Australia Telescope National Facility

HI in Early-Type Galaxies
Tom Oosterloo, Raffaella Morganti, Elaine Sadler, PASA, 16 (1), in press.

Next Section: References
Title/Abstract Page: HI in Early-Type Galaxies
Previous Section: HI Morphology and Kinematics

Contents Page: Volume 16, Number 1

Connection with other properties

The range of HI properties observed in early-type galaxies is quite large, but it appears that there are a few systematic trends in the data, and in particular that some of the HI properties may be connected to properties observed at other wavebands.

Many low-luminosity early-type galaxies that have HI, have this HI in a regularly rotating disk. In the optical, these galaxies also show a disky morphology and are rotationally supported. One possibility is that the HI disk observed is the normal gas counterpart of the stellar disk structure in these galaxies. The central surface densities of the HI are high enough for star formation to occur, and indeed star formation is observed in the centres of many of these galaxies, and the optical spectrum of the emission lines is that of HII regions. The higher densities of the HI could also be related to the steeper cores that are observed in low-luminosity galaxies, although also other mechanisms could be responsible for that. It appears that the HI properties of low-luminosity early-type galaxies fit in with other properties of these galaxies, indicating that the ISM in these galaxies has played an important role in determining the structure of these galaxies.

**Figure 6:** Radial HI surface density profiles of the low-luminosity elliptical ESO 118-G34 *(left)* and the luminous E4 galaxy NGC 807 *(right)*
$\begin{figure} \centerline{\psfig{file=fig6a.eps,height=9cm,width=7.cm,angle=-90} \hss \psfig{file=fig6b.eps,height=9cm,width=7.cm,angle=-90}}\end{figure}$

The regular HI disks/rings observed in the more luminous galaxies could be similar in origin and character to the ones observed in the smaller galaxies (e.g. Morganti et al. 1998b), except that some mechanism must be responsible for keeping the central surface density of the HI low. A clue to this mechanism could be that the centres of these regular structures are often filled up with a disk of ionized gas that shows similar kinematics. The conditions appear to be such that high HI surface densities cannot build up because the HI in the centre gets ionized. This could be connected to the fact that more luminous early-type galaxies often have a halo of hot has, that could interact with the HI and ionize it (e.g. Goudfrooij 1998). This could also explain the different excitation of the optical gas that is observed.

In a few low-luminosity galaxies, there is still evidence that the HI has accreted recently, a process that appears to occur more often in more luminous galaxies. The different characteristics of the HI in these galaxies suggest that the accretion of the HI occurs in a different way in low-luminosity galaxies compared to the more luminous ones. In low-luminosity galaxies, acccretion appears to result in a more regular HI structure. This could also be due to interactions with a halo of hot gas playing a role in luminous galaxies. If HI falls into a more luminous galaxy, the HI could get partially ionized and may not have time to settle in a disk-like structure. In NGC 4696 such an interaction could be occurring (Sparks et al. 1989; de Jong et al. 1990), although this galaxy is in a cluster and it may not be representative for the galaxies we have studied.

Another factor affecting the way gas is accreted could be the environment. Several of the more luminous galaxies we studied are in small groups of galaxies, while the low-luminosity galaxies are more isolated. Interactions and accretions are of course more common in small groups and less relaxed HI structures should be more common. Also the luminous galaxies with regular HI structures tend to be more isolated, consistent with the idea that environment plays an important role in the evolution of HI in early-type galaxies.

Acknowledgements. The optical images shown in this paper are taken from the Digital Sky Survey. These image are based on photographic data obtained using The UK Schmidt Telescope. The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright (c) the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.

Next Section: References
Title/Abstract Page: HI in Early-Type Galaxies
Previous Section: HI Morphology and Kinematics

Contents Page: Volume 16, Number 1

ASKAP

Public