What fuels star formation in galaxies? (part 1)

I've only just realized that back in when I did Physics with Astrophysics as an undergraduate student in Brum (1991-1994, in case you were wondering) we never touched on what fuels star formation in galaxies. Star formation requires interstellar gas (specifically cold dense molecular gas), and while we touched on the standard Jeans requirements for cloud collapse we never seem to have addressed the issues (a) where does the gas come from, and (b) what controls the star formation rate in a galaxy with a given amount of gas?

Back then we were taught about the monolithic collapse scenario for the formation of elliptical galaxies, and that spiral galaxies formed stars on a longer timescale, specifically a time scale long enough for their gas to have collapsed into a disk. Mergers may have been hot in the literature, but they weren't yet part of the intro astro courses I attended. A few years later and mergers were the shiznit (so to speak). Certainly I came away from conferences and the literature with the idea that it was mergers all the way down (i.e. galaxy evolution depended primarily on the merger history, and all growth was due to the accretion of satellite galaxies). The monolithic collapse folks were experiencing a hard time at conferences.

This is a lengthy introduction to a small series of posts that will discuss a recent paper by Keres et al (2005, MNRAS 363, 2) entitled "How do galaxies get their gas?", which is the culmination of work previously discussed in less detail by the authors in various conference proceedings over the last few years.