HI in Abell 3128

Abstract

We discuss Australia Telescope Compact Array (ATCA) HI 21 cm data for the galaxy cluster A3128. Our observations are intentionally relatively shallow, and a blind search through our data cube yields (tentative) detections of only two galaxies, of which one is probably spurious. A3128 is part of the ESO Nearby Abell Cluster Survey (ENACS) (1996); redshifts are available for 193 galaxies in the A3128 region. For 148 of these galaxies the redshifts are such that the HI emission (if any) would lie within our data cube. We use the known redshifts of these galaxies to coadd their spectra and thus improve our sensitivity to HI emission. The technique is fairly successful -the coadded spectra allow detection of an average mass content of ~9 × 10⁸M_⊙, almost an order of magnitude lower than for direct detection (by which we mean a 5σ detection after smoothing to 90" × 90"and 300 km s⁻¹ resolution) of individual objects. By dividing the total galaxy sample into subsamples we find that the gas content of late type galaxies that lie outside the X-ray emitting core of the cluster is substantially higher than that of those within the core. The fact that for disk galaxies the average gas content is higher for galaxies outside the X-ray emitting region as compared to those inside implies that these galaxies are not well mixed in the cluster potential. Even outside the X-ray emitting region the distribution of gas-rich galaxies in the cluster is not uniform, we find that gas-rich galaxies are concentrated in the east of the cluster. This is consistent with earlier analyses of the kinematics of the galaxies in A3128 which indicate the presence of subclustering. In summary we find that coadding spectra is a powerful tool for the study of HI in cluster galaxies, and suggest that this technique could be applied to substantially increase the redshift range over which such observations could be carried out.