Galaxies in X-Ray Groups. Ii. A Weak Lensing Study of Halo Centering

Abstract

Locating the centers of dark matter halos is critical for understanding the mass profiles of halos, as well as the formation and evolution of the massive galaxies that they host. The task is observationally challenging because we cannot observe halos directly, and tracers such as bright galaxies or X-ray emission from hot plasma are imperfect. In this paper, we quantify the consequences of miscentering on the weak lensing signal from a sample of 129 X-rayselected galaxy groups in the COSMOS field with redshifts 0 < z < 1 and halo masses in the range 10 13 -10 14 M . By measuring the stacked lensing signal around eight different candidate centers (such as the brightest member galaxy, the mean position of all member galaxies, or the X-ray centroid), we determine which candidates best trace the center of mass in halos. In this sample of groups, we find that massive galaxies near the X-ray centroids trace the center of mass to 75 kpc, while the X-ray position and centroids based on the mean position of member galaxies have larger offsets primarily due to the statistical uncertainties in their positions (typically ∼50-150 kpc). Approximately 30% of groups in our sample have ambiguous centers with multiple bright or massive galaxies, and some of these groups show disturbed mass profiles that are not well fit by standard models, suggesting that they are merging systems. We find that halo mass estimates from stacked weak lensing can be biased low by 5%-30% if inaccurate centers are used and the issue of miscentering is not addressed.

SARCS strong-lensing galaxy groups

“…3.2, using the same COSMOS data, George et al (2012) confirmed the findings of Jeltema et al (2007) about the BGG/BCG in groups and clusters: an early-type galaxy, but not necessarily a dominant one, and with observed shifts up to 100 kpc with respect to the X-ray peak (see also Hoekstra et al 2012, and their sample of 50 massive galaxy clusters). In our study of the SARCS sample, we observed the same behaviour in some specific cases such as the multi-modal groups where there is not a single early-type galaxy dominating the light distribution, and/or where the strong-lensing system is not associated to the brightest member.…”

“…For instance, Jeltema et al 2007 have studied a sample of seven X-ray-loud galaxy groups at intermediate redshifts (0.2 < z < 0.6), and for two of them, the brightest galaxy presents an offset ∼100 h −1 70 kpc with the X-ray emission peak. They also found two groups where no dominant elliptical galaxy is present, but rather several ones with comparable luminosities (see also the more recent work by George et al 2012, of the COSMOS X-ray-selected groups). In such cases, the assumption of tracing the mass centre by the position of the most luminous galaxy might be wrong as well.…”

“…However, the question of the centre remains as a source of uncertainties: substantial miscentering can lead to weak-lensing masses underestimated up to 30% (e.g. George et al 2012). …”

The 400d Galaxy Cluster Survey weak lensing programme

“…Dietrich et al 2012;George et al 2012) that WL masses using lensing cluster centres are biased high due to random noise with respect to those based on independently obtained cluster centres, e.g. the ROSAT centres we employ.…”

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

“…One important aspect of how LRGs trace dark matter is how often they coincide with the centers of dark matter halos. The identification of central galaxies in cluster samples is critical for a variety of studies, including the determination of cluster halo masses with weak gravitational lensing (George et al 2012), and RSD studies (Hikage & Yamamoto 2013). Because LRGs are luminous and trace massive overdensities, it is often assumed that central galaxies are also LRGs.…”