galaxy mass and environment

Wu and I had a long chat today with Frank van den Bosch (MPIA), who is working on galaxy environments with his high-test group catalog constructed from the SDSS data. His results are all in general agreement with ours, including especially that all environment effects come in at the group scale or smaller, and that a lot of what is happening is driven by the differences between central and satellite galaxies in the groups (where by these terms we usually mean highest stellar mass and lower stellar mass, observationally, since group centers are notoriously hard to determine).

What is new about van den Bosch's work is that he finds that almost all of the information in the environment relations—once you make the central–satellite split—comes from mass segregation among the satellites! The higher mass satellites are more concentrated towards the centers of the groups, and the more massive groups have more massive satellites. Since star-formation rate and everything else is strongly related to galaxy mass, these mass effects drive most of the action. This is remarkable, but not in conflict with our results, and the starting point for some new observational experiments.

Of course, like with our work, the van den Bosch results are easy to misinterpret, because they involve controlling for multiple variables and looking at dependences on remaining variables, which is a subtle business. Also, vdB showed that some of the environment effect conflicts in the literature come from the use of different simple scalar statistics to describe the complex variation of distribution functions.

This is all further motivation for writing a review of galaxy environments that assembles and synthesizes this!