  Observationally, the fraction of blue satellite galaxies decreases steeply
with host halo mass, and their radial distribution around central galaxies is
significantly shallower in massive (M_* >10e11M_sun) than in Milky Way like
systems. Theoretical models, based primarily on semi-analytical techniques,
have had a long-standing problem with reproducing these trends, instead
predicting too few blue satellites in general but also estimating a radial
distribution that is too shallow, regardless of primary mass. In this Letter,
we use the Illustris cosmological simulation to study the properties of
satellite galaxies around isolated primaries. For the first time, we find good
agreement between theory and observations. We identify the main source of this
success relative to earlier work to be a consequence of the large gas contents
of satellites at infall, a factor ~5-10 times larger than in semi-analytical
models. Because of their relatively large gas reservoirs, satellites can
continue to form stars long after infall, with a typical timescale for
star-formation to be quenched ~2 Gyr in groups but more than ~5 Gyr for
satellites around Milky Way like primaries. The gas contents we infer are
consistent with z=0 observations of HI gas in galaxies, although we find large
discrepancies among reported values in the literature. A testable prediction of
our model is that the gas-to-stellar mass ratio of satellite progenitors should
vary only weakly with cosmic time.
