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

TL;DR

This study reports the detection of HI 21-cm absorption in an extremely inverted spectrum radio source using uGMRT, revealing high HI column density and supporting free-free absorption as the cause of its spectral inversion.

Contribution

First detection of HI 21-cm absorption in an EISERS source, providing insights into its absorption properties and spectral origin.

Findings

Detected HI absorption in one EISERS source with high optical depth

HI column density measured at approximately 3.48×10^{21} cm^{-2}

No HI absorption detected in the second source

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)'. EISERS conceivably form a special sub-class 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 $\alpha=+2.5$ (S($\nu$) $\propto$ $\nu^{\alpha}$). We have 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$=0.4040) implies an optical depth of 0.178$\pm$0.02 corresponding to an HI column density of 34.8$\pm$2.9 $\times$10$^{20}$ 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 GPC/CSS 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.