2014A&A...565A.120A

Using deep two-color near-infrared HST imaging and unbiased grism spectroscopy, we present a detailed study of the z=1.803 JKCS041 cluster. We confirm, for the first time for a high-redshift cluster, a mass of logM≥14.2 in solar units using four different techniques based on the X-ray temperature, the X-ray luminosity, the gas mass, and the cluster richness. JKCS041 is thus a progenitor of a local system like the Coma cluster. Our rich dataset and the abundant population of 14 spectroscopically confirmed red-sequence galaxies allows us to explore the past star formation history of this system in unprecedented detail. Our most interesting result is a prominent red sequence down to stellar masses as low as logM/M_☉=9.8, corresponding to a mass range of 2dex. These quiescent galaxies are concentrated around the cluster center with a core radius of 330 kpc. There are only few blue members and avoid the cluster center. In JKCS041 quenching was therefore largely completed by a look-back time of 10Gyr, and we can constrain the epoch at which this occurred via spectroscopic age-dating of the individual galaxies. Most galaxies were quenched about 1.1Gyr prior to the epoch of observation. The less-massive quiescent galaxies are somewhat younger, corresponding to a decrease in age of 650Myr per mass dex, but the scatter in age at fixed mass is only 380Myr (at logM/M_☉=11). There is no evidence for multiple epochs of star formation across galaxies. The size-mass relation of quiescent galaxies in JKCS041 is consistent with that observed for local clusters within our uncertainties, and we place an upper limit of 0.4dex on size growth at fixed stellar mass (95% confidence). Comparing our data on JKCS041 with 41 clusters at lower redshift, we find that the form of the mass function of red sequence galaxies has hardly evolved in the past 10Gyr, both in terms of its faint-end slope and characteristic mass. Despite observing JKCS041 soon after its quenching and the three-fold expected increase in mass in the next 10Gyr, it is already remarkably similar to present-day clusters.