What Aspects of Galaxy Environment Matter?

We determine what aspects of the density field surrounding galaxies mostaffect their properties. For Sloan Digital Sky Survey galaxies, we measure thegroup environment, meaning the host group luminosity and the distance from thegroup center (hereafter, ``groupocentric distance''). For comparison, wemeasure the surrounding density field on scales ranging from 100 kpc/h to 10Mpc/h. We use the relationship between color and group environment to test thenull hypothesis that only the group environment matters, searching for aresidual dependence of properties on the surrounding density. Generally, redgalaxies are slightly more clustered on small scales (about 100--300 kpc/h)than the null hypothesis predicts, possibly indicating that substructure withingroups has some importance. At large scales (> 1 Mpc/h), the actual projectedcorrelation functions of galaxies are biased at less than the 5% level withrespect to the null hypothesis predictions. We exclude strongly the conversenull hypothesis, that only the surrounding density (on any scale) matters.These results generally encourage the use of the halo model description ofgalaxy bias, which models the galaxy distribution as a function of host halomass alone. We compare these results to proposed galaxy formation scenarioswithin the Cold Dark Matter cosmological model.Comment: 23 pages, 10 figures, submitted to Ap