Properties at other wavelengths

There are several properties of early-type galaxies that systematically vary with luminosity:

Stellar rotation. Some early-type galaxies are `pressure supported', while in other galaxies the rotation of the stellar component is important for the dynamics of the system. In general, in luminous galaxies the random motions dominate while in lower luminosity systems the stellar rotation becomes more important (e.g., Davies et al. 1982).

Isophotal shape. In many early-type galaxies, the isophotes are not perfect ellipses. In lower luminosity galaxies, the isophotes are more often disky, while in luminous systems they tend to be more often box shaped.

Core properties. Imaging studies performed with HST have shown that the central density distributions vary systematically with luminosity (e.g., Lauer 1997). Low-luminosity systems usually have steeper cores that more luminous systems

Excitation of the ionized gas. Many early-type galaxies have optical emission lines in their spectrum. The character of these lines changes systematically with luminosity. In low-luminosity systems, the spectrum is usually that of HII regions. In luminous galaxies it corresponds to a liner spectrum (Sadler 1987). This indicates that the ionization mechanism is different in these two types of galaxies.

Star formation history. The star formation history appears to change systematically with luminosity. For example, the relative abundance of Mg with respect to Fe correlates with velocity dispersion. Luminous galaxies typically have [Mg/Fe] 0.4, while fainter galaxies have values around 0. This indicates that the enrichment history of the ISM changes systematically with luminosity. Lower-luminosity galaxies also show a larger spread in the Mg- relation (e.g., Bender 1996), again pointing to a different star formation history. Many low-luminosity ellipticals in fact display star formation in the central parts of the galaxies. It appears that disky galaxies have stronger H indices, indicating that some star formation occurred recently (de Jong & Davies 1997).

X-ray emission. The amount of X-ray emission correlates strongly with optical luminosity. In large ellipticals, part of the X-ray emission originates from a halo of hot gas, while in smaller ellipticals the X-ray is due only to X-ray binaries (e.g., Canizares et al. 1987).

Many of these differences between different galaxies can be explained by different amounts of gas present in the formation/evolution, and in many models for galaxy formation the gas supply is a key factor (e.g., Kauffmann 1996). For example, the differences between boxy and disky galaxies, the importance of rotation vs. anisotropic galaxies, and the different central density distributions are possible a consequence of the relative importance of gas. Obviously, since stars form from gas, the different star formation histories must be related to different gas contents during the evolution.

Considering the relation between gas and these different properties, it is worthwhile to investigate the systematics of the HI properties of early-type galaxies.

© Copyright Astronomical Society of Australia 1997