EDP Sciences
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Volume 388, Number 2, June III 2002
Page(s) 383 - 388
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20020513

A&A 388, 383-388 (2002)
DOI: 10.1051/0004-6361:20020513

HI 21 cm imaging of a nearby damped Lyman- $\alpha$ system

J. N. Chengalur and N. Kanekar

National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India

(Received 6 November 2001 / Accepted 3 April 2002)

We present Giant Metrewave Radio Telescope (GMRT) HI 21 cm emission images of the z=0.009 damped Lyman- $\alpha$ (DLA) absorber towards the QSO HS 1543+5921. The DLA has been earlier identified as a low surface brightness galaxy SBS 1543+593 (Reimers & Hagen 1998; Bowen et al. 2001a) with small (~ 400 pc) impact parameter to the QSO line of sight. The extremely low redshift of the absorber allows us to make, for the first time, spatially resolved images of the 21 cm emission; besides the HI mass, this also enables us to determine the velocity field of the galaxy and, hence, to estimate its dynamical mass. We obtain a total HI mass of ~ $1.4 \times 10^9$  $M_\odot$, considerably smaller than the value of  $M_{\rm HI}^*$ determined from blind 21 cm emission surveys. This continues the trend of low HI mass in all low redshift DLAs for which HI emission observations have been attempted. We also find that the QSO lies behind a region of low local HI column density in the foreground galaxy. This is interesting in view of suggestions that DLA samples are biased against high HI column density systems. Finally, the dynamical mass of the galaxy is found to be  $M_{\rm dyn} \sim 5 \times 10^9$  $M_\odot$.

Key words: galaxies: evolution -- galaxies: formation -- galaxies: ISM -- cosmology: observations -- radio lines: galaxies

Offprint request: J. N. Chengalur, chengalu@ncra.tifr.res.in

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