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We present the discovery of two galaxy overdensities in the Hubble SpaceTelescope UDF: a proto-cluster, HUDFJ0332.4-2746.6 at $z = 1.84 \pm 0.01$, anda group, HUDFJ0332.5-2747.3 at $z =1.90 \pm 0.01$. Assuming viralization, thevelocity dispersion of HUDFJ0332.4-2746.6 implies a mass of $M_{200}= (2.2 \pm1.8) \times 10^{14} M_{\odot}$, consistent with the lack of extended X-rayemission. Neither overdensity shows evidence of a red sequence. About $50\%$ oftheir members show interactions and/or disturbed morphologies, which aresignatures of merger remnants or disk instability. Most of their ETGs have bluecolors and show recent star-formation. These observations reveal for the firsttime large fractions of spectroscopically confirmed star-forming blue ETGs inproto-clusters at $z\approx 2$. These star-forming ETGs are most likely amongthe 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 theselower redshifts, their size would evolve according to a similar mass-sizerelation. It is noteworthy that quiescent ETGs in clusters at $z=1.8-2$ also donot show any significant size evolution over this redshift range, contrary tofield ETGs. The ETG fraction is $\lesssim 50\%$, compared to the typicalquiescent ETG fraction of $\approx 80\%$ in cluster cores at $z< 1$. Thefraction, masses, and colors of the newly discovered ETGs imply that othercluster ETGs will be formed/accreted at a later time.

astrophysical journal/the astrophysical journal

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