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

Abstract

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 $z\sim 0.7-1.5$. 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 $\lesssim 50\%$, 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.

33 pages, 13 figures. ApJ, in press

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