JKCS041: a Coma cluster progenitor at z=1.803
  Using deep two-color near-infrared HST imaging and unbiased grism
spectroscopy we present a detailed study of the z=1.803 JKCS041 cluster.
Uniquely, for a high redshift cluster, we confirm a mass of $\log M=14.2$ in
solar units using four different techniques based on the X-ray temperature, the
X-ray luminosity 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.
Remarkably, we find a prominent red sequence down to stellar masses as low as
$\log M=9.8$, corresponding to a mass range of 2 dex. These quiescent galaxies
are concentrated around the cluster center with a core radius of 330 kpc. Blue
members are few and avoid the cluster center. In JKCS041 quenching was
therefore largely completed by a look-back time of 10 Gyr and we can constrain
the epoch at which this occurred via spectroscopic age-dating of the individual
galaxies. Most galaxies were quenched about 1.1 Gyr prior to the epoch of
observation. The less massive quiescent galaxies are somewhat younger,
corresponding to a decrease in age of 650 Myr per mass dex, but the scatter in
age at fixed mass is only 380 Myr (at $\log M=11$). The size-mass relation of
quiescent galaxies in JKCS041 is consistent with that observed for local
clusters within our uncertainties. 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 10 Gyr, 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 10 Gyr,
it is already remarkably similar to present-day clusters.
