Published in "Secular Evolution of Galaxies", by J.
Falcón-Barroso, and Johan H. Knapen, Cambridge, UK: Cambridge
University Press, 2013, p.1
For a PDF version of the article, click here.
http://arxiv.org/abs/1311.2609
Abstract: Self-gravitating systems evolve toward the most tightly bound configuration that is reachable via the evolution processes that are available to them. They do this by spreading – the inner parts shrink while the outer parts expand – provided that some physical process efficiently transports energy or angular momentum outward. The reason is that self-gravitating systems have negative specific heats. As a result, the evolution of stars, star clusters, protostellar and protoplanetary disks, black hole accretion disks and galaxy disks are fundamentally similar. How evolution proceeds then depends on the evolution processes that are available to each kind of self-gravitating system. These processes and their consequences for galaxy disks are the subjects of my lectures and of this Canary Islands Winter School.
I begin with a review of the formation, growth and death of bars. Then I review the slow ("secular") rearrangement of energy, angular momentum, and mass that results from interactions between stars or gas clouds and collective phenomena such as bars, oval disks, spiral structure and triaxial dark halos. The "existence-proof" phase of this work is largely over: we have a good heuristic understanding of how nonaxisymmetric structures rearrange disk gas into outer rings, inner rings and stuff dumped onto the center. The results of simulations correspond closely to the morphology of barred and oval galaxies. Gas that is transported to small radii reaches high densities. Observations confirm that many barred and oval galaxies have dense central concentrations of gas and star formation. The result is to grow, on timescales of a few Gyr, dense central components that are frequently mistaken for classical (elliptical-galaxy-like) bulges but that were grown slowly out of the disk (not made rapidly by major mergers). The resulting picture of secular galaxy evolution accounts for the richness observed in galaxy structure.
We can distinguish between classical and pseudo bulges because the latter retain a "memory" of their disky origin. That is, they have one or more characteristics of disks: (1) flatter shapes than those of classical bulges, (2) correspondingly large ratios of ordered to random velocities, (3) small velocity dispersions σ with respect to the Faber-Jackson correlation between σ and bulge luminosity, (4) spiral structure or nuclear bars in the "bulge" part of the light profile, (5) nearly exponential brightness profiles and (6) starbursts. So the cleanest examples of pseudobulges are recognizable. However, pseudo and classical bulges can coexist in the same galaxy.
I review two important implications of secular evolution:
The existence of pseudobulges highlights a problem with our theory of galaxy formation by hierarchical clustering. We cannot explain galaxies that are completely bulgeless. Galaxy mergers are expected to happen often enough so that every giant galaxy should have a classical bulge. But we observe that bulgleless giant galaxies are common in field environments. We now realize that many dense centers of galaxies that we used to think are bulges were not made by mergers; they were grown out of disks. So the challenge gets more difficult. This is the biggest problem faced by our theory of galaxy formation.
Pseudobulges are observed to contain supermassive black holes (BHs), but they do not show the well known, tight correlations between BH mass and the mass and velocity dispersion of the host bulge. This leads to the suggestion that there are two fundamentally different BH feeding processes. Rapid global inward gas transport in galaxy mergers leads to giant BHs that correlate with host ellipticals and classical bulges, whereas local and more stochastic feeding of small BHs in largely bulgeless galaxies evidently involves too little energy feedback to result in BH-host coevolution. It is an important success of the secular evolution picture that morphological differences can be used to divide bulges into two types that correlate differently with their BHs.
I review environmental secular evolution – the transformation of gas-rich, star-forming spiral and irregular galaxies into gas-poor, "red and dead" S0 and spheroidal ("Sph") galaxies. I show that Sph galaxies such as NGC 205 and Draco are not the low-luminosity end of the structural sequence (the "fundamental plane") of elliptical galaxies. Instead, Sph galaxies have structural parameters like those of low-luminosity S+Im galaxies. Spheroidals are continuous in their structural parameters with the disks of S0 galaxies. They are bulgeless S0s. S+Im → S0+Sph transformation involves a variety of internal (supernova-driven baryon ejection) and environmental processes (e. g., ram-pressure gas stripping, harassment, and starvation). Finally, I summarize how hierarchical clustering and secular processes can be combined into a consistent and comprehensive picture of galaxy evolution.
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