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

3.4.4. Properties and Evolution of Star Formation in ULIRGs

In the framework of starburst models with a Salpeter IMF and upper mass cutoffs of 50 to 100 M (Krabbe et al 1994, Leitherer & Heckman 1995), the ULIRG data are fit with burst ages/durations of t ~ t ~ 5 to 100 × 10⁶ years (Genzel et al 1998). In the case of NGC 6240, Tecza et al (2000) have directly determined the age of the most recent star formation activity [t = (2 ± 0.5) × 10⁷ years] from the fact that the K-band light in both nuclei appears to be dominated by red supergiants. For that age and the measured (low) Br equivalent width, the duration of the burst t has to be about 5 ± 2 × 10⁶ years (Tecza et al 2000). Rigopoulou et al (1999a) found that there is no correlation between the gas content/mass of a ULIRG and the merger phase, as measured by the separation of their nuclei, in contrast to infrared galaxies of somewhat lower luminosities (LIRGs; Gao and Solomon 1999). Even very compact ULIRGs such as Arp 220 and Mrk 231 still have a lot of molecular hydrogen.

Burst ages in ULIRGs thus are similar to those in other starbursts (Section 3.2.2), are comparable to the dynamical time scale (between peri-approaches), and are much smaller than the gas exhaustion time (Section 3.2.2) and overall merger age. There are likely several bursts during the merger evolution (Section 3.2.1). A burst may be triggered as the result of the gravitational compression of the gas during the encounters of the nuclei (Mihos & Hernquist 1996). It is terminated within an O-star lifetime by the negative feedback of supernovae and superwinds. The models of Mihos & Hernquist suggest that by far the most powerful bursts occur when the nuclei finally merge and the circumnuclear gas is compressed the most. The data indeed suggest that ULIRGs switch on only in the last ~ 20% of the merger history (Mihos 2000). However, the data do not indicate that the most powerful starbursts in ULIRGs occur in the last phases with very small separations. Far-IR luminosities of ULIRGs do not change significantly with nuclear separation from separations of a few hundred parsecs to a few tens of kpc (Rigopoulou et al 1999a).