Copyright © 2004 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 2004. 42:
603-683 Copyright © 2004 by Annual Reviews. All rights reserved |
9.1. A Preliminary Prescription for Recognizing Pseudobulges
Any prescription must cope with the expected continuum from pure classical bulges built by mergers and rapid collapse through objects with some E-like and some disk-like characteristics to pseudobulges built completely by secular processes. Uncertainties are inevitable when we deal with transition objects. Keeping them in mind, a preliminary list of pseudobulge characteristics suggested by the previous sections include:
1 - The candidate pseudobulge is seen to be a disk in images, e.g., its apparent flattening is similar to that of the outer disk.
2 - It is or it contains a nuclear bar (in relatively face-on galaxies).
3 - It is box-shaped (in edge-on galaxies).
4 - It has Sérsic index
n 
1 to 2.
5 - It is more rotation-dominated than are the classical bulges in the
Vmax /
-
diagram; e.g.,
(Vmax /
)*
> 1.
6 - It is a low- outlier
in the
Faber-Jackson (1976)
correlation between (pseudo)bulge luminosity and velocity dispersion.
7 - It is dominated by Population I material (young stars, gas, and dust), but there is no sign of a merger in progress.
If any of these characteristics are extreme or very well developed, it seems safe to identify the central component as a pseudobulge. The more of 1 - 7 apply, the safer the classification becomes. If several of 1 - 7 apply but all are relatively subtle, then the central component may be a pseudobulge or it may be a transition object.
Small bulge-to-total luminosity ratios B / T do not guarantee that the galaxy in question contains a pseudobulge, but if B / T 1/3 to 1/2, it seems safe to conclude that the galaxy contains a classical bulge.
Based on these criteria, galaxies with classical bulges include M 31,
M 81, NGC 2841, NGC 3115, and NGC 4594. Galaxies with prototypical
pseudobulges include
NGC 1291
(Figures 2,
14),
NGC 1512
(Figures 3,
8),
NGC 1353
(Figure 10),
NGC 1365
(Figure 7),
NGC 3945
(Figures 5,
14,
17),
NGC 4371
(Figure 17),
NGC 4736
(Figures 2,
8,
17,
20), and
NGC 5377
(Figure 11).
The classification of the bulge of our Galaxy is ambiguous; the
observation that it is box shaped strongly favors a pseudobulge, but the
stellar population age
and 
-element
overabundance are most easily understood if the bulge is
classical (Section 8.1).