The cosmic evolution survey (COSMOS): A large-scale structure at z=0.73 and the relation of galaxy morphologies to local environment

We have identified a large-scale structure at z similar or equal to 0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak- lensing convergence map, and the distribution of extended X- ray sources in a mosaic of XMM- Newton observations. The main peak seen in these maps corresponds to a rich cluster with T-X = 3.51(-0.46)(+0.60) keV and L-X =( 1.56 +/- 0.04)x10(44) ergs s(-1) ( 0.1-2.4 keV band). We estimate an X- ray mass within r(500) corresponding to M-500 similar or equal to 1.6x 10(14) M-circle dot and a total lensing mass ( extrapolated by fitting a NFW profile) M-NFW =(6 +/- 3)x 10(15) M-circle dot. We use an automated morphological classification of all galaxies brighter than I-AB = 24 over the structure area to measure the fraction of early-type objects as a function of local projected density Sigma(10), based on photometric redshifts derived from ground-based deep multiband photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions'' of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to ( the colder side of) a possible shock region detected in the X- ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two subclusters.Our analysis reveals the value of combining various measures of the projected mass density to locate distant structures and their potential for elucidating the physical processes at work in the transformation of galaxy morphologies.