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تسجيل مستخدم جديدLARgE Survey -- II. The Dark Matter Halos and the Progenitors and Descendants of Ultra-Massive Passive Galaxies at Cosmic Noon
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We use a 27.6 deg$^2$ survey to measure the clustering of $gzK_s$-selected quiescent galaxies at $zsim1.6$, focusing on ultra-massive quiescent galaxies. We find that $zsim1.6$ Ultra-Massive Passively Evolving Galaxies (UMPEGs), which have $K_s(AB)<19.75$ (stellar masses of $M_{stars}$ $>sim 10^{11.4}M_{odot}$ and mean $<$$M_{stars}$$>$ = $10^{11.5}M_{odot}$), cluster more strongly than any other known galaxy population at high redshift. Comparing their correlation length, $r_0 = 29.77 pm 2.75$ $ h^{-1}$Mpc, with the clustering of dark matter halos in the Millennium XXL N-body simulation suggests that these $zsim1.6$ UMPEGs reside in dark matter halos of mass $M_{h}sim10^{14.1}h^{-1}M_{odot}$. Such very massive $zsim1.6$ halos are associated with the ancestors of $zsim0$ massive galaxy clusters such as the Virgo and Coma clusters. Given their extreme stellar masses and lack of companions with comparable mass, we surmise that these UMPEGs could be the already-quenched central massive galaxies of their (proto)clusters. We conclude that with only a modest amount of further growth in their stellar mass, $zsim1.6$ UMPEGs could be the progenitors of some of the massive central galaxies of present-day massive galaxy clusters observed to be already very massive and quiescent near the peak epoch of the cosmic star formation.
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We study the environments of a sample of 61 extremely rare z~1.6 Ultra-Massive Passively Evolving Galaxies (UMPEGs: stellar masses M_stars >10^11.5 M_sun) which -- based on clustering analysis presented in Cheema et al. (2020) -- appear to be associa
ted with very massive (M_halo ~ 10^14.1 h^-1 M_sun) dark matter halos that are likely to be the progenitors of z~0 massive (Coma- and Virgo-like) galaxy clusters. We find that UMPEGs on average have fewer than one satellite galaxy with mass ratio M_sat : M_UMPEG >~ 1:5 (i.e., M_sat >~ 10^10.8 M_sun) within 0.5 Mpc; the large mass gap that we observe between the typical UMPEG and its most massive satellite implies that the z~1.6 UMPEGs assembled through major mergers. Using observed satellite counts with merger timescales from the literature, we estimate the growth rate due to mergers with mass ratio of >~ 1:4 to be ~13% Gyr^-1 (with a ~2x systematic uncertainty). This relatively low growth rate is unlikely to significantly affect the shape of the massive end of the stellar mass function, whose evolution must instead be driven by the quenching of new cohorts of ultra-massive star-forming galaxies. However, this growth rate is high enough that, if sustained to z~0, the typical z~1.6 M_UMPEG=10^11.6 M_sun UMPEG can grow into a M_stars~10^12 M_sun brightest cluster galaxy (BCG) of a present-day massive galaxy cluster. Our observations favour a scenario in which our UMPEGs are main-branch progenitors of some of the present-day BCGs that have first assembled through major mergers at high redshifts and grown further through (likely minor) merging at later times.
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