Global Patterns Downsizing

In piecing together how galaxies evolve from the observations, we are in a position like that of paleontologists. We have snapshots of the situation at various times and in different environments, variously filtered by observational limitations and what the Universe has provided for us to see. As the timespans grow and our sampling becomes sparser, it becomes less clear how we should connect the populations in these snapshots. The biological analogy uses a framework of cladistics— piecing together probably lines or trees of descent based on the least changes in those characteristics that are most robust to adaptation, and so are most diagnostic of an organism's classification. A set of rules for galaxy cladistics would include some limitations or constraints on how galaxies change with time. Generally, we do not expect the stellar mass in a galaxy to decrease with time, and the mass of heavy elements representing the metallicity of the stars should not decrease with time (although the metallicity history can be complicated by both winds and infall of pristine gas). In general, galaxies can grow in mass but not shrink. Fraix-Burnet has made progress with this approach.

Several distinct kinds of data now point to a common theme in galaxy evolution—downsizing. This may be expressed in several equivalent ways. The characteristic mass of star-forming galaxies has been declining monotonically since early epochs. Alternately, the star-formation history of lower-mass galaxies has continued longer. At least in the sense of the ages of the dominant stellar populations, massive galaxies formed earlier and completed the process more quickly than low-mass galaxies. These tie together the early appearance of a distinct red sequence of galaxies in clusters and the star-formation histories we deduce from the stellar makeup of nearby galaxies.

These are direct statements from multiple surveys and thus are very robust, but the physics behind the pattern remains unclear. Feedback could be important, especially if it is more likely to shut down star formation in more massive systems. Hierarchical buildup must fit this pattern, so that recent buildup either applies mostly to low-mass galaxies or involves systems that are already gas-poor ("dry mergers"). Especially for massive systems, it may be as instructive to trace the occurrence of the shutdown in star formation as the prior history of starbirth (a point particularly stressed by Dressler in the context of SO galaxies). Slow accretion of surrounding cool gas and merger-driven star formation seem to have been important only for relatively low-mass galaxies.

The growth of massive black holes, as traced by deep surveys and particularly X-ray demographics, has paralleled this downsizing pattern. The typical mass of black holes responsible for most of the ongoing accretion has declined over cosmic time, more or less in the same way as the mass of galaxies hosting active star formation. This coincidence may reflect the fact that cool gas is important for both, or may be telling us something deeper about the connection between galaxies and the central black holes.

As seen in Chapter 5, the minimum evolution for a galaxy is the passive case, which gives a monotonic fading and reddening after a short time. Many galaxies will have a more eventful history, with active star formation and perhaps forced evolution due to gravitational interaction with another galaxy, internal dynamics such as a bar redistributing gas, or the cluster environment. In seeking the processes of galaxy formation, we need to look beyond these historical events to the initial state of the galaxies when they formed the first stars we can still see.

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