What will the HI multibeam survey see?

The Parkes HI multibeam survey (Staveley-Smith 1996) will cover the entire southern sky, and thus will include many early-type galaxies.

The number of new HI detections for `large' elliptical galaxies is difficult to predict, because the HI content of these galaxies is largely decoupled from any other observable property. The multibeam survey volume (out to 100 Mpc) will contain at least 5,000 giant elliptical galaxies. A spectacular HI-rich elliptical like NGC 5266 (Morganti et al. 1996), with more than 10 M of neutral gas, should be detectable throughout this volume. NGC 5266 has an apparent magnitude of B = 12.1, making it one of the 100 brightest early-type galaxies in the southern sky. If we therefore assume that 1/100 ellipticals in a randomly-chosen sample will contain as much HI as NGC 5266, we would expect the multibeam survey to detect up to 50 HI-rich giant elliptical galaxies. These would be a valuable resource for follow-up dynamical studies.

The number of HI detections for `small' elliptical galaxies can be estimated if we make the following assumptions:

• The survey covers the region south of declination 0, i.e. half the sky.
• Within this region, the space density of `small' early-type galaxies (M to -18) is 9.5 Mpc based on Figure 1.
• We can make rough estimates (see col. 4 of table 1) of the likely detection rates for various HI mass limits, based on the few galaxies for which HI data are available.

This calculation is probably somewhat conservative, since it neglects the smallest galaxies (M fainter than -16) whose optical luminosity function is uncertain. It nevertheless suggests that the multibeam survey should detect at least 300 HI-rich, low-luminosity early-type galaxies. Since most of these galaxies will have apparent magnitudes brighter than B, their optical identification, classification and follow-up optical spectroscopy should be straightforward.

 
Table 1: Estimated multibeam detections for small early-type galaxies (M -16.0 to -18.0), assuming an HI velocity width = 100 km s, 600 s integration time, 3 detection limit and H = 100 km s Mpc.

© Copyright Astronomical Society of Australia 1997