The HI absorption “Zoo”

Abstract

We present an analysis of the H I 21 cm absorption in a sample of 101 flux-selected radio AGN (S 1.4 GHz > 50 mJy) observed with the Westerbork Synthesis Radio Telescope (WSRT). We detect H I absorption in 32 objects (30% of the sample). In a previous paper, we performed a spectral stacking analysis on the radio sources, while here we characterize the absorption spectra of the individual detections using the recently presented busy function. The H I absorption spectra show a broad variety of widths, shapes, and kinematical properties. The full width half maximum (FWHM) of the busy function fits of the detected H I lines lies in the range 32 km s −1 < FWHM < 570 km s −1 , whereas the full width at 20% of the peak absorption (FW20) lies in the range 63 km s −1 < FW20 < 825 km s −1 . The width and asymmetry of the profiles allows us to identify three groups: narrow lines (FWHM < 100 km s −1 ), intermediate widths (100 km s −1 < FWHM < 200 km s −1 ), and broad profiles (FWHM > 200 km s −1 ). We study the kinematical and radio source properties of each group, with the goal of identifying different morphological structures of H I. Narrow lines mostly lie at the systemic velocity and are likely produced by regularly rotating H I disks or gas clouds. More H I disks can be present among galaxies with lines of intermediate widths; however, the H I in these sources is more unsettled. We study the asymmetry parameter and blueshift/redshift distribution of the lines as a function of their width. We find a trend for which narrow profiles are also symmetric, while broad lines are the most asymmetric. Among the broadest lines, more lines appear blueshifted than redshifted, similarly to what was found by previous studies. Interestingly, symmetric broad lines are absent from the sample. We argue that if a profile is broad, it is also asymmetric and shifted relative to the systemic velocity because it is tracing unsettled H I gas. In particular, besides three of the broadest (up to FW20 = 825 km s −1 ) detections, which are associated with gas-rich mergers, we find three new cases of profiles with blueshifted broad wings (with FW20 > ∼ 500 km s −1 ) in high radio power AGN. These detections are good candidates for being HI outflows. Together with the known cases of outflows already included in the sample (3C 293 and 3C 305), the detection rate of H I outflows is 5% in the total radio AGN sample. Because of the effects of spin temperature and covering factor of the outflowing gas, this fraction could represent a lower limit. However, if the relatively low detection rate is confirmed by more detailed observations, it would suggest that, if outflows are a characteristic phenomenon of all radio AGN, they would have a short depletion timescale compared to the lifetime of the radio source. This would be consistent with results found for some of the outflows traced by molecular gas. Using stacking techniques, in our previous paper we showed that compact radio sources have higher τ, FWHM, and column density than extended sources. ...

Connecting X-ray absorption and 21 cm neutral hydrogen absorption in obscured radio AGN

“…Gereb et al 2015). The symmetry and width of the line are consistent with the compact radio source illuminating a regular gaseous structure (such as a disk) near to the systemic velocity of the host galaxy.…”

“…This has been supplemented by studies showing a higher detection rate of Hi in these kinds of galaxies, particularly in the local Universe (e.g. Gereb et al 2015;Maccagni et al 2017). However, different processes may dominate in particular AGN depending on their accretion rates, jet power, density of the surrounding medium and, importantly, evolutionary age.…”

Exceptional AGN-driven turbulence inhibits star formation in the 3C 326N radio galaxy

“…The spatial distribution of the outflowing gas, and the distribution of the filling factor of the cold gas are unlikely to be homogeneous and isotropic, which could skew line profiles. Asymmetric Hi absorption line profiles associated with off-nuclear outflows are indeed observed in radio galaxies (e.g., Morganti et al 2005a;Gereb et al 2015). In fact, the high degree of symmetry seen in the broad line profiles towards e.g., highredshift quasars , and often interpreted as outflows, is remarkable.…”

Atomic and molecular absorption in redshifted radio sources

“…3), which is used as the FWHM to obtain the integrated optical depth limit, thus giving the N H I f /T spin limit per channel (see Curran 2012). Allison et al (2012Allison et al ( , 2015; Yan et al (2012Yan et al ( , 2016; Geréb et al (2015); Srianand et al (2015); Aditya et al (2016Aditya et al ( , 2017. 8 The absorption redshift of z abs = 1.343 (Kanekar et al 2009) corresponds to a velocity offset of ∆v = 64 km s −1 relative to z QSO = 1.3425 and so we deem this as associated rather than intervening.…”

“…However, there exists an additional constraint when searching for absorption by gas in the hosts of radio galaxies and quasars -specifically, that neutral hydrogen has never been detected where the ultra-violet (λ = 912Å) luminosity of the source exceeds LUV ∼ 10 23 W Hz −1 . This "critical" luminosity applies to all redshifts for various heterogeneous (unbiased) samples, as seen through the non-detection of H I 21-cm absorption at LUV > ∼ 10 23 W Hz −1 by Curran et al (2008Curran et al ( , 2011aCurran et al ( , 2013bCurran et al ( ,c, 2016a; Allison et al (2012);Geréb et al (2015); Aditya et al (2016); Grasha et al (2017). This is interpreted as the flux limited optical spectroscopic surveys, which yield the redshift, selecting the most ultra-violet luminous objects at high redshift (Curran et al 2008), where the corresponding ionising photon rates of QH I > ∼ 3 × 10 56 sec −1 are sufficient to ionise all of the neutral gas in a large spiral galaxy (Curran & Whiting 2012).…”