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~=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 TX=3.51+0.60-0.46 keV and LX=(1.56+/-0.04)×1044 ergs s-1 (0.1-2.4 keV band). We estimate an X-ray mass within r500 corresponding to M500~=1.6×1014 Msolar and a total lensing mass (extrapolated by fitting a NFW profile) MNFW=(6+/-3)×1015 Msolar. We use an automated morphological classification of all galaxies brighter than IAB=24 over the structure area to measure the fraction of early-type objects as a function of local projected density Σ10, 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 also based on data collected using: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.; and MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii. Based in part on data products produced at TERAPIX and CADC