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

9. CONCLUSION

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. There are still many uncertainties. Keeping them in mind, a preliminary list of pseudobulge characteristics suggested by the previous sections are

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 a Sérsic index n 1 to 2.

5. It is more rotation-dominated than are the classical bulges in the V_max / - diagram; e.g., (V_max / )* < 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 very well developed, it seems safe to identify the central component as a pseudobulge. The more that any of these characteristics apply, the safer the classification becomes. If several characteristics 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 M31, M81, 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).

9.2. Perspective

Secular evolution provides a new collection of physical processes that we need to take into account when we try to understand galaxies. Doing so has already led to significant progress. Thirty years ago, Hubble classification was in active and successful use. However, we also knew about a long list of commonly observed, regular features in disk galaxies, including lenses, boxy bulges, nuclear bars, and nuclear star clusters, that were not understood and not included in the classification schemes. In addition, we knew about uniquely peculiar galaxies (e.g., Arp 1966) that were completely outside the classification process. Now, almost all of the common features and peculiar galaxies have candidate explanations within one of two paradigms of galaxy evolution that originated in the late 1970s. The peculiar objects have turned out mostly to be interacting and merging galaxies. And many of the previously unexplained but common features of disk galaxies now are fundamental to our growing realization that galaxies continue to evolve secularly after the spectacular fireworks of galaxy mergers, dissipative collapse, and their attendant nuclear activity have died down.

ACKNOWLEDGMENTS

We are indebted to Ron Buta and Jerry Sellwood; to scientific editors Geoffrey Burbidge and Allan Sandage; and to the Nuker team, especially Sandy Faber and Scott Tremaine, for penetrating comments on the draft that resulted in important improvements in the final paper. We thank Ralf Bender, Martin Bureau, Andi Burkert, Leo Blitz, Ken Freeman, Sheila Kannappan, Mike Rich, and Guy Worthey for helpful discussions or for permission to quote results before publication. We are exceedingly grateful to Ron Buta and Marcello Carollo for making available many of the digital images used in the construction of figures. Additional figures were kindly made available by Lia Athanassoula, Martin Bureau, Ortwin Gerhard, and Linda Sparke. The color image of NGC 1326 in Figure 8 was constructed for this review by Zolt Levay of STScI from UBV RI and H, HST PC images provided by Ron Buta. We are most grateful to Mary Kormendy for extensive editorial help. Also, we thank Mark Cornell for technical support and for permission to quote results before publication. This review has made extensive use of the NASA/IPAC Extragalactic Database (NED), which is operated by JPL and Caltech under contract with NASA. We acknowledge the support of NSF grant AST-0307386 and NASA grant NAG5-8426.