A Giant Metrewave Radio Telescope search for associated H i 21 cm absorption in high-redshift flat-spectrum sources

Abstract

We report results from a Giant Metrewave Radio Telescope search for "associated" redshifted HI 21 cm absorption from 24 active galactic nuclei (AGNs), at 1.1 < z < 3.6, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. 22 out of 23 sources with usable data showed no evidence of absorption, with typical 3σ optical depth detection limits of ≈ 0.01 at a velocity resolution of ≈ 30 km s −1 . A single tentative absorption detection was obtained at z ≈ 3.530 towards TXS 0604+728. If confirmed, this would be the highest redshift at which HI 21 cm absorption has ever been detected.Including 29 CJF sources with searches for redshifted HI 21 cm absorption in the literature, mostly at z < 1, we construct a sample of 52 uniformly-selected flat-spectrum sources. A Peto-Prentice two-sample test for censored data finds (at ≈ 3σ significance) that the strength of HI 21 cm absorption is weaker in the high-z sample than in the low-z sample; this is the first statistically significant evidence for redshift evolution in the strength of HI 21 cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21 cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at ≈ 3.4σ significance. The fact that the higher-luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI 21 cm opacities in high-redshift, high-luminosity active galactic nuclei.

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.…”

“…A decrease in the detection rate with redshift was first shown by Curran et al (2008), which was interpreted as the high redshifts selecting UV luminosities sufficient to excite the gas to below the detection limit. 10 Be- cause of the Malmquist bias, it can be difficult to ascertain whether the decreasing detection rate is caused by an evolutionary effect at high redshift (Curran & Whiting 2010;Aditya et al 2016), alluminosity in their comparison with the highest redshift (z = 3.40) H I 21-cm detection (Uson et al 1991). This, B2 0902+34, has L UV ≈ 3 × 10 22 W Hz −1 , cf.…”

“…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).…”

Hi absorption in nearby compact radio galaxies

“…For instance, there is mounting evidence in the literature that the detection rate of Hi absorption in compact 'FR-0' radio galaxies is higher than that of their older, extended FR-I/II counterparts (e.g. Pihlström et al 2003;Curran et al 2013a;Aditya et al 2016). Curran et al proposed that this is due to a higher covering factor for compact sources, although different levels of sensitivity reached between surveys is another consideration.…”

“…The detection rate reaches zero at ionising photon rates of QHi ∼ 3×10 56 sec −1 (LUV ∼ 10 23 W Hz −1 at λ = 912Å), at which point all of the neutral gas in the host galaxy is believed to be ionised (Curran & Whiting 2012). This critical UV luminosity limit has been observed several times since, through the non-detection of Hi above this value (Curran et al 2011a(Curran et al , 2013bCurran et al 2016a;Grasha & Darling 2011;Allison et al 2012a;Aditya et al 2016).…”