Galaxies in X-ray Groups. II. A Weak Lensing Study of Halo Centering

TL;DR

This study investigates how different methods of locating galaxy group centers affect weak lensing measurements, revealing that miscentering can bias mass estimates and that certain galaxy positions better trace true halo centers.

Contribution

It provides a quantitative analysis of halo centering methods in X-ray galaxy groups and assesses their impact on weak lensing mass estimates.

Findings

Massive galaxies near X-ray centers trace halo centers within ~75 kpc.

Miscentering can bias weak lensing mass estimates by 5-30%.

Groups with ambiguous centers often show disturbed profiles, indicating merging systems.

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-ray selected galaxy groups in the COSMOS field with redshifts 0<z<1 and halo masses in the range 10^13 - 10^14 M_sun. 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 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.