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

3.2. Starburst Galaxies

3.2.1. The Dark Side of Star Formation

The beautiful HST image of the famous Antennae system of colliding galaxies (NGC 4038 / 39; Figure 6b, see color insert, from Whitmore et al 1999) summarizes one of the paradigms about active star formation on galactic scales: Spectacular fireworks of star formation with hundreds of young star clusters are triggered by strong gravitational disturbances. The interaction/collision and eventual merging of two gas-rich galaxies is the most effective perturbation of this kind. Bar-driven inflow is another such perturbation. ISOCAM studies of several samples of interacting, barred, and collisional ring galaxies confirm this paradigm (Appleton et al 1999, Charmandaris et al 1999, Roussel et al 1999b, Wozniak et al 1999). The optical/UV data in the Antennae tell a fascinating story with evidence for a large age spread in star cluster populations. Several phases of active star formation have occurred since the first interaction ~ 500 million years ago. However, the UV/optical data do not reveal the whole story. Superposed on the HST image in Figure 6b are the contours of 15 µm mid-IR emission (Mirabel et al 1998). The 15 µm map is an (approximate) tracer of the entire infrared emission that contains ~ 80% of the bolometric luminosity of the system. About half of the entire luminosity emerges from the optically "dark" interaction region between the two galaxy nuclei. This region also contains most of the molecular gas (Stanford et al 1990, Lo et al 2000). A compact source at the southern edge of the interaction region accounts for ~ 15% of the entire luminosity, yet on the HST image there is only a faint and very red compact cluster at this position. This knot is the site of the most recent star formation activity in the Antennae (see Section 3.2.3; Vigroux et al 1996, Kunze et al 1996).

Another example is the luminous merger Arp 299 (NGC 3690 / IC694, Gehrz et al 1983, Satyapal et al 1999). In this source, as in the Antennae, more than 50% of the star-forming activity orginates outside the (bright) nuclei, much of it in a highly obscured, compact knot 500 pc southeast of the nucleus of NGC 3690 (Gallais et al 1999). Finally, Thuan et al (1999) have observed with ISOCAM the blue compact dwarf galaxy SBS0335-052, which has the second lowest metallicity known in the local Universe (Z = Z / 41). They find that this starburst system is remarkably bright in the mid-IR (L_{12 µm} / L_B = 2). The mid-IR spectrum can be fitted by a ~ 260 K blackbody with strong 9.7/18 µm silicate absorption features and no UIB emission. Therefore, a significant fraction of the starburst activity of SBS0335-052 must be dust enshrouded - potentially a very important clue for the understanding of low metallicity starbursts in the Early Universe (Section 4).