Probing radio source environments via H i and OH absorption

Abstract

We present the results of H I and OH absorption measurements towards a sample of radio sources using the Arecibo 305-m telescope and the Giant Metrewave Radio Telescope (GMRT). In total, 27 radio sources were searched for associated 21-cm H I absorption. One totally new H I absorption system was detected against the radio galaxy 3C258, while five previously known H I absorption systems, and one galaxy detected in emission, were studied with improved frequency resolution and/or sensitivity. Our sample included 17 gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) objects, four of which exhibit H I absorption. This detection rate of ∼25 per cent compares with a value of ∼40 per cent by Vermeulen et al. for similar sources. We detected neither OH emission nor absorption towards any of the sources that were observed at Arecibo. We are, however, able to estimate a limit on the abundance ratio of N(H I)/N(OH) 4 × 10 6 for 3C258.We have combined our results with those from other available H I searches to compile a heterogeneous sample of 96 radio sources consisting of 27 GPS, 35 CSS, 13 compact flat spectrum (CFS) and 21 large (LRG) sources. The H I absorption detection rate is highest (∼45 per cent) for the compact GPS sources and least for the LRG sources. We find H I column density to be anticorrelated with source size, as reported earlier by Pihlström et al., a trend which is consistent with the results of optical spectroscopy. The H I column density shows no significant dependence on either redshift or luminosity, which are themselves strongly correlated. These results suggest that the environments of radio sources on GPS/CSS scales are similar at different redshifts. Further, in accordance with the unification scheme, the GPS/CSS galaxies have an H I detection rate of ∼40 per cent which is higher than the detection rate (∼20 per cent) towards the GPS/CSS quasars. Also, the principal (strongest) absorption component detected towards GPS sources appears blueshifted in ∼65 per cent of the cases. This is in agreement with the growing evidence for jet-cloud interactions playing an important role in determining the ionization and kinematical properties of the ambient gas.

Radio properties of CSSs and GPSs

“…Further results from samples (Gupta et al 2006) are: i) no correlations between N H and redshift or luminosity; ii) detection rate higher (40%) in galaxies than in quasars (20%) and, among galaxies, higher in the more compact ones; iii) line velocity distribution peaked on the systemic velocity but skewed towards negative values in GPSs, indication of out-flowing gas (probably clouds accelerated by the jet, see also Sect. 5).…”

“…Pihlström et al (2003), taking into account also the upper limits, give the relation N H = 7.2 × 10 19 LS −0.43 cm −2 (LS in kpc and T spin = 100 K). The extension to LSOs by Gupta et al (2006) would be useful to improve the fit, but it has too many upper limits, mostly above the value predicted by the previous relation, hence it doesn't constrain the parameters any further. The exact dependence of N H on LS has to be taken with caution since for the smallest sources T spin could be up to ∼ 8000 K and hence N H proportionally higher, while the largest sources could have c f < 1 and again N H could be higher.…”

“…More sources are reported by Mack et al (2009). OH: Peck et al (1999), O'Dea et al (2005), Gupta et al (2006) observed, in total, a dozen objects without success. Their upper limits on OH column density are in the range 10 14 -10 15 cm −2 which is at least ten times less than in the galactic Giant Molecular Clouds and less than in other types of (few) radio galaxies.…”

Hi absorption in nearby compact radio galaxies

“…Instead of selecting by spectral index, our survey targeted compact radio galaxies selected at 20 GHz, including both steep and flat-spectrum sources, and so our source selection differed from similar surveys, such as Gupta et al (2006) and Chandola et al (2011), which targeted GPS and CSS radio sources. Of the full sample, 38 (58%) are classified as flat-spectrum sources (-0.5 < α < 0.5 between 1.4/0.843 GHz and 20 GHz; Table 2).…”

“…with detection rates of ∼20 -40% in Vermeulen et al 2003; Gupta et al 2006;Chandola et al 2011). Fur-thermore, Curran et al (2016c) shows that the Hi detection rate is proportional to the turnover frequency, which is inversely proportional to source size.…”