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

We present new Goddard High Resolution Spectrograph observations of Lyα absorption lines associated with a group of galaxies toward the QSO 1545 + 2101. We have identified eight distinct Lyα absorption features in the spectrum of QSO 1545 + 2101 at a mean redshift of 〈z〉 = 0.2648 with a velocity dispersion of 163 km s^-1. A group of galaxies is detected in the vicinity of this QSO at a mean redshift of 〈z〉 = 0.2645 and velocity dispersion of 239 km s^-1. The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse intragroup medium. Our analysis suggests that the Lyα absorption lines are associated with individual galaxies in the group, although a one-to-one relationship between absorbers and galaxies is difficult to establish in such a dense environment.