A Divided Universe: Red and Blue Galaxies and their Preferred Environments

(Summary) Making use of scaling relations between the central and the total galaxy luminosity of a dark matter halo as a function of the halo mass, and the scatter in these relations, we present an empirical model to describe the luminosity function (LF) of galaxies. We extend this model to describe relative statistics of early-type, or red, and late-type, or blue, galaxies, with the fraction of early type galaxies at halo centers, relative to the total sample, determined only by the halo mass and the same fraction in the case of satellites is taken to be dependent on both the halo mass and the satellite galaxy luminosity. This simple model describes the conditional luminosity functions, LF of galaxies as a function of the halo mass, measured with the 2dF galaxy group catalog from cluster to group mass scales. Using 2dF measurements, we extract information related to conditional mass function for halos from extreme voids to dense regions in terms of the galaxy overdensity. We also calculate the probability distribution function of halo mass, as a function of the galaxy overdensity, and use these probabilities to address preferred environments of red and blue galaxies. Our model also allow us to make predictions, for example, galaxy bias as a function of the galaxy type and luminosity, the void mass function, and the average galaxy luminosity as a function of the density environment.