Building Blocks of the Milky Way's Accreted Spheroid
  In the $\Lambda$CDM model of structure formation, a stellar spheroid grows by
the assembly of smaller galaxies, the so-called building blocks. Combining the
Munich-Groningen semi-analytical model of galaxy formation with the high
resolution Aquarius simulations of dark matter haloes, we study the assembly
history of the stellar spheroids of six Milky Way-mass galaxies, focussing on
building block properties such as mass, age and metallicity. These properties
are compared to those of the surviving satellites in the same models. We find
that the building blocks have higher star formation rates on average, and this
is especially the case for the more massive objects. At high redshift these
dominate in star formation over the satellites, whose star formation timescales
are longer on average. These differences ought to result in a larger
$\alpha$-element enhancement from Type II supernovae in the building blocks
(compared to the satellites) by the time Type Ia supernovae would start to
enrich them in iron, explaining the observational trends. Interestingly, there
are some variations in the star formation timescales of the building blocks
amongst the simulated haloes, indicating that [$\alpha$/Fe] abundances in
spheroids of other galaxies might differ from those in our own Milky Way.
