The massive galaxy cluster XMMU J1230.3+1339 at z ∼ 1: colour–magnitude relation, Butcher–Oemler effect, X‐ray and weak lensing mass estimates★

We present results from the multiwavelength study of XMMU J1230.3+1339 at z ∼ 1. We analyse deep multiband wide-field images from the Large Binocular Telescope (LBT), multi-object spectroscopy observations from VLT, as well as space-based serendipitous observations, from the GALEX and Chandra X-ray observatories. We apply a Bayesian photometric redshift code to derive the redshifts using the far-UV (FUV), near-UV (NUV) and the deep U, B, V, r, i, z data. We make further use of spectroscopic data from FORS2 to calibrate our photometric redshifts, and investigate the photometric and spectral properties of the early-type galaxies. We achieve an accuracy of Δz/(1 + z) = 0.07 (0.04) and the fraction of catastrophic outliers is η = 13 (0) per cent, when using all (secure) spectroscopic data, respectively. The i ― z against z colour-magnitude relation of the photo-z members shows a tight red sequence with a zero-point of 0.935 mag, and slope equal to -0.027. We observe evidence for a truncation at the faint end of the red-cluster-sequence and the Butcher-Oemler effect, finding a fraction of blue galaxies f b ≈ 0.5. Further, we conduct a weak lensing analysis of the deep 26 x 26 arcmin r-band LBC image. The observed shear is fitted with a Single-Isothermal-Sphere and a Navarro-Frenk-White model to obtain the velocity dispersion and the model parameters, respectively. Our best-fitting values are, for the velocity dispersion σ SIS = 1308 ± 284 km s ―1 , concentration parameter c = 4.0 +4 ―2 and scale radius r s = 345 +50 ―57 kpc. From a 38 ks Chandra X-ray observation we obtain an independent estimate of the cluster mass. In addition, we create a signal-to-noise ratio (S/N) map for the detection of the matter mass distribution of the cluster using the mass-aperture technique. We find excellent agreement of the mass concentration identified with weak lensing and the X-ray surface brightness. Combining our mass estimates from the kinematic, X-ray and weak lensing analyses we obtain a total cluster mass of M tot 200 = (4.56 ± 2.3) x 10 14 M ⊙ . This study demonstrates the feasibility of ground-based weak lensing measurements of galaxy clusters up to z ∼ 1.