Using deep two-color near-infrared HST imaging and unbiased grism spectroscopy , we present a detailed study of the z = 1.803 JKCS 041 cluster .
We confirm , for the first time for a high–redshift cluster , a mass of \log M \gtrsim 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 .
JKCS 041 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 \log M / M _ { \odot } = 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 .
There are only few blue members and avoid the cluster center .
In JKCS 041 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 / M _ { \odot } = 11 ) .
There is no evidence for multiple epochs of star formation across galaxies .
The size–mass relation of quiescent galaxies in JKCS 041 is consistent with that observed for local clusters within our uncertainties , and we place an upper limit of 0.4 dex on size growth at fixed stellar mass ( 95 % confidence ) .
Comparing our data on JKCS 041 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 JKCS 041 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 .