The gaseous extent of galaxies and the origin of Ly alpha absorption systems. IV. Ly alpha absorbers arising in a galaxy group

We present new Goddard High Resolution Spectrograph observations of Ly alph a absorption lines associated with a group of galaxies toward the QSO 15452101. We have identified eight distinct Ly alpha absorption features in the spectrum of QSO 1545+2101 at a mean redshift of [z] = 0.2648 with a veloci ty dispersion of 163 km s(-1). A group of galaxies is detected in the vicin ity of this QSO at a mean redshift of [z]= 0.2645 and velocity dispersion o f 239 km s(-1). The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse int ragroup medium. Our analysis suggests that the Ly alpha absorption lines ar e associated with individual galaxies in the group, although a one-to-one r elationship between absorbers and galaxies is difficult to establish in suc h a dense environment.