uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources

Abstract

We report the detection of HI 21-cm absorption in a member of the rare and recently discovered class of compact radio sources: extremely inverted spectrum extragalactic radio sources (EISERS). The EISERS conceivably form a special subclass of the inverted spectrum radio galaxies since the spectral index of the optically thick part of the spectrum for these sources crosses the synchrotron self-absorption limit of α = +2.5 (S(ν) ∝ να). We searched for HI absorption in two EISERS using the recently upgraded Giant Metrewave Radio Telescope (uGMRT) and detected an absorption feature in one of them. The strong associated HI absorption detected against the source J1209−2032 (z = 0.4040) implies an optical depth of 0.178 ± 0.02, corresponding to an HI column density of 34.8 ± 2.9 × 1020 cm−2, for an assumed HI spin temperature of 100 K and covering factor of 1. This is among the highest known optical depth and HI column densities found for compact radio sources of peaked spectrum type and supports the free-free absorption model for the steeply inverted radio spectrum of this source. For the other source, J1549+5038 (z = 2.171), no HI absorption was detected in our observations.

The First Large Absorption Survey in H i (FLASH): I. Science goals and survey design

“…This hypothesis is supported by observational evidence that the absorption signal strength is inversely related to the UV luminosity, and that very few 21-cm absorbers are detected in AGN for (e.g. Curran et al 2013; Aditya & Kanekar 2018b; Curran et al 2019; Grasha et al 2019; Chowdhury et al 2020b; Mhaskey et al 2020, see also Aditya et al 2021).…”

FLASH pilot survey: detections of associated 21 cm H i absorption in GAMA galaxies at 0.42 < z < 1.00

“…As noted above, the detection rate here is 2.9 +9.7 −2.6 %. For high-redshift searches at 𝑧 > 1, we explored almost all the available literature (Aditya et al 2016(Aditya et al , 2017Aditya & Kanekar 2018a,b;Aditya et al 2021;Carilli et al 2007;Chowdhury et al 2020;Curran et al 2006Curran et al , 2008Curran et al , 2011Curran et al , 2013bCurran et al ,a, 2016Curran et al , 2017de Waard et al 1985;Dutta et al 2020;Grasha et al 2019;Gupta et al 2006Gupta et al , 2021Ishwara-Chandra et al 2003;Kanekar et al 2009;Mhaskey et al 2020;Röttgering et al 1999;Uson et al 1991;Yan et al 2016) and found three detections with 𝜏 pk > 0.1 out of 221 sources for which 3𝜎 detections could have been made at this level. This gives a detection rate DR(𝜏 pk > 0.1) of 1.4 +2.2 −1.0 %.…”

Resolved simulations of jet–ISM interaction: Implications for gas dynamics and star formation

“…Which of the above processes dominate is still a question of debate, with both giving equally acceptable interpretation of observations (O'Dea & Saikia 2021). However, recently, several radio galaxies with steeply inverted optically thick spectral indices >2.5 have been discovered (Callingham et al 2015; Gopal‐Krishna et al 2014; Mhaskey et al 2019, 2020). This supports a free–free process as the absorbing mechanism and implies the presence of dense gas surrounding the jets.…”