HI 21cm imaging of a nearby damped Lyman-α system

Abstract

We present Giant Metrewave Radio Telescope (GMRT) HI 21 cm emission images of the z=0.009 damped Lyman-α (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 × 10⁹ M_⊙, considerably smaller than the value of M* _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_dyn ~ 5 × 10⁹ M_⊙.