Constraints on the Alignment of Galaxies in Galaxy Clusters from $\sim$14,000 Spectroscopic Members

TL;DR

This study uses a large spectroscopic catalog of 14,576 cluster members (9,054 within $r_{200}$) from 90 massive clusters to measure three intrinsic alignment signals (satellite radial alignment toward the cluster center, satellite–BCG alignment, and satellite–satellite alignment) with two independent shape estimators (KSB and GALFIT). Across the full radius range up to at least $3r_{200}$ and a variety of galaxy and cluster subsamples, the authors find no statistically significant intrinsic alignments, with results remaining null even when including red sequence members (total sample ~23,041 members with ~8% RS contamination). They then place these constraints into a halo-model context, showing a 1-halo IA term would alter the angular power spectrum by at most about 10% for KiDS-like surveys, and that a linear alignment model remains a robust description for current and near-future cosmic-shear analyses, though larger surveys may require more detailed IA modeling. Overall, the work provides robust observational limits on satellite IA in massive halos and clarifies the level of systematic risk for weak-lensing studies from intrinsic alignments.

Abstract

Torques acting on galaxies lead to physical alignments, but the resulting ellipticity correlations are difficult to predict. As they constitute a major contaminant for cosmic shear studies, it is important to constrain the intrinsic alignment signal observationally. We measured the alignments of satellite galaxies within 90 massive galaxy clusters in the redshift range 0.05<z<0.55 and quantified their impact on the cosmic shear signal. We combined a sample of 38,104 galaxies with spectroscopic redshifts with high-quality data from the Canada-France-Hawaii Telescope. We used phase-space information to select 14,576 cluster members, 14,250 of which have shape measurements and measured three different types of alignment: the radial alignment of satellite galaxies toward the brightest cluster galaxies (BCGs), the common orientations of satellite galaxies and BCGs, and the radial alignments of satellites with each other. Residual systematic effects are much smaller than the statistical uncertainties. We detect no galaxy alignment of any kind out to at least 3 r200. The signal is consistent with zero for both blue and red galaxies, bright and faint ones, and also for subsamples of clusters based on redshift, dynamical mass, and dynamical state. These conclusions are unchanged if we expand the sample with bright cluster members from the red sequence. We augment our constraints with those from the literature to estimate the importance of the intrinsic alignments of satellites compared to those of central galaxies, for which the alignments are described by the linear alignment model. Comparison of the alignment signals to the expected uncertainties of current surveys such as the Kilo-Degree Survey suggests that the linear alignment model is an adequate treatment of intrinsic alignments, but it is not clear whether this will be the case for larger surveys.

Figures (13)

• Figure 1: Left: redshift distributions of all MENeaCS+CCCP clusters (blue histogram) and clusters used in this study (gray filled histogram). Right: distributions of number of spectroscopic members, $N_m$ (gray filled histogram), and number of spectroscopic members within $r_{200}$, $N_{200}$ (blue histogram). Abell 2142, with $N_m=1052$ and $N_{200}=731$, is not shown.
• Figure 2: Distribution of spectroscopic members. Left: as a function of rest-frame absolute r-band magnitude. Right: as a function of cluster-centric distance in units of $r_{200}$. The dotted black lines show the distribution of the full sample; the solid lines show the distribution split into three redshift bins of approximately equal number of clusters.
• Figure 3: Top: comparison between velocity dispersions calculated from spectroscopic members in this work with those in rines13, and with velocity dispersions calculated by fitting a single isothermal sphere to the weak lensing profile hoekstra12 for CCCP clusters used in this work. Errorbars are not shown for the latter for clarity. Squares and circles show relaxed and disturbed clusters, respectively. The black line shows the one-to-one relation, and the top axis shows $E(z)M_{200}$ for a given $\sigma_{200}$ from the evrard08 relation. Bottom: distribution of velocity dispersions of the full sample. The gray histogram shows the total distribution, with the blue and red histograms showing the distributions for relaxed and disturbed clusters, respectively.
• Figure 4: Purity of the red sequence (filled symbols with solid lines) and spectroscopic completeness within the red sequence (open symbols with dashed lines). Filled symbols with dotted lines show the fraction of galaxies that are not selected as members but that are within $\Delta z=0.03(1+z)$ of the cluster, which represents the contamination in an unbiased photometric redshift selection. Left: as a function of cluster-centric distance, for different luminosity limits. Middle: as a function of absolute magnitude, for different radial apertures. Right: as a function of redshift for different luminosity limits, at an aperture of 1 Mpc. Note that points within a given line are independent (each line is a differential distribution), but lines of the same type are not independent from each other.
• Figure 5: Distribution of the distance ratio, $D_{ls}/D_s$, for red sequence members that are confirmed to be nonmembers of the clusters from spectroscopic redshifts. The gray filled histogram shows red sequence galaxies from all clusters; the blue and red (empty) histograms show the distributions for clusters at low and high redshift, respectively. For illustration, the top axis shows the source redshift for a cluster at $z=0.15$.