Giant Metrewave Radio Telescope detection of two new HI 21cm absorbers at $z\approx 2$

Abstract

I report the detection of HI 21cm absorption in two high column density damped Lyman-$\alpha$ absorbers (DLAs) at $z\approx 2$ using new wide-band $250-500$ MHz receivers onboard the Giant Metrewave Radio Telescope. The integrated HI 21cm optical depths are $0.85\pm 0.16$ km/s (TXS1755+578) and $2.95\pm 0.15$ km/s (TXS1850+402). For the $z=1.9698$ DLA towards TXS1755+578, the difference in HI 21cm and CI profiles and the weakness of the radio core suggest that the HI 21cm absorption arises towards radio components in the jet, and that the optical and radio sightlines are not the same. This precludes an estimate of the DLA spin temperature. For the $z=1.9888$ DLA towards TXS1850+402, the absorber covering factor is likely to be close to unity, as the background source is extremely compact, with all the 5 GHz emission arising from a region of size $\le 1.4$ mas. This yields a DLA spin temperature of ${\mathrm{T}}_{\mathrm{s}}=(372\pm 18)\times (f/1.0)$ K, lower than typical ${\mathrm{T}}_{\mathrm{s}}$ values in high-$z$ DLAs. This low spin temperature and the relatively high metallicity of the $z=1.9888$ DLA ([Zn/H] $=(-0.68\pm 0.04)$) are consistent with the anti-correlation between metallicity and spin temperature that has been earlier found in damped Lyman-$\alpha$ systems.

5 pages, 2 figures; in press in Astrophysical Journal Letters. References updated and a typo removed, to match final ApJL version

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• A comparative study of intervening and associated HI 21-cm absorption profiles in redshifted galaxies
• GASKAP Pilot Survey Science II: ASKAP Zoom Observations of Galactic 21-cm Absorption
• Incidence of HI 21-cm absorption in strong FeII systems at $0.5<z<1.5$
• Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies
• The Expanded Giant Metrewave Radio Telescope
• A statistical measurement of the HI spin temperature in DLAs at cosmological distances
• The evolution of cold neutral gas and the star formation history
• Probing galaxy evolution through HI 21-cm emission and absorption: current status and prospects with the Square Kilometre Array
• The potential of tracing the star formation history with HI 21-cm in intervening absorption systems