A large variance exists in the amplitude of the galaxy cluster Stellar Mass – Halo Mass (SMHM) relation. We find that the magnitude gap between the brightest central galaxy (BCG) and its fourth brightest neighbor accounts for this variance. At fixed halo mass, clusters with higher magnitude gaps have larger BCG stellar masses. This stratification is also observed in semi-analytic simulations of low-redshift clusters; this suggests that this stratification results from the hierarchical growth of BCGs and may link assembly of the halo with BCG growth. We quantify the impact of the magnitude gap in the SMHM relation using a multiplicative stretch factor, which we measure to be significantly non-zero. Including the magnitude gap also significantly reduces the intrinsic scatter in the BCG stellar mass at fixed halo mass. Additionally, hierarchical growth predicts that at higher redshifts fewer mergers have occurred, indicating that the BCG’s stellar mass and magnitude gap should decrease with increasing lookback time. This suggests that the slope and magnitude gap stretch factor may evolve with redshift. We test this prediction using SDSS-redMaPPer clusters to measure the SMHM relation’s parameters as a function of redshift to z < 0.3. Contrary to expectations from semi-analytic galaxy evolution models, we find no evolution. We will discuss our results in the context of hierarchical growth and prior measurements of BCG growth over the redshift range of our sample.

Talks will be given each Wednesday and will be 30 minutes in length, with time after for questions. Lunch will be served at 11:45 and talks will begin at 12:00. Unless otherwise noted, they will be held in 340 West Hall.