Kavli Affiliate: Michael McDonald
| First 5 Authors: Ian McConachie, Gillian Wilson, Ben Forrest, Z. Cemile Marsan, Adam Muzzin
| Summary:
We examine the quiescent fractions of massive galaxies in six $zgtrsim3$
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 $z=3.122^{+0.007}_{-0.004}$ 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
ultra-massive galaxies ($log(M_{star}/{rm M}_odot)>11$; UMGs), the most
massive of which has $log(M_{star}/{rm M}_odot)=11.15^{+0.05}_{-0.06}$.
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 $2.25^{+1.55}_{-0.65}times10^{14}~{rm M}_{odot}$ 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-forming 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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