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

Mo, H. J. ; Yang, Xiaohu ; Bosch, Frank C. ; Jing, Y. P.

In: Monthly Notices of the Royal Astronomical Society, 2004, vol. 349, no. 1, p. 205-212

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    Summary
    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 (δ) 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 ∼109h−2 L⊙ (bJ-band) in virtually all environments. The characteristic luminosity, L⋆, increases moderately with δ. The faint-end slope, α, on the other hand, is quite independent of δ. However, when splitting the galaxy population into early and late types, it is found that for late types, α is virtually constant, whereas for early types, α increases from ∼−0.3 in underdense regions (δ∼−0.5) to ∼−0.8 in highly overdense regions (δ∼ 10). The luminosity function at < 109h−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