Small-Scale Structure in the SDSS and LCDM: Isolated L* Galaxies with Bright Satellites

Abstract: We use a volume-limited spectroscopic sample of isolated galaxies in the Sloan Digital Sky Survey (SDSS) to investigate the frequency and radial distribution of luminous (M_r <~ -18.3) satellites like the Large Magellanic Cloud (LMC) around ~L* Milky Way analogs and compare our results object-by-object to LCDM predictions based on abundance matching in simulations. We show that 12% of Milky Way-like galaxies host an LMC-like satellite within 75 kpc (projected), and 42 % within 250 kpc (projected). This implies ~10% have a satellite within the distance of the LMC, and ~40% of L* galaxies host a bright satellite within the virialized extent of their dark matter halos. Remarkably, the simulation reproduces the observed frequency, radial dependence, velocity distribution, and luminosity function of observed secondaries exceptionally well, suggesting that LCDM provides an accurate reproduction of the observed Universe to galaxies as faint as L~10^9 Lsun on ~50 kpc scales. When stacked, the observed projected pairwise velocity dispersion of these satellites is sigma~160 km/s, in agreement with abundance-matching expectations for their host halo masses. Finally, bright satellites around L* primaries are significantly redder than typical galaxies in their luminosity range, indicating that environmental quenching is operating within galaxy-size dark matter halos that typically contain only a single bright satellite. This redness trend is in stark contrast to the Milky Way's LMC, which is unusually blue even for a field galaxy. We suggest that the LMC's discrepant color might be further evidence that it is undergoing a triggered star-formation event upon first infall.