Astronomy Tea Talk

It is generally assumed that the central galaxy in a dark matter halo, that is, the galaxy with the lowest specific potential energy, is also the brightest halo galaxy (BHG), and that it resides at rest at the center of the dark matter potential well. This central galaxy paradigm (CGP) is an essential assumption made in various fields of astronomical research. I test the validity of the CGP using a galaxy group catalog constructed from the Sloan Digital Sky Survey, and many mock group catalogs (Skibba et al. 2011). For each group I compute two statistics, R and S, which quantify the offsets of the line-of-sight velocities and projected positions of brightest group galaxies relative to the other group members. By comparing the cumulative distributions of R and S to those obtained from the mock catalogs, I rule out the null-hypothesis that the CGP is correct, and I constrain possible explanations of this, such as central galaxies in motion, and prevalent massive substructures. Instead, the data indicate that in a non-zero fraction of haloes the BHG is not the central galaxy, but instead, a satellite galaxy. In particular, I find that this fraction increases from 0.25 in low-mass haloes to 0.40 in massive haloes. I show that these values are uncomfortably high compared to predictions from halo occupation statistics and from semi-analytical models of galaxy formation. I end by discussing various implications of these results.