GMRT Observations of Low z Damped Lyman-alpha Absorbers

Details GMRT Observations of Low z Damped Lyman-alpha Absorbers GMRT Observations of Low z Damped Lyman-alpha Absorbers

1998-11-04

arxiv.org/abs/astro-ph/9811068v1

Astronomy and Astrophysics

Astrophysics

4 pages, 2 figures, accepted for publication in MNRAS (letters)

Scientific paper

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted HI 21cm absorption in two low redshift (z=0.2212, z=0.0912) damped Lyman-alpha systems seen towards the gigahertz peaked source OI 363 (z_em = 0.630). The object at z=0.0912 is the lowest redshift damped Lyman-alpha system known to date. Ground based imaging (Rao & Turnshek, 1998) shows that at neither redshift is there a large spiral galaxy at low impact parameter to the line of sight to OI 363, in contradiction with the suggestion that these systems are large proto-disks. Since OI 363 is a highly compact, core dominated source, the covering factor of the HI gas is likely to be unity. Nonetheless, the spin temperatures derived from the 21cm optical depth (and using the N_HI measured from HST spectra, Rao & Turnshek, 1998) are high, viz. 1120 +/- 200 K and 825 +/- 110 K for the high and low redshift systems respectively. These values are considerably higher than typical values (100 - 200 K) measured in our Galaxy and Andromeda and are, in fact, similar to those obtained in high redshift damped Lyman-alpha systems. Our observations hence suggest that evolutionary effects may not be crucial in understanding the difference in derived spin temperature values between local spiral disks and high redshift damped Lyman-alpha systems.

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