Satellites and haloes of dwarf galaxies

Abstract

We study the abundance of satellite galaxies as a function of primary stellar mass using the Sloan Digital Sky Survey/Data Release 7 (SDSS/DR7) spectroscopic catalogue. In contrast with previous studies, which focused mainly on bright primaries, our central galaxies span a wide range of stellar mass, 107.5 ⩽ Mpri*/M⊙ ⩽ 1011, from dwarfs to central cluster galaxies. Our analysis confirms that the average number of satellites around bright primaries, when expressed in terms of satellite-to-primary stellar mass ratio (msat*/M*pri), is a strong function of Mpri*. On the other hand, satellite abundance is largely independent of primary mass for dwarf primaries (Mpri* < 1010 M⊙). These results are consistent with galaxy formation models in the Λ cold dark matter (ΛCDM) scenario. We find excellent agreement between SDSS data and semianalytic mock galaxy catalogues constructed from the Millennium-II Simulation. Satellite galaxies trace dark matter substructure in ΛCDM, so satellite abundance reflects the dependence on halo mass, M200, of both substructure and galaxy stellar mass (M*). Since dark matter substructure is almost scale free, the dependence of satellite abundance on primary mass results solely from the well-defined characteristic mass in the galaxy mass-halo mass relation. On dwarf galaxy scales, where models predict a power-law scaling, M*∝M2.5200, similarity is preserved and satellite abundance is independent of primary mass. For primaries brighter than the characteristic mass of the M*–M200 relation, satellite abundance increases strongly with primary mass. Our results provide strong support for the steep, approximately power-law dependence of dwarf galaxy mass on halo mass envisioned in ΛCDM galaxy formation models.