Types of Galaxies found at 21-cm

Using the 21-cm line of HI to search for galaxies is a relatively new technique, and it is important to understand the types of objects which will be discovered by large HI blind searches, such as those to be conducted with the multibeam system. Will a 21-cm-selected sample consist of the same types of galaxies which are found in optical- or IR-selected samples, or will they differ? This is especially important to know when comparing studies at different wavelengths in the ZOA, and also is of general astrophysical interest.

Because a large fraction of this HI-selected sample lies in optically opaque regions, we are forced to consider only the HI properties of the sample versus HI properties of optical- and IR-selected comparison samples. For this analysis, the distribution of HI linewidths is considered, a sensitive indicator of galaxy morphological type. The possible bias against broad-lined galaxies has been considered, and does not appear to be important. A fuller discussion of this analysis has appeared recently (Henning 1995). In summary, the linewidth distribution of the HI-selected sample is statistically indistinguishable from that of the optically-selected sample of dwarf and other low surface brightness galaxies studied by Schneider et al. (1990). So while the HI search did not discover a new class of previously unknown objects, it does turn up galaxies which are difficult to study optically, and are under-represented in optical catalogs and redshift surveys.

A related point for consideration when tracing large scale structure into the ZOA is that galaxies discovered in HI are drawn from a different population than those found by IRAS, which consist of broader linewidth, relatively massive spirals. These two methods of searching at low latitudes are complementary, and it is important to realize that they use different tracers to map structures.

© Copyright Astronomical Society of Australia 1997