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