A new 21-cm absorber identified with an L ~ L* galaxy

Abstract

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted 21-cm absorption from the z=0.437 metal line absorption system towards PKS 1243-072. HI absorption is clearly detected; the absorption profile has a velocity spread of ~20 km s⁻¹. Detection of 21-cm absorption indicates that the absorber has an HI column density large enough to be classified as a damped Lyman-α system. Follow up ground based optical imaging and spectroscopy allow us to identify the absorber with an L ~ L* galaxy at an impact parameter of ~9.8 kpc from the line of sight to the QSO. The absorbing galaxy is unusual in that it has bright emission lines. On the basis of the optical spectrum we are unable to uniquely classify the galaxy since its emission line ratios lie in the transition region between starburst and Seyfert II type spectra.