Robert Braun

HI in Abell 3128

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) (Katgert et al. 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 of9 10 8 M ,a lmost an order of magnitude lower than for direct detection (by which we mean a 5 detection after smoothing to 90 00 90 00 and 300 km s 1 resolution) of individual objects. By dividing the total galaxy sample into subsamples we nd 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 nd 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 nd 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.

A Radio Search for High Redshift HI Absorption

The rare damped Ly α systems seen in absorption against the UV continuum of high redshift QSOs nonetheless dominate the observed neutral gas at high redshift. HI 21cm absorption studies of these systems are challenging and only a handful of have been detected hitherto (Carilli et al., these proceedings). HI 21cm observations are however of great value because they potentially yield physical information (velocity dispersion, spin temperature, size), that cannot be obtained by other means.

Morphological Characteristics of Compact High-Velocity Clouds Revealed by High-Resolution WSRT Imaging

A class of compact, isolated high-velocity clouds which plausibly represents a homogeneous subsample of the HVC phenomenon in a single physical state was objectively identified by Braun and Burton (1999). Six examples of the CHVCs, unresolved in single-dish data, have been imaged with the Westerbork Synthesis Radio Telescope. The high-resolution imaging reveals the morphology of these objects, including a core/halo distribution of fluxes, signatures of rotation indicating dark matter, and narrow linewidths constraining the kinetic temperature of several opaque cores. In these regards, as well as in their kinematic and spatial deployment on the sky, the CHVC objects are evidently a dynamically cold ensemble of dark-matter-dominated HI clouds accreting onto the Local Group in a continuing process of galactic evolution.