2015ApJ...804..117M

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 ± 0.01, and a group, HUDFJ0332.5-2747.3 at z = 1.90 ±0.01. Assuming viralization, the velocity dispersion of HUDFJ0332.4-2746.6 implies a mass of M₂₀₀ = (2.2 ± 1.8) x 10¹⁴M_☉, 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 ≈ 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 ∼ 0.7 -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 < e 50%, compared to the typical quiescent ETG fraction of ≈ 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.