environments

I discussed and worked on measurements of environments—the environments of galaxies and the environments of groups of galaxies. Risa Wechsler (Chicago) is visiting and she has a prediction about the relative clustering of old and young groups. Berlind confirms it (clustering is a strong function of apparent group age, based on galaxy colors). Blanton doesn't (looking at the densities of group environments). We need to resolve this.

Interestingly, there may be evidence that the color distribution in each of Berlind's groups is not consistent with a Poisson sampling of (ie, independent draws from) the color distribution of galaxies. To do this test correctly, you have to use not the color distribution of all galaxies, but rather the color distribution of galaxies in the same environments as those in the groups. There may be, truly, red groups and blue groups. This relates to an old problem we assigned to Masjedi, but never finished.

Also I have an old paper about the environments of post-starburst galaxies that I am re-working. Are the environmental measures affected by the 55 arcsec spectrograph constraint of SDSS? No, it turns out. How did I determine this? By re-running my analysis using only the roughly 40 percent of the sky which the spectrograph covers more than once.