We examine galaxy formation in a cosmological AMR simulation , which includes two high resolution boxes , one centered on a 3 \times 10 ^ { 14 } \mathrm { M } _ { \odot } cluster , and one centered on a void .
We examine the evolution of 611 massive ( M _ { * } > 10 ^ { 10 } \mathrm { M } _ { \odot } ) galaxies .
We find that the fraction of the final stellar mass which is accreted from other galaxies is between 15 and 40 \% and increases with stellar mass .
The accreted fraction does not depend strongly on environment at a given stellar mass , but the galaxies in groups and cluster environments are older and underwent mergers earlier than galaxies in lower density environments .
On average , the accreted stars are \sim 2.5 Gyrs older , and \sim 0.15 dex more metal poor than the stars formed in-situ .
Accreted stellar material typically lies on the outskirts of galaxies ; the average half-light radius of the accreted stars is 2.6 times larger than that of the in-situ stars .
This leads to radial gradients in age and metallicity for massive galaxies , in qualitative agreement with observations .
Massive galaxies grow by mergers at a rate of approximately 2.6 \% / \mathrm { Gyr ^ { -1 } } .
These mergers have a median ( mass-weighted ) mass ratio less than 0.26 \pm 0.21 , with an absolute lower limit of 0.20 , for galaxies with M _ { * } \sim 10 ^ { 12 } \mathrm { M } _ { \odot } .
This suggests that major mergers do not dominate in the accretion history of massive galaxies .
All of these results agree qualitatively with results from SPH simulations by .