DWARF galaxy clustering and missing satellites

Abstract

At redshifts around 0.1 the Canada-France-Hawaii Telescope Legacy Survey Deep fields contain some 6 × 104 galaxies spanning the mass range from 105 to 1012 M ⊙. We measure the stellar mass dependence of the two-point correlation using angular measurements to largely bypass the errors, approximately 0.02 in the median, of the photometric redshifts. Inverting the power-law fits with Limber's equation we find that the autocorrelation length increases from a very low 0.4 h -1 Mpc at 105.5 M ⊙ to the conventional 4.5 h -1 Mpc at 1010.5 M ⊙. The power-law fit to the correlation function has a slope which increases from γ ≃ 1.6 at high mass to γ ≃ 2.3 at low mass. The spatial cross-correlation of dwarf galaxies with more massive galaxies shows fairly similar trends, with a steeper radial dependence at low mass than predicted in numerical simulations of subhalos within galaxy halos. To examine the issue of "missing satellites" we combine the cross-correlation measurements with our estimates of the low-mass galaxy number density. We find on the average there are 60 20 dwarfs in subhalos with M(total)>107 M ⊙ for a typical Local Group M(total)/M(stars) = 30, corresponding to M/LV ≃ 100 for a galaxy with no recent star formation. The number of dwarfs per galaxy is about a factor of 2 larger than currently found for the Milky Way. Nevertheless, the average dwarf counts are about a factor of 30 below lambda cold dark matter (LCDM) simulation results. The divergence from LCDM predictions is one of the slope of the relation, approximately dN/dln M ≃ -0.5 rather than the predicted -0.9, not sudden onset at some characteristic scale. The dwarf galaxy star formation rates span the range from passive to bursting, which suggests that there are few completely dark halos. © 2009 The American Astronomical Society.