We study the Local Group ( LG ) dwarf galaxy population using cosmological simulations of volumes selected to resemble the surroundings of the Milky Way ( MW ) and Andromeda ( M31 ) galaxies .
The simulations , from the APOSTLE project , indicate that the total mass within 3 Mpc of the pair ’ s midpoint ( M _ { 3 Mpc } ) typically exceeds \sim 3 times the sum of the virial masses of the two primaries and that the dwarf galaxy formation efficiency per unit mass is uniform throughout the volume .
This suggests that MW and M31 satellites should make up fewer than one third of all LG dwarfs within 3 Mpc .
This is consistent with the fraction ( 28 per cent ) of galaxies with stellar mass M _ { * } > 10 ^ { 7 } { M } _ { \odot } that are satellites .
In addition , the total number of such galaxies ( 42 ) suggests , for the APOSTLE galaxy mass-halo mass relation , a total LG mass of M _ { 3 Mpc } \sim 10 ^ { 13 } { M } _ { \odot } .
At lower galaxy masses , however , the satellite fraction is substantially higher ( 42 per cent for M _ { * } > 10 ^ { 5 } { M } _ { \odot } , or 42 out of 99 ) .
If this is due to incompleteness in the field sample , then \sim 50 dwarf galaxies at least as massive as the Draco dwarf spheroidal are missing from our current LG field dwarf inventory .
The incompleteness interpretation is supported by the pronounced flattening of the LG luminosity function below M _ { * } \sim 10 ^ { 7 } { M } _ { \odot } , and by the scarcity of low-surface brightness LG field galaxies : whereas \sim 40 per cent of all M _ { * } > 10 ^ { 5 } { M } _ { \odot } MW/M31 satellites have effective surface brightness below \Sigma _ { eff } = 26.5 m _ { V } /arcsec ^ { 2 } , only 1 out of all field dwarfs does .
The simulations indicate that most missing dwarfs should lie near the virial boundaries of the two LG primaries , and predict a trove of nearby dwarfs that await discovery by upcoming wide-field imaging surveys .