An extreme case of galaxy and cluster co-evolution at $z$=0.7
  We report the discovery of eMACSJ0252.4$-$2100 (eMACSJ0252), a massive and
highly evolved galaxy cluster at $z=0.703$. Our analysis of Hubble Space
Telescope imaging and VLT/MUSE and Keck/DEIMOS spectroscopy of the system finds
a high velocity dispersion of 1020$^{+180}_{-190}$ km s$^{-1}$ and a high (if
tentative) X-ray luminosity of $(1.2\pm 0.4)\times10^{45}$ erg s$^{-1}$
(0.1$-$2.4 keV). As extreme is the system's brightest cluster galaxy, a giant
cD galaxy that forms stars at a rate of between 85 and 300 M$_\odot$ yr$^{-1}$
and features an extended halo of diffuse [OII] emission, as well as evidence of
dust. Its most remarkable properties, however, are an exceptionally high
ellipticity and a radially symmetric flow of gas in the surrounding
intracluster medium, potential direct kinematic evidence of a cooling flow. A
strong-lensing analysis, anchored by two multiple-image systems with
spectroscopic redshifts, finds the best lens model to consist of a single
cluster-scale halo with a total mass of $(1.9\pm0.1)\times 10^{14}$ M$_\odot$
within 250 kpc of the cluster core and, again, an extraordinarily high
ellipticity of $e=0.8$. Although further, in-depth studies across the
electromagnetic spectrum (especially in the X-ray regime) are needed to
conclusively determine the dynamical state of the system, the properties
established so far suggest that eMACSJ0252 must have already been highly
evolved well before $z\sim 1$, making it a prime target to constrain the
physical mechanisms and history of the co-evolution or dark-matter halos and
baryons in the era of cluster formation.
