Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

2.3. Early-Type Galaxies

IRAS observations have shown that a significant fraction of early-type galaxies contains detectable infrared emission (e.g. Knapp et al 1989, 1992). With ISO, researchers have improved their understanding of the origin of this emission. One key question is whether the mid-IR emission originates mainly in the photospheres and shells of cool asymptotic giant branch (AGB) stars or in the interstellar medium, or whether an active galactic nucleus plays a role. Several studies with ISOCAM and ISOPHOT in more than 40 early-type galaxies have addressed this issue (Madden et al 1999, Vigroux et al 1999, Knapp et al 1996, Fich et al 1999, Malhotra et al 1999b). Exploiting the imaging capability of ISOCAM at different wavelengths, these studies show that all three components play a role. Madden et al (1999) found that the 4.5 / 6.7 µm and 6.7 / 15 µm colors of about 60% of their sample galaxies are consistent with the notion of a largely stellar origin for the mid-IR emission. The other galaxies are dominated by UIB emission (17%) or an AGN (22%). Half of the sample exhibits a mid-IR excess resulting from hot interstellar dust at some level. The interstellar dust emission is associated with the optical dust lane in NGC 5266 (Madden et al 1999) and in Cen A (Mirabel et al 1999).

The detection of UIB features in early-type galaxies is of general significance because the radiation field from the old stellar population is too soft to excite the UIB emission with UV photons (Section 2.1.1). Either lower-energy photons in the visible band are sufficient to excite the mid-IR features (Boulade et al 1996, Uchida et al 1998, Vigroux et al 1999, Madden et al 1999), or there must be a young stellar component producing a stronger UV radiation field. Evidence for the latter explanation comes from the detection of 158 µm [CII] (and 63 µm [OI]) far-IR line emission in NGC 1155, NGC 1052, and NGC 6958 (Malhotra et al 1999b), and in Cen A (Madden et al 1995, Unger et al 2000). Excitation of [CII] line emission (Sections 3.1, 3.2.3) definitely requires 11.3 eV photons, and the old stellar population in these galaxies cannot account for the required far-UV radiation field (Malhotra et al 1999b). A fair amount of the [CII] emission may come from a diffuse neutral HI medium. However, an additional obscured source of UV photons is required and is probably associated with massive star formation in dense interstellar clouds. The [OI] / [CII] ratio in NGC 1155 indicates that the radiation field is about 100 times greater than that in the solar neighborhood, and the gas density is ~ 100 cm^-3, which is characteristic of moderately dense PDRs associated with molecular clouds (Section 3.2.3). Most of the [CII] emission in Cen A originates in the central dust disk, where ISOCAM observations provide strong evidence for ongoing star-forming activity (Mirabel et al 1999).