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

6. COSMOLOGICAL IMPLICATIONS OF SECULAR EVOLUTION

6.1. Evolution Along the Hubble Sequence

The qualitative arguments in Sections 2 - 4 and the star formation measures in Section 5 imply that secular evolution increases bulge-to-total luminosity ratios B / T. How much evolution along the Hubble sequence (e.g., Pfenniger 1996b) is plausible?

This question is too important to be postponed, but we warn readers that the results of this section are very uncertain. To address the question, we compare predicted B / T ratios with the distribution of values observed by Simien & de Vaucouleurs (1986). They decomposed the B-band surface brightness profiles of 98 galaxies to measure B / T as a function of RC2 type. They added up all of the central light in excess of the inward extrapolation of exponentials fitted to the outer disks, so B / T measures the sum of bulge and pseudobulge light. They found that B / T is typically 2 % in Sd galaxies, 9 % in Sc galaxies, 16 % in Sbc galaxies, 24 % in Sb galaxies, and 41 % in Sa galaxies. The scatter around these values is large. In Sections 5.1 and 5.2, we estimated that circumnuclear star-forming rings grow pseudobulges with masses of ~ 10⁹ M. The plausible range of these masses is also large, from ~ 10⁷ to 10¹⁰ M. The total stellar masses of these galaxies are of order (1 - 5) × 10¹⁰ M. Therefore secular evolution can reasonably be expected to have produced pseudobulges with masses ranging from 0 % to > 10 % of the total stellar masses of the systems. This is comparable to the B / T value in Sc galaxies, consistent with our conclusion that Scs contain pseudobulges. Evolution of one Hubble stage - e.g., from Sd to Sc - is plausible at the late end of the Hubble sequence. Evolution from Sc to Sbc is also plausible.

However, it is less easy for secular processes to form the more massive bulges of S0 - Sb galaxies. The B / T ratio in these galaxies is large, and the galaxies themselves tend to be very massive. Total bulge masses are at least 10¹⁰ - 10¹¹ M. The evidence from stellar populations (Section 8.1) is that the stars in these bulges formed quickly and long ago. We conclude that the stars in these bulges formed mostly during hierarchical clustering. That is, S0 - Sb galaxies mainly contain classical bulges. Secular processes can contribute modestly to the growth of classical bulges, but evolution by as much as half of a Hubble stage is expected to be unusual. Based on present star formation rates, Sab galaxies like NGC 4736 that have dominant pseudobulges should be rare.

In fact, they are not rare, and even some S0s have pseudobulges. The above estimates are lower limits for at least two reasons. First, typical disk galaxies presumably contained more gas in the past. Second, some secular processes, such as buckling instabilities, do not depend on concurrent star formation. They elevate pre-existing disk stars into the pseudobulge.