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, anACS weak-lensing convergence map and the distribution of extended X-ray sourcesin a mosaic of XMM observations. The main peak seen in these maps correspondsto a rich cluster with Tx= 3.51+0.60/-0.46 keV and Lx=(1.56+/-0.04) x 10^{44}erg/s ([0.1-2.4] keV band). We estimate an X-ray mass within $r500$corresponding to M500~1.6 x 10^{14} Msun and a total lensing mass (extrapolatedby fitting a NFW profile) M(NFW)=(6+/-3) x 10^15 Msun. We use an automatedmorphological classification of all galaxies brighter than I_AB=24 over thestructure area to measure the fraction of early-type objects as a function oflocal projected density Sigma_10, based on photometric redshifts derived fromground-based deep multi-band photometry. We recover a robust morphology-densityrelation at this redshift, indicating, for comparable local densities, asmaller fraction of early-type galaxies than today. Interestingly, thisdifference is less strong at the highest densities and becomes more severe inintermediate environments. We also find, however, local "inversions'' of theobserved global relation, possibly driven by the large-scale environment. Inparticular, we find direct correspondence of a large concentration of diskgalaxies to (the colder side of) a possible shock region detected in the X-raytemperature map and surface brightness distribution of the dominant cluster. Weinterpret this as potential evidence of shock-induced star formation inexisting galaxy disks, during the ongoing merger between two sub-clusters.