Intrinsic alignments of group and cluster galaxies in photometric surveys

Abstract

Intrinsic alignments of galaxies have been shown to contaminate weak gravitational lensing observables on linear scales, r > 10 h −1 Mpc, but studies of alignments in the non-linear regime have thus far been inconclusive. We present an estimator for extracting the intrinsic alignment signal of galaxies around stacked clusters of galaxies from multiband imaging data. Our estimator removes the contamination caused by galaxies that are gravitationally lensed by the clusters and scattered in redshift space due to photometric redshift uncertainties. It uses posterior probability distributions for the redshifts of the galaxies in the sample and it is easily extended to obtain the weak gravitational lensing signal while removing the intrinsic alignment contamination. We apply this algorithm to groups and clusters of galaxies identified in the Sloan Digital Sky Survey 'Stripe 82' coadded imaging data over ∼ 150 deg 2 . We find that the intrinsic alignment signal around stacked clusters in the redshift range 0.1 < z < 0.4 is consistent with zero. In terms of the tidal alignment model of Catelan et al. (2001), we set joint constraints on the strength of the alignment and the bias of the lensing groups and clusters on scales between 0.1 and 10. This constrains the contamination fraction of alignment to lensing signal to the range between [−18, 23] per cent below scales of 1 h −1 Mpc at 95 per cent confidence level, and this result depends on our photometric redshift quality and selection criteria used to identify background galaxies. Our results are robust to the choice of photometric band in which the shapes are measured (i and r) and to centring on the Brightest Cluster Galaxy or on the geometrical centre of the clusters.

Constraints on the alignment of galaxies in galaxy clusters from ~14 000 spectroscopic members

“…Including red sequence members roughly doubles the number of galaxies used and confirms the latter result, with + = −0.0022 ± 0.0020 and + = 0.0000 ± 0.0026 with KSB and GALFIT, respectively. Our results are consistent with the nondetection of satellite radial alignments in massive clusters at z > 0.5 (Hung & Ebeling 2012), based on ∼500 spectroscopic members in the inner ∼500 kpc of clusters, using imaging from the Hubble Space Telescope (HST), and also with measurements at smaller masses from photometrically-selected galaxy groups from SDSS (Hao et al 2011;Chisari et al 2014) and spectroscopically-selected galaxy groups from the Galaxy And Mass Assembly (GAMA) survey (Schneider et al 2013). Our results suggest that the stars in galaxies within clusters do not feel a strong enough tidal torque to be aligned toward the center of the cluster, in contrast with results from simulations which find strong alignments even when accounting for differences in the response between stars and dark matter which naturally occurs in hydrodynamical simulations (Pereira & Bryan 2010;Tenneti et al 2014;Velliscig et al, in prep.).…”

“…Schneider et al (2013) also used a sample of spectroscopically-confirmed group members, plus a shape measurement method that was specifically calibrated to weak lensing measurements (Mandelbaum et al 2005), and found no significant evidence for alignments. Finally, Chisari et al (2014) measured galaxy alignments in photometrically-selected galaxy groups and clusters in SDSS Stripe 82, fully accounting for photometric redshift uncertainties, and constrain alignments to similar values as those found here.…”

“…The fact that all recent measurements that use high-quality imaging and properly calibrated shape measurements have yielded null detections (Hao et al 2011;Hung & Ebeling 2012;Schneider et al 2013;Chisari et al 2014, plus the present study) leads us to conclude that there is no evidence for intrinsic alignments of satellite galaxies in galaxy groups or clusters to the level of uncertainty achievable with current datasets (both statistical and systematic).…”

Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations

“…8). In this case the average value of g+ for distances that are smaller than the host virial radius is consistent with zero, in agreement with the observations of Chisari et al (2014) and Sifón et al (2015). Using deeper observations, in order to probe the lower surface brightness parts of satellite galaxies, could represent a way to reveal the alignment that is seen in observations when all stars bounded to subhaloes are considered.…”

“…They constrained the average ellipticity, within the host virial radius, to be g+ = (− 3.7 ± 2.7) × 10 −3 or g+ = (0.4 × ±3.1) × 10 −3 depending on the shape estimation method employed. Chisari et al (2014) Sifón et al (2015).…”