Annu. Rev. Astron. Astrophys. 1998. 36: 189-231 Copyright © 1998 by Annual Reviews. All rights reserved

5.2. Evolution of Circumnuclear Star Formation

The SFRs in the circumnuclear regions are largely decoupled from those of disks and show no strong relationship to either the gas contents or the bulge/disk properties of the parent galaxies. Instead, the nuclear SFRs are closely associated with dynamical influences such as gas transport by bars or external gravitational perturbations, which stimulate the flow of gas into the circumnuclear regions.

The temporal properties of the star formation in the nuclear regions show a wide variation. Approximately 80-90% of spiral nuclei in optically selected samples exhibit modest levels of Balmer emission, with an average H emission-line equivalent width of 20-30 Å (Stauffer 1982, Kennicutt et al 1989b, Ho et al 1997a, b). This is comparable to the average value in the disks of late-type spiral galaxies and is in the range expected for constant star formation over the age of the disk (Kennicutt 1983a, Kennicutt et al 1994). Hence, most nuclei show SFRs consistent with steady-state or declining star formation, though it is likely that some of these nuclei are observed in a quiescent stage between major outbursts.

Starbursts are clearly the dominant mode of star formation in IR-selected samples of nuclei. The typical gas consumption times are in the range of 10⁸-10⁹ years (Figure 7), so the high SFRs can only be sustained for a small percentage of the Hubble time. These time scales can be extended if a steady supply is introduced from the outside, for example by a strong dissipative bar. The most luminous nuclear starbursts (L_bol 10¹² L) are singular events. Maintaining such luminosities for even 10⁸ years requires a total gas mass on the order of 10¹⁰-10¹¹ M, equivalent to the total gas supply in most galaxies. Violent interactions and mergers are the only events capable of triggering such a catastrophic mass transfer.