A tale of two populations: surviving and destroyed dwarf galaxies and
  the build up of the Milky Way's stellar halo
  We use magneto-hydrodynamical simulations of Milky Way-mass haloes from the
Auriga project to examine the properties of surviving and destroyed dwarf
galaxies that are accreted by these haloes over cosmic time. We show that the
combined luminosity function of surviving and destroyed dwarfs at infall is
similar in the various Auriga haloes, and is dominated by the destroyed dwarfs.
There is, however, a strong dependence on infall time: destroyed dwarfs have
typically early infall times, $t_{infall}<6$ Gyr, whereas the majority of
dwarfs accreted at $t_{infall}>10$ Gyr have survived to the present day.
Because of their late infall the surviving satellites today had higher
metallicites at infall than their destroyed counterparts of similar infall
mass; the difference is even more pronounced for the present-day metallicites
of satellites, many of which continue to form stars after infall. In agreement
with previous work, we find that a small number of relatively massive destroyed
dwarf galaxies dominate the mass of the stellar haloes. However, there is a
significant radial dependence: while 90 per cent of the mass in the inner
regions ($<\,20\,$kpc) is contributed, on average, by only 3 massive
progenitors, the outer regions ($>\,100\,$kpc) typically have $\sim8$ main
progenitors of relatively lower mass. Finally, we show that a few massive
progenitors dominate the metallicity distribution of accreted stars, even at
the metal poor end. Contrary to common assumptions in the literature, dwarf
galaxies of mass $M_{*}<10^7 \, M_{\odot}$ make up less than 10 per cent of the
accreted, metal poor stars ([Fe/H] $<\,-3$) in the inner $50\,$kpc.
