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 \sim 8 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 _ { star } < 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 .