Annu. Rev. Astron. Astrophys. 2004. 42: 603-683 Copyright © 2004 by Annual Reviews. All rights reserved

6.2. Merger-Induced Versus Secular Star Formation in Bulges

Finally, we make a preliminary comparison of the relative importance of secular and merger-induced star formation in the present universe. As shown above, it is plausible that most of the stars in Sc - Sm (pseudo)bulges and a significant fraction of the stars in Sb - Sbc (pseudo)bulges formed as a result of secular evolution. Given the relative numbers of early- and late-type galaxies, classical bulges and pseudobulges are not very different in number. However, the masses of (mostly) classical bulges in early-type galaxies are at least 1 - 2 orders of magnitude larger than the masses of pseudobulges in late-type spirals. Integrated over the history of the universe, star formation caused by secular processes has contributed at most a few percent of bulge stars. The vast majority of bulge stars are believed to have formed in collapse and merger events.

However, most of these events occurred in the distant past. Observations show that merger rates increase dramatically with increasing cosmic lookback time (Patton et al. 2002; Conselice et al. 2003). The fractional contribution of galaxy interactions and mergers to the total present-day SFR in the universe has been estimated by numerous workers, starting with a classic study by Larson & Tinsley (1978). Kennicutt et al. (1987) estimate that 6 % ± 3 % of current star formation is induced by galaxy-galaxy interactions. This contains two, partly compensating uncertainties. It underestimates dust-extincted star formation in bulges, but it overestimates bulge star formation because it includes a contribution from distant tidal interactions as well as mergers. How does this value compare to the contribution from secular evolution? In Section 5.3, we noted that ~ 10 % of intermediate-type spirals contain circumnuclear disks or rings and that their star formation accounts for 10 %- 80 % of the current SFR in those galaxies. These same intermediate-type spirals dominate the current total cosmic SFR (Brinchmann et al. 2003). Combining these numbers suggests that a few percent of present-day star formation is attributable to secular processes. The hint is that galaxy mergers and secular evolution produce comparable star formation in the present universe. Both contributions are small compared to the dominant source of star formation at z = 0, i.e., namely the quiescent star formation in the disks of spiral and irregular galaxies. Nevertheless, as argued in the Introduction, it appears that we live approximately at the epoch of transition when secular processes are overtaking mergers as the primary mechanism that forms stars in the central parts of galaxies.

We emphasize that all of the estimates in Sections 6.1 and 6.2 are very uncertain. We include them primarily to stimulate further work.