Abstract
We present a survey of serendipitous extended X-ray sources and optical cluster candidates from the Chandra Multiwavelength Project (ChaMP). Our main goal is to make an unbiased comparison of X-ray and optical cluster detection methods. In 130 archival Chandra pointings covering 13 deg2, we use a wavelet decomposition technique to detect 55 extended sources, of which 6 are nearby single galaxies. Our X-ray cluster catalog reaches a typical flux limit of about ~10-14 ergs cm-2 s-1, with a median cluster core radius of 21''. For 56 of the 130 X-ray fields, we use the ChaMP's deep NOAO 4 m MOSAIC g', r', and i' imaging to independently detect cluster candidates using a Voronoi tessellation and percolation (VTP) method. Red-sequence filtering decreases the galaxy fore- and background contamination and provides photometric redshifts to z ~ 0.7. From the overlapping 6.1 deg2 X-ray/optical imaging, we find 115 optical clusters (of which 11% are in the X-ray catalog) and 28 X-ray clusters (of which 46% are in the optical VTP catalog). The median redshift of the 13 X-ray/optical clusters is 0.41, and their median X-ray luminosity (0.5-2 keV) is LX = × 1043 ergs s-1. The clusters in our sample that are only detected in our optical data are poorer on average (~4 σ) than the X-ray/optically matched clusters, which may partially explain the difference in the detection fractions.
Highlights
A primary goal of modern astronomy is to study the formation and evolution of galaxies
Our final sample of optical cluster candidates selected using our red-sequence Voronoi tessellation and percolation (VTP) technique—independent of X-ray detections—contains 115 sources measured from 36 mosaic optical fields
This sample contains only optical VTP detections from regions that overlap with the Chandra sky coverage and excludes Chandra PI targets
Summary
A primary goal of modern astronomy is to study the formation and evolution of galaxies. Clusters of galaxies provide us with laboratories in which galaxy evolution can be studied over a large range in cosmic look-back time. Interactions, mergers, and dynamical effects (e.g., tidal forces and ram pressure stripping) may play significant roles in shaping galaxy evolution in these type of locales (e.g., Dubinski 1998; Moore et al 1999). Clusters play a key role constraining fundamental cosmological parameters such as m (the matter-density parameter) and 8 (the rms density fluctuation on a scale of 8 hÀ1 Mpc). The number density of clusters as a function of mass and redshift strongly depends on m and 8 (see Rosati et al 2002 and references therein).
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