Satellites and haloes of dwarf galaxies

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, 10(7.5) <= M-*(pri)/M-circle dot <= 10(11), 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 (m(*)(sat)/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) < 10(10) M-circle dot). These results are consistent with galaxy formation models in the Lambda cold dark matter (Lambda CDM) 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 Lambda CDM, so satellite abundance reflects the dependence on halo mass, M-200, 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-* proportional to M-200(2.5), similarity is preserved and satellite abundance is independent of primary mass. For primaries brighter than the characteristic mass of the M-*-M-200 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 Lambda CDM galaxy formation models.