Are brightest halo galaxies central galaxies?

Abstract

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 centre of the dark matter potential well. This central galaxy paradigm (CGP) is an essential assumption made in various fields of astronomical research. In this paper, we test the validity of the CGP using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey. For each group, we compute two statistics, and , 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 and to those obtained from detailed mock group catalogues, we rule out the null hypothesis that the CGP is correct. Rather, the data indicate that in a non‐zero fraction fBNC(M) of all haloes of mass M the BHG is not the central galaxy, but instead a satellite galaxy. In particular, we find that fBNC increases from ∼0.25 in low‐mass haloes (1012 h−1≤M≲ 2 × 1013 h−1 M⊙) to ∼0.4 in massive haloes (M≳ 5 × 1013 h−1 M⊙). We show that these values of fBNC are uncomfortably high compared to predictions from halo occupation statistics and from semi‐analytical models of galaxy formation. We end by discussing various implications of a non‐zero fBNC(M), with an emphasis on the halo masses inferred from satellite kinematics.

Luminous red galaxies in clusters: central occupation, spatial distributions and miscentring

“…We also derive P BNC , the probability that the brightest cluster member is not the central galaxy. We find P BNC ≈ 20 − 30% which is a factor of ∼ 2 lower than the value found by Skibba et al (2011). Finally, we study the radial offsets of bright non-central LRGs from cluster centers and show that bright noncentral LRGs follow a different radial distribution compared to red cluster members, which follow a Navarro-Frank-White profile.…”

“…Furthermore, it is usually assumed that BCGs are central galaxies (van den Bosch et al 2004;Weinmann et al 2006;Budzynski et al 2012). However, recent results suggest that the central galaxy is not the BCG in all clusters (van den Bosch et al 2005;Coziol et al 2009;Sanderson et al 2009;Skibba et al 2011;Einasto et al 2011Einasto et al , 2012Sehgal et al 2013;Lauer et al 2014). For example, by analyzing differences between the velocities and positions of BCGs relative to other cluster members, Skibba et al (2011) found that 40% of BCGs may in fact be satellite galaxies.…”

“…While we rely on DR8 data, Skibba et al (2011) use SDSS DR4 photometry. Improvements in the sky subtraction in DR8 typically increased the measured luminosities of central galaxies more than those of satellites (Bernardi et al 2013), thus decreasing the probability that a satellite is brighter than the central galaxy.…”

Measurement of differential magnification

“…There is often a large offset between the true mass centre and the bright centre galaxy (e.g. Sanderson et al 2009;Skibba et al 2011). Moreover, other aspects can also cause systematics, such as the irregular shape, massive substructures and multiple critical curves.…”

maggie: Models and Algorithms for Galaxy Groups, Interlopers and Environment

“…This galaxy is assumed to be the central galaxy and at rest relative to the group. Although the most massive group galaxies can be offset and not at rest with the group (e.g., Skibba et al 2011), we prefer this definition to the barycentre, since the galaxy number density profiles in clusters are known to be less cuspy when clusters are centered on their barycentres (Beers & Tonry 1986), and indeed most analyses adopt the central galaxy as the position of the group centre.…”