We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of to examine the accretion of galaxies into a large sample of groups and clusters , covering a wide range in halo mass ( 10 ^ { 12.9 } to 10 ^ { 15.3 } h ^ { -1 } ~ { } M _ { \odot } ) , and selected from each of four redshift epochs ( z=0 , 0.5 , 1.0 and 1.5 ) .
We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups .
A 10 ^ { 14.5 } h ^ { -1 } ~ { } M _ { \odot } mass cluster at z=0 has , on average , accreted \sim 40 \% of its galaxies ( M _ { stellar } > 10 ^ { 9 } h ^ { -1 } ~ { } M _ { \odot } ) from halos with masses greater than 10 ^ { 13 } h ^ { -1 } ~ { } M _ { \odot } .
Further , the galaxies which are accreted through groups are more massive , on average , than galaxies accreted through smaller halos or from the field population .
We find that at a given epoch , the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass .
In contrast , we find that observing a cluster of the same halo mass at each redshift epoch implies different accretion rates of isolated galaxies , from 5-6 \% per Gyr at z=0 to 15 \% per Gyr at z=1.5 .
We find that combining the existence of a Butcher Oemler effect at z=0.5 and the observations that galaxies within groups display significant environmental effects with galaxy accretion histories justifies striking conclusions .
Namely , that the dominant environmental process must begin to occur in halos of 10 ^ { 12 } – 10 ^ { 13 } h ^ { -1 } ~ { } M _ { \odot } , and act over timescales of > 2 Gyrs .
This argues in favor of a mechanism like “ strangulation ” , in which the hot halo of a galaxy is stripped upon infalling into a more massive halo .
This simple model predicts that by z=1.5 galaxy groups and clusters will display little to no environmental effects .
This conclusion may limit the effectiveness of red sequence cluster finding methods at high redshift .