The masses and metallicities of stellar haloes reflect galactic merger
  histories
  There is increasing observational and theoretical evidence for a correlation
between the metallicity and the mass of the stellar halo for galaxies with
Milky Way-like stellar masses. Using the Illustris cosmological hydrodynamical
simulations, we find that this relationship arises because a single massive
massive progenitor contributes the bulk of the mass to the accreted stellar
component as well as sets its metallicity. Moreover, in the Illustris
simulations, this relationship extends over 3 orders of magnitude in accreted
stellar mass for central galaxies. We show that for Milky Way-like mass
galaxies, the scatter in accreted metallicity at a fixed accreted stellar mass
encodes information about the stellar mass of the dominant accreted progenitor,
while the radial density and metallicity gradients of the accreted stellar
component encodes information about the time of accretion of the dominant
progenitor. We demonstrate that for Milky Way-like mass galaxies, the Illustris
simulations predict that the metallicity and the stellar mass of the total
accreted stellar component can be reconstructed from aperture measurements of
the stellar halo along the minor axis of edge-on disk galaxies. These
correlations highlight the potential for observational studies of stellar halos
to quantify our understanding of the most dominant events in the growth history
of galaxies. We explore the implications of our model for our understanding of
the accretion histories of the Milky Way, M31 and NGC 5128. In particular, a
relatively late and massive accretion is favoured for M31; additionally, we
provide a first estimate of the accreted stellar mass for NGC 5128.
