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

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SECULAR EVOLUTION AND THE FORMATION OF PSEUDOBULGES IN DISK GALAXIES

John Kormendy 1 and Robert C. Kennicutt, Jr. 2


1 Department of Astronomy, University of Texas, Austin, Texas 78712; email: kormendy@astro.as.utexas.edu
2 Department of Astronomy, Steward Observatory, University of Arizona, Tucson, Arizona 85721; email: rkennicutt@as.arizona.edu


Abstract. The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular - the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges.

We begin with an "existence proof" - a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and plausibly feeds star formation. In the observations, many barred and oval galaxies have dense central concentrations of gas and star formation. Optical colors and spectra often imply young stellar populations. So the formation of pseudobulges is well supported by theory and observations. It is embedded in a broader evolution picture that accounts for much of the richness observed in galaxy structure.

If secular processes built dense central components that masquerade as bulges, how can we distinguish them from merger-built bulges? Observations show that pseudobulges retain a memory of their disky origin. That is, they have one or more characteristics of disks: (a) flatter shapes than those of classical bulges, (b) correspondingly large ratios of ordered to random velocities, (c) small velocity dispersions sigma with respect to the Faber-Jackson correlation between sigma and bulge lumi nosity, (d) spiral structure or nuclear bars in the "bulge" part of the light profile, (e) nearly exponential brightness profiles, and (f) starbursts. All these structures occur preferentially in barred and oval galaxies, where secular evolution should be most rapid. So the cleanest examples of pseudobulges are recognizable.

Are their formation timescales plausible? We use measurements of central gas densities and star-formation rates to show that pseudobulges of the observed densities form on timescales of a few billion years.

Thus a large variety of observational and theoretical results lead to a new picture of galaxy evolution that complements hierarchical clustering and merging. Secular evolution consists of more than the aging of stellar populations. Every galaxy is dynamically evolving.


Table of Contents

INTRODUCTION
What is a Bulge? Classical and Physical Morphology

SECULAR EVOLUTION OF BARRED GALAXIES
Morphology of Barred Galaxies
Dynamics of Barred Galaxies: The Importance of Resonances
Bar-Driven Radial Transport of Gas: The Formation of Rings

THE SECULAR EVOLUTION OF UNBARRED GALAXIES
Many Apparently Unbarred Galaxies Show Bars in the Infrared
Oval Galaxies
The Demise of Bars
Global Pattern Spirals
Conclusion

THE OBSERVED PROPERTIES OF PSEUDOBULGES
Embedded Disks, Spiral Structure, and Star Formation
Exponential Bulges
Some Bulges Are As Flat As Disks
Bars Within Bars
Box-Shaped Bulges
Bulges with the Dynamics of Disks: The V / sigma - epsilon Diagram
Velocity Dispersions and the Faber-Jackson Relation
Pseudobulges and the Fundamental Plane Correlations
Nuclei
In Which Pseudobulges Fade Out into Disks

CENTRAL STAR FORMATION AND PSEUDOBULGE GROWTH
Case Studies: NGC 1326, NGC 1512, NGC 4314, and NGC 5248
General Properties of Circumnuclear Regions
Constraining Evolution Timescales and Pseudobulge Growth

COSMOLOGICAL IMPLICATIONS OF SECULAR EVOLUTION
Evolution Along the Hubble Sequence
Merger-Induced Versus Secular Star Formation in Bulges

COMPLICATIONS
Pseudobulges Do Not Have To Be Flat
Pseudobulges Do Not Have To Be Young

CAVEATS
Stellar Populations in Classical Bulges and Pseudobulges
Can Minor Accretion Events Mimic Pseudobulge Growth?
Disky Distortions in Elliptical Galaxies

CONCLUSION
A Preliminary Prescription for Recognizing Pseudobulges
Perspective

REFERENCES

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