These lectures review the slow ("secular") evolution of disk galaxies, both internally and environmentally driven. As a heuristic introduction at a 2011 winter school, they emphasize a qualitative and intuitive understanding of physical processes. This provides a useful complement to Kormendy & Kennicutt (2004), which is a more complete review of technical details and the literature. Since this is a school, my lectures will be as self-contained as possible. There will therefore be some overlap with the above review and with Kormendy (1981, 1982b, 1993b, 2008a, b); Kormendy & Cornell (2004); Kormendy & Fisher (2005, 2008) and Kormendy & Bender (2012).
The secular evolution of disk galaxies has deep similarities to the evolution of all other kinds of self-gravitating systems. I begin by emphasizing these similarities. In particular, the growth of pseudobulges in galaxy disks is as fundamental as the growth of stars in protostellar disks, the growth of black holes in black hole accretion disks and the growth of proto-white-dwarf cores in red giant stars. A big-picture understanding of these similarities is conceptually very important. The associated physics allows us to understand what kinds of galaxies evolve secularly and what kinds do not. This review discusses only disk galaxies; secular evolution of ellipticals is also important but is less thoroughly studied.
Galaxy bars are important as "engines" that drive secular evolution, so I provide a heuristic introduction to how bars grow and how they die. Then I review in some detail the evolution processes that are driven by bars and by oval disks and the formation of the various kinds of structures that are built by these processes. I particularly emphasize the growth and properties of pseudobulges. Based on this, I summarize how we recognize pseudobulges and connect them up with our overall picture of galaxy formation.
Two consequences (among many) of secular evolution are particularly important. I review the problem of understanding pure-disk galaxies. These are galaxies that do not contain classical bulges. We infer that they have not experienced a major merger since the first substantial star formation. Many have barely experienced secular evolution. We do not know how these galaxies are formed. Second, I review evidence that classical bulges coevolve with their supermassive black holes but pseudobulges do not.
Next, I discuss secular evolution that is environmentally driven. Here, I concentrate on the evidence that gas-rich, star forming spiral and irregular galaxies are transformed into gas-poor, "red and dead" S0 and spheroidal galaxies. I particular emphasize the properties of spheroidals – that is, tiny dwarfs such as Fornax, Draco and UMi and larger systems such as NGC 147, NGC 185 and NGC 205. These are, in essence, bulgeless S0 galaxies. And I review the various transformation processes that may make these objects.
Finally, I tie together our picture of galaxy formation by hierarchical clustering and galaxy merging (lectures by Isaac Shlosman, Nick Scoville and Daniela Calzetti) and the secular evolution that is the theme of this School.
1.2. Fast versus slow processes of galaxy evolution
Kormendy & Kennicutt (2004) emphasize that the Universe is in transition from early times when galaxy evolution was dominated by fast processes – hierarchical clustering and galaxy merging – to a future when merging will largely be over and evolution will be dominated by slow processes (Fig. 1).
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Figure 1. Processes of galaxy evolution updated from Kormendy (1982b) and from Kormendy & Kennicutt (2004). Processes are divided vertically into fast (top) and slow (bottom). Fast evolution happens on a free-fall ("dynamical") timescale, tdyn ~ (G ρ)-1/2, where ρ is the density of the object produced and G is the gravitational constant. Slow means many galaxy rotation periods. Processes are divided horizontally into ones that happen internally in one galaxy (left) and ones that are driven by environmental effects (right). The processes at center are aspects of all types of galaxy evolution. My lectures are about slow processes, both internal (Sections 2 – 6) and environmentally driven (Section 7). |
We have a well developed picture of galaxy formation that mostly involves the processes in the upper-right corner of Fig. 1. Quantum density fluctuations in non-baryonic, dynamically cold dark matter form immediately after the Big Bang and then get stretched by the expansion of the Universe. Gravity drives hierarchical clustering that causes these fluctuations to grow, separate out from the expansion of the Universe, collapse and form galaxy halos. The baryons first go along for the ride and later cool inside the halos to form stars and visible galaxies. Spiral galaxies form when halos quiescently accrete gas that dissipates and forms disks. Ellipticals form when galaxies collide and merge; then dynamical violence scrambles disks into ellipsoidal ellipticals. It is a convincing story, developed in Toomre (1977a); White & Rees (1978); Kauffmann et al. (1993); Steinmetz & Navarro (2002, 2003), and many other papers. Quoting Binney (2004), "Cold Dark Matter theory has now reached the point at which it should be admitted as a Candidate Member to the Academy of Established Theories, so that it can sit alongside the established theories of Maxwell, Einstein, and Heisenberg."
Now we are making a transition to a time in the far future when the Universe will have expanded so much that most mergers that ever will happen will already have happened. Even now, major mergers – defined as ones in which the less-massive progenitor is within a factor of (say) 5 – 10 of the mass of the bigger progenitor – are uncommon. Minor mergers remain common now but will also get less common in the future. As this happens, more and more galaxies spend more and more of their time not undergoing fast and violent evolution events. And between such events – that is, between galaxy collisions and mergers – galaxies in isolation do not just sit and age their stars. Instead, galaxies evolve on slow timescales; that is, timescales that are much longer than the crossing time or the collapse time. We call such slow processes "secular". At present, both fast and slow processes are important. It is easy to find (especially in cluster environments) good examples of galaxies whose histories have been dominated by fast processes. They are the ellipticals and the classical bulges of disk galaxies. Elsewhere (particularly in field environments), it is easy to find galaxies whose evolution has almost entirely been secular. Both of these types of galaxies are relatively easy to recognize. But it is also important to understand that both kinds of processes can be important in a single galaxy, and in particular, that a galaxy can contain both a classical (merger-built) bulge and a pseudo (secularly built) bulge. Recognizing this is difficult and indeed not always possible. We will spend some considerable effort on understanding how to differentiate classical and pseudo bulges.
Beginning with the seminal paper of Toomre (1977a), most work on galaxy formation over the last 35 years has concentrated on hierarchical clustering. The idea of secular evolution got its start at almost the same time; some of the earliest papers on the subject are Kormendy (1979a, 1979b, 1981) and Combes & Sanders (1981). But research on this subject remained for many years a series of largely isolated "cottage industries" that did not penetrate the astronomical folklore. This changed very rapidly in the last decade. In particular, Kormendy & Kennicutt (2004) aimed to combine the cottage industries into a general and well articulate paradigm on secular evolution that complements hierarchical clustering. Now, this subject has become a major industry. Whole meetings have been devoted to it. This is the motivation that underlies the present Canary Islands Winter School.
1.3. A comment about the name "pseudobulges"
As in Kormendy & Kennicutt (2004), I use the name "pseudobulge" for all high-density, distinct central components of galaxies that are grown slowly out of disks and not rapidly by galaxy mergers. They divide themselves into at least three subtypes that involve different formation processes. Boxy bulges in edge-on galaxies are bars seen side-on (Combes & Sanders 1981). Bars are disk phenomena. I will call these "boxy pseudobulges". Second, dense central components are grown out of disks when nonaxisymmetries transport gas toward the center where it feeds starbursts. Often, these are recognizably more disky than merger-built bulges. But they are not guaranteed to be flat. Still, it is often useful to refer to them as "disky pseudobulges" when we need to differentiate them from boxy pseudobulges. Third, nuclear bars are a recognizably distinct subset of disky pseudobulges. What problem am I trying to solve by calling all of these "pseudobulges"?
Astronomers have a long history of inventing awkward names for things. For historical reasons, we have two different names for the products of major galaxy mergers. If a merger remnant does not have a disk around it, we call it an "elliptical". But if an elliptical subsequently accretes cold gas and grows a new disk (e. g., Steinmetz & Navarro 2002), then we call it a "bulge". In particular, I will call it a "classical bulge". This is inconvenient when one wants to refer to all elliptical-galaxy-like merger remnants without prejudice as to whether they have associated disks. Some authors call these "spheroidal galaxies". This is exceedingly misleading, because the same name is used for dwarf galaxies such as Draco and Fornax that are essentially bulgeless S0s (see Section 7 here). Another alternative is to call them "hot stellar systems". This is misleading, because some ellipticals have lower velocity dispersions than some pseudobulges and indeed also than some lenses (which certainly are disk components). I could define the term "ellipsoidals" to mean both classical bulges and ellipticals. But this name has no constituency. So I will explicitly say "bulges and ellipticals" when I need to refer to both at once.
I do not want the same problem to happen with pseudobulges.