![]() | Annu. Rev. Astron. Astrophys. 2004. 42:
603-683 Copyright © 2004 by Annual Reviews. All rights reserved |
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.