ChaMP serendipitous galaxy cluster survey

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 L_X = (2.65 ± 0.19) × 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.