Understanding the observed evolution of the galaxy luminosity function from z = 6–10 in the context of hierarchical structure formation

Abstract

Recent observations of the Lyman-break galaxy (LBG) luminosity function (LF) from z ≈ 6–10 show a steep decline in abundance with increasing redshift. However, the LF is a convolution of the mass function of dark matter halos (HMF) — which also declines sharply over this redshift range — and the galaxy-formation physics that maps halo mass to galaxy luminosity. We consider the strong observed evolution in the LF from z ≈ 6–10 in this context and determine whether it can be explained solely by the behavior of the HMF. From z ≈ 6–8, we find a residual change in the physics of galaxy formation corresponding to a ∼ 0.5 dex increase in the average luminosity of a halo of fixed mass. On the other hand, our analysis of recent LF measurements at z ≈ 10 shows that the paucity of detected galaxies is consistent with almost no change in the average luminosity at fixed halo mass from z ≈ 8. The LF slope also constrains the variation about this mean such that the luminosity of galaxies hosted by halos of the same mass are all within about an order-of-magnitude of each other. We show that these results are well-described by a simple model of galaxy formation in which cold-flow accretion is balanced by star formation and momentum-driven outflows. If galaxy formation proceeds in halos with masses down to 108M⊙, then such a model predicts that LBGs at z ≈ 10 should be able to maintain an ionized intergalactic medium as long as the ratio of the clumping factor to the ionizing escape fraction is C/fesc≲10.