COSMOS2020: Discovery of a protocluster of massive quiescent galaxies at $z=2.77$

分类： 天文学 >> 天文学 提交时间： 2023-02-19

Kei Ito Masayuki Tanaka Francesco Valentino Sune Toft Gabriel Brammer Katriona M. L. Gould Olivier Ilbert Nobunari Kashikawa Mariko Kubo Yongming Liang Henry J. McCracken John R. Weaver

摘要： Protoclusters of galaxies have been found in the last quarter century. However, most of them have been found through the overdensity of star-forming galaxies, and there had been no known structures identified by multiple spectroscopically confirmed quiescent galaxies at $z>2.5$. In this letter, we report the discovery of an overdense structure of massive quiescent galaxies with the spectroscopic redshift $z=2.77$ in the COSMOS field, QO-1000. We first photometrically identify this structure as a $4.2\sigma$ overdensity with 14 quiescent galaxies in $7\times4\ {\rm pMpc^2}$ from the COSMOS2020 catalog. We then securely confirm the spectroscopic redshifts of 4 quiescent galaxies by detecting multiple Balmer absorption lines with Keck/MOSFIRE. All the spectroscopically confirmed members are massive ($\log{(M_\star/M_\odot)}>11.0$) and located in a narrow redshift range ($2.7668$ times denser than in the general field. In addition, we confirm that they form a red sequence in the $J-K_s$ color. This structure's halo mass is estimated as $\log{(M_{\rm halo}/M_\odot)}>13.2$ from their stellar mass. Similar structures found in the IllustrisTNG simulation are expected to evolve into massive galaxy clusters with $\log{(M_{\rm halo}/M_\odot)}\geq14.8$ at $z=0$. These results suggest that QO-1000 is a more mature protocluster than the other known protoclusters. It is likely in a transition phase between the star-forming protoclusters and the quenched galaxy clusters.

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