The Swift AGN and Cluster Survey. II. Cluster Confirmation with SDSS Data

TL;DR

This study confirms 104 galaxy clusters from the Swift survey using SDSS data, analyzing their properties, redshift distribution, and correlations, and demonstrates the survey's effectiveness for cluster evolution and cosmology research.

Contribution

First comprehensive validation of Swift X-ray cluster candidates with SDSS data, establishing a statistically complete cluster sample up to z~0.4 and identifying many new clusters.

Findings

104 clusters confirmed with >3sigma galaxy over-density

Red sequences detected in ~85% of confirmed clusters

Sample is complete for z <~ 0.3 and 80% complete up to z ~ 0.4

Abstract

We study 203 (of 442) Swift AGN and Cluster Survey extended X-ray sources located in the SDSS DR8 footprint to search for galaxy over-densities in three dimensional space using SDSS galaxy photometric redshifts and positions near the Swift cluster candidates. We find 104 Swift clusters with a >3sigma galaxy over-density. The remaining targets are potentially located at higher redshifts and require deeper optical follow-up observations for confirmation as galaxy clusters. We present a series of cluster properties including the redshift, BCG magnitude, BCG-to-X-ray center offset, optical richness, and X-ray luminosity. We also detect red sequences in ~85% of the 104 confirmed clusters. The X-ray luminosity and optical richness for the SDSS confirmed Swift clusters are correlated and follow previously established relations. The distribution of the separations between the X-ray centroids and the most likely BCG is also consistent with expectation. We compare the observed redshift distribution of the sample with a theoretical model, and find that our sample is complete for z <~ 0.3 and is still 80% complete up to z ~= 0.4, consistent with the SDSS survey depth. These analysis results suggest that our Swift cluster selection algorithm has yielded a statistically well-defined cluster sample for further studying cluster evolution and cosmology. We also match our SDSS confirmed Swift clusters to existing cluster catalogs, and find 42, 23 and 1 matches in optical, X-ray and SZ catalogs, respectively, so the majority of these clusters are new detections.