A divided Universe: red and blue galaxies and their preferred environments

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 centres, relative to the total sample, determined only by the halo mass; 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 LF (the LF of galaxies as a function of the halo mass) measured with the 2dF galaxy group catalogue from cluster to group mass scales. Given the observational measurements of the LF as a function of the environment using the 2dF, with environment defined by the galaxy overdensity measured over a given volume, we extend our model to describe environmental LFs. Using 2dF measurements, we extract information related to the conditional mass function for haloes 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 allows us to make predictions, e.g. 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. The extension of the halo model to construct conditional and environmental LFs of galaxies is a powerful approach in the era of wide‐field large‐scale structure surveys, given the ability to extract information beyond the average LF.