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 M pri*. On the other hand, satellite abundance is largely independent of primary mass for dwarf primaries (M pri* < 1010M⊙). 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* α 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 powerlaw dependence of dwarf galaxy mass on halo mass envisioned in λCDM galaxy formation models. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.