Volume 643, November 2020
|Number of page(s)||8|
|Published online||20 November 2020|
uGMRT HI 21-cm absorption observations of two extremely inverted spectrum sources⋆
Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
2 Department of Physics, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India
3 Inter University Centre for Astronomy and Astrophysics (IUCAA), Pune 411007, India
4 UM-DAE Centre for Excellence in Basic Sciences (CEBS), Univ. of Mumbai Campus, Vidya Nagari, Mumbai 400098, India
5 ARIES, Manora Peak, Nainital, 263002 Uttarakhand, India
Accepted: 26 September 2020
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.
Key words: radiation mechanisms: non-thermal / galaxies: active / galaxies: ISM / radio lines: ISM
The reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A174
© ESO 2020
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