I spent a very enjoyable day at Johns Hopkins, crashing the GALEX science team meeting, where they asked me lead a discussion on galaxy environments. I worked through the usual issues: How do you measure environment and what are the trade-offs? (You either get well-defined physical scale or else good s/n but never both!) At what scale is the environment most connected to galaxy properties? (1 Mpc or so.) What is most closely connected to environment, in an information sense? (Star-formation rate.) How do you perform the most sensitive possible experiments? (By averaging low s/n quantities as a function of high s/n quantities and not the other way around!)
This all started some pretty lively discussion, and there are some great new directions from GALEX, including but not limited to: GALEX colors have much more dynamic range among red galaxies than optical colors, so you can hope to do more sensitive things in the
green valley of galaxies that are neither completely red nor completely blue. GALEX can tell you about the spatial distribution and extent of star formation in low-redshift galaxies, which might show interesting environment dependences. Because GALEX is so sensitive to dust extinction, and dust is mixed with gas, in principle it might be possible to use GALEX to measure, maybe indirectly, the gas resevoirs in galaxies with low star-formation rates. In general, the fact that star-formation rate is so tied to environment means that GALEX is the right tool for the job.