Star-forming blue ETGs in two newly discovered galaxy overdensities in
  the HUDF at z=1.84 and 1.9: unveiling the progenitors of passive ETGs in
  cluster cores
  We present the discovery of two galaxy overdensities in the Hubble Space
Telescope UDF: a proto-cluster, HUDFJ0332.4-2746.6 at $z = 1.84 \pm 0.01$, and
a group, HUDFJ0332.5-2747.3 at $z =1.90 \pm 0.01$. Assuming viralization, the
velocity dispersion of HUDFJ0332.4-2746.6 implies a mass of $M_{200}= (2.2 \pm
1.8) \times 10^{14} M_{\odot}$, consistent with the lack of extended X-ray
emission. Neither overdensity shows evidence of a red sequence. About $50\%$ of
their members show interactions and/or disturbed morphologies, which are
signatures of merger remnants or disk instability. Most of their ETGs have blue
colors and show recent star-formation. These observations reveal for the first
time large fractions of spectroscopically confirmed star-forming blue ETGs in
proto-clusters at $z\approx 2$. These star-forming ETGs are most likely among
the progenitors of the quiescent population in clusters at more recent epochs.
Their mass-size relation is consistent with that of passive ETGs in clusters at
$z\sim0.7-1.5$. If these galaxies are the progenitors of cluster ETGs at these
lower redshifts, their size would evolve according to a similar mass-size
relation. It is noteworthy that quiescent ETGs in clusters at $z=1.8-2$ also do
not show any significant size evolution over this redshift range, contrary to
field ETGs. The ETG fraction is $\lesssim 50\%$, compared to the typical
quiescent ETG fraction of $\approx 80\%$ in cluster cores at $z< 1$. The
fraction, masses, and colors of the newly discovered ETGs imply that other
cluster ETGs will be formed/accreted at a later time.
