HI content

Before discussing the HI properties of early-type galaxies, it is important to define which type of galaxies are considered and in which environment the galaxies are. Table 1 lists the detection rates for different kinds of early-type galaxies. A few things are evident from this table. First, only a small fraction of `pure' elliptical galaxies have detectable amounts of HI. But as soon as the optical morphology shows some peculiarity, the probability of detecting HI increases dramatically. This result has often been interpreted to mean that the origin of the HI in elliptical galaxies is external (e.g. Knapp et al. 1985). It implies that if we investigate the characteristics of the HI in these galaxies (morphology and kinematics), one is considering a subset of the whole population of early-type galaxies, namely those for which it is likely that some interaction/accretion in the recent past has occurred, and it is important to keep this in mind. Nevertheless it is important not to restrict samples to `pure' ellipticals with no optical peculiarities, since the HI-rich galaxies may represent an important phase in the evolution of many early-type galaxies.

The table does however suggest that there may be a second origin for the HI in early-type galaxies. The detection rate also depends strongly on how much stellar disk is present in a galaxy. This could imply that the presence of HI in early-type galaxies is not always due to a recent accretion. The fact that the HI content is related to the fundamental structure of a galaxy could suggest that some of the disky galaxies may have a long-lived ISM. It appears that many early-type galaxies, especially those in the field, indeed often have an ISM with similar characteristics as the ISM in spirals, the main difference being that early-type galaxies have less of it (see e.g. Knapp 1998).

Figure 1: HI detection rate of E and E/S0 galaxies as function of absolute magnitude. The shaded histogram gives the distribution of the detections, the unshaded histogram of the non-detections. Data from Bregman et al. 1992

It is often stated that low-luminosity early-type galaxies are more likely to have HI. This is usually based on a study done by Lake and Schommer (1984) who observed a small sample of low-luminosity early-type galaxies and found that the detection rate of their sample was significantly higher than that of more luminous early-type galaxies as it was known at the time. However, when using larger samples of HI data on early-type galaxies that are available now, the situation appears to be somewhat different from that suggested by Lake and Schommer.

Figure 2: left: Contours of the total HI image of NGC 802 as obtained with ATCA using uniform weighting. right: HI velocity field of NGC 802 from the natural weighted ATCA data. Contour values (heliocentric, km ) are indicated. The optical image in both figures is taken from the Digital Sky Survey

In figure 1 we show the detection rates of early-type galaxies as function of luminosity as it can be derived from the compilation of data of Bregman et al. (1992). The figure shows that galaxies brighter than absolute magnitude -22 appear to be poorer in HI than galaxies fainter than this limit. However, for galaxies in the magnitude range -16 to -21.5, the detection rate appears to be reasonably flat. There is no strong evidence that low-luminosity galaxies (absolute magnitude between -16 and -19) are richer in HI than galaxies in the range -19 to -22. A similar conclusion was obtained by Knapp et al. (1985). It appears that a more correct statement about HI content would be that the most luminous galaxies are poor in HI. Galaxies fainter than M_B = -16 appear to have more often HI, although the number of galaxies for which data are available is small. It is however, somewhat difficult to derive strong conclusions from the compilation of Bregman et al., because it consists of a mix of field and cluster galaxies, and differential environmental effects are possibly important.

© Copyright Astronomical Society of Australia 1997