The dependence of the galaxy luminosity function on large-scale environment

A basic assumption in the current halo-occupation model is that the properties of a galaxy depend only on the mass of its dark matter halo. An important consequence of this is that the segregation of the galaxy population by large-scale environment is entirely due to the environmental dependence of the halo population. In this paper, we use such a model to predict how the galaxy luminosity function depends on large-scale environment. The latter is represented by the density contrast (delta) averaged over a spherical volume of radius R = 8 h(-1) Mpc. The model predicts that the Schechter function is a good approximation to the luminosity functions of galaxies brighter than similar to10(9) h(-2) L-. (b(J)-band) in virtually all environments. The characteristic luminosity, L*, increases moderately with delta. The faint-end slope, alpha, on the other hand, is quite independent of. However, when splitting the galaxy population into early and late types, it is found that for late types, is virtually constant, whereas for early types, increases from similar to -0.3 in underdense regions (delta similar to -0.5) to similar to -0.8 in highly overdense regions (delta similar to 10). The luminosity function at L-bJ < 10(9) h(-2) L-. is significantly steeper than the extrapolation of the Schechter function that fits the brighter galaxies. This steepening is more significant for early types and in low-density environments. The model also predicts that the luminosity density and the mass density are closely correlated. The relation between the two is monotonic but highly non-linear. This suggests that one can use the luminosity density, averaged over a large volume, to rank the mass density. This, in turn, allows the environmental effects predicted here to be tested by observations.