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We examine the quiescent fractions of massive galaxies in six z>3 spectroscopically confirmed protoclusters in the COSMOS field, one of which is newly confirmed and presented here. We report the spectroscopic confirmation of MAGAZ3NE J100143+023021 at by the Massive Ancient Galaxies At z>3 NEar-infrared (MAGAZ3NE) survey. MAGAZ3NE J100143+023021 contains a total of 79 protocluster members (28 spectroscopic and 51 photometric). Three spectroscopically confirmed members are star-forming ultramassive galaxies (UMGs; >11), the most massive of which has . Combining Keck/MOSFIRE spectroscopy and the COSMOS2020 photometric catalog, we use a weighted Gaussian kernel density estimator to map the protocluster and measure its total mass in the dense "core" region. For each of the six COSMOS protoclusters, we compare the quiescent fraction to the status of the central UMG as star-forming or quiescent. We observe that galaxies in these protoclusters appear to obey galactic conformity: Elevated quiescent fractions are found in protoclusters with UVJ-quiescent UMGs and low quiescent fractions are found in protoclusters containing UVJ star-frming UMGs. This correlation of star formation/quiescence in UMGs and the massive galaxies nearby in these protoclusters is the first evidence for the existence of galactic conformity at z>3. Despite disagreements over mechanisms behind conformity at low redshifts, its presence at these early cosmic times would provide strong constraints on the physics proposed to drive galactic conformity.
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Spectroscopic Confirmation of a Protocluster at z = 3.37 with a High Fraction of Quiescent Galaxies
Abstract We report the discovery of MAGAZ3NE J095924+022537, a spectroscopically confirmed protocluster at z = 3.3665 − 0.0012 + 0.0009 around a spectroscopically confirmed UVJ -quiescent ultramassive galaxy (UMG; M ⋆ = 2.34 − 0.34 + 0.23 × 10 11 M ⊙ ) in the COSMOS UltraVISTA field. We present a total of 38 protocluster members (14 spectroscopic and 24 photometric), including the UMG. Notably, and in marked contrast to protoclusters previously reported at this epoch that have been found to contain predominantly star-forming members, we measure an elevated fraction of quiescent galaxies relative to the coeval field ( 73.3 − 16.9 + 26.7 % versus 11.6 − 4.9 + 7.1 % for galaxies with stellar mass M ⋆ ≥ 10 11 M ⊙ ). This high quenched fraction provides a striking and important counterexample to the seeming ubiquitousness of star-forming galaxies in protoclusters at z > 2 and suggests, rather, that protoclusters exist in a diversity of evolutionary states in the early universe. We discuss the possibility that we might be observing either “early mass quenching” or nonclassical “environmental quenching.” We also present the discovery of MAGAZ3NE J100028+023349, a second spectroscopically confirmed protocluster, at a very similar redshift of z = 3.3801 − 0.0281 + 0.0213 . We present a total of 20 protocluster members, 12 of which are photometric and eight spectroscopic including a poststarburst UMG ( M ⋆ = 2.95 − 0.20 + 0.21 × 10 11 M ⊙ ). Protoclusters MAGAZ3NE J0959 and MAGAZ3NE J1000 are separated by 18′ on the sky (35 comoving Mpc), in good agreement with predictions from simulations for the size of “Coma”-type cluster progenitors at this epoch. It is highly likely that the two UMGs are the progenitors of Brightest Cluster Galaxies seen in massive virialized clusters at lower redshift.
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- Award ID(s):
- 1815475
- PAR ID:
- 10326888
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 926
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 37
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac2b9f
