow observations, we obtained a direct detection rate of ∼29%, comparable with deeper studies of radio galaxies. Furthermore, detections are found at every S 1.4 GHz flux level, showing that H i absorption detections are not biased toward brighter sources. The stacked profiles of detections and non-detections reveal a clear dichotomy in the presence of H i, with the 27 detections showing an average peak τ = 0.02 corresponding to N(H i) ∼ (7.4 ± 0.2) × 10 18 (T spin /c f ) cm −2 , while the 66 non-detections remain undetected upon stacking with a peak optical depth upper limit τ < 0.002 corresponding to N(H i) < (2.26 ± 0.06) × 10 17 (T spin /c f ) cm −2 (using a FWHM of 62 km s −1 , derived from the mean width of the detections). Separating the sample into compact and extended radio sources increases the detection rate, optical depth, and FWHM for the compact sample. The dichotomy for the stacked profiles of detections and non-detections still holds between these two groups of objects. We argue that orientation effects connected to a disk-like distribution of the H i can be partly responsible for the dichotomy that we see in our sample. However, orientation effects alone cannot explain all the observational results, and some of our galaxies must be genuinely depleted of cold gas. A fraction of the compact sources in the sample are confirmed by previous studies as likely young radio sources (compact steep spectrum and gigahertz peaked spectrum 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 embedded in a medium that is rich in atomic gas. Part of our motivation is to probe for the presence of faint H i outflows at low optical depth using stacking. However, the stacked profiles do not reveal any significant blueshifted wing. We are currently collecting more data to investigate the presence of outflows. The results presented in this paper are particularly relevant for future surveys in two ways. The lack of bias toward bright sources is encouraging for the search for H i in sources with even lower radio fluxes planned by such surveys. The results also represent a reference point when searching for H i absorption at higher redshifts.

Hi absorption in nearby compact radio galaxies

“…Our optical depth upper limit of τ < 0.004 is consistent with the stacking result by Geréb et al (2014), of τ < 0.002 for 66 sources (Fig. 5).…”

“…Their compact sources exhibited the broadest line features and were more often seen to have asymmetric lines. Geréb et al (2014) suggest that this trend in compact sources is due to the presence of unsettled gas, which is typically traced by asymmetric and redshifted/blueshifted lines. These properties point to interactions between the AGN and the circumnuclear medium in such compact galaxies -the feedback from the newly triggered AGN which is still propagating through this medium would be the cause of the unsettled gas.…”

The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?

“…The H I profile is broader than that typically found in radio galaxies (Gereb et al 2014), but resembles what is observed in some other clusters; we discuss this in more details in Sect. 4.1.…”

“…However, the H I profile detected in 4C 35.06 appears to be broader than in typical radio galaxies (Gereb et al 2014). However, it resembles some of the profiles found in other clusters (in particular Abell 2597 and Abell 1795 mentioned above) that show relatively broad wings in the H I absorption profile, perhaps a signature of the more disturbed kinematics of the gas due to the cluster environment, as seen also in molecular gas by McNamara et al (2014) derived from ALMA data of Abell 1835.…”

Correlation between X-Ray and Radio Absorption in Compact Radio Galaxies

“…Several HI absorption surveys revealed that compact radio galaxies display a significant excess in the detection rate with respect to extended radio sources (Conway 1997;Morganti et al 2001;Pihlström et al 2003;Vermeulen et al 2003;Gupta et al 2006;Chandola et al 2011;Curran et al 2013;Geréb et al 2014).…”