Interpreting the Relationship between Galaxy Luminosity, Color, and Environment

We study the relationship between galaxy luminosity, color, and environmentin a cosmological simulation of galaxy formation. We compare the predictedrelationship with that found for SDSS galaxies and find that the modelsuccessfully predicts most of the qualitative features seen in the data, butalso shows some interesting differences. Specifically, the simulation predictsthat the local density around bright red galaxies is a strong increasingfunction of luminosity, but does not depend much on color at fixed luminosity.Moreover, we show that this is due to central galaxies in dark matter haloswhose baryonic masses correlate strongly with halo mass. The simulation alsopredicts that the local density around blue galaxies is a strong increasingfunction of color, but does not depend much on luminosity at fixed color. Weshow that this is due to satellite galaxies in halos whose stellar agescorrelate with halo mass. Finally, the simulation fails to predict theluminosity dependence of environment observed around low luminosity redgalaxies. However, we show that this is most likely due to the simulation'slimited resolution. A study of a higher resolution, smaller volume simulationsuggests that this dependence is caused by the fact that all low luminosity redgalaxies are satellites in massive halos, whereas intermediate luminosity redgalaxies are a mixture of satellites in massive halos and central galaxies inless massive halos.Comment: 8 pages including 5 figures (emulateapj). Submitted to Ap