Tilted CDM versus WDM in the Subgalactic Scuffle

Abstract

Although the currently favored cold dark matter plus cosmological constant model (LCDM) has proven to be remarkably successful on large scales, on subgalactic scales it faces some potentially fatal difficulties; these include over-producing dwarf satellite galaxies and predicting excessive central densities in dark halos. Among the most natural cosmological solutions to these problems is to replace cold dark matter with a warm species (LWDM). The warm component acts to reduce the small-scale power, resulting in fewer galactic subhalos and lower halo central densities. An alternative model with a mild ``tilt'' in the inflationary power spectrum (TLCDM; n =0.9) similarly reduces the central densities of dark halos, although the substructure abundance remains relatively high. Here I argue that because dwarf galaxy formation should be suppressed in the presence of a strong ionizing background, favored LWDM models will generally under-predict the observed abundance of dwarf galaxies. The satellite count for TLCDM fairs much better, as long as the photoionization effect is taken into account. TLCDM provides a more successful alternative to LWDM on subgalactic scales with the added attraction that it relies on only a minor, natural adjustment to the standard framework of CDM.