A Massive Quiescent Galaxy in a Group Environment at z = 4.53

Takumi Kakimoto*, Masayuki Tanaka, Masato Onodera, Rhythm Shimakawa, Po Feng Wu, Katriona M.L. Gould, Kei Ito, Shuowen Jin, Mariko Kubo, Tomoko L. Suzuki, Sune Toft, Francesco Valentino, Kiyoto Yabe

*Corresponding author for this work

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We report on the spectroscopic confirmation of a massive quiescent galaxy at z spec = 4.53 in the COSMOS field. The object was first identified as a galaxy with suppressed star formation at z phot ∼ 4.65 from the COSMOS2020 catalog. The follow-up spectroscopy with Keck/MOSFIRE in the K band reveals faint [O ii] emission and the Balmer break, indicative of evolved stellar populations. We fit the spectral energy distribution using photometry and a spectrum to infer physical properties. The obtained stellar mass is high (M * ∼ 1010.8 M ) and the current star formation rate is more than 1 dex below that of main-sequence galaxies at z = 4.5. Its star formation history suggests that this galaxy experienced rapid quenching from z ∼ 5. The galaxy is among the youngest quiescent galaxies confirmed so far at z spec > 3 with z form ∼ 5.2 (200 Myr ago), which is the epoch when 50% of the total stellar mass was formed. A unique aspect of the galaxy is that it is in an extremely dense region; there are four massive star-forming galaxies at 4.4 < z phot < 4.7 located within 150 physical kpc from the galaxy. Interestingly, three of them have virial radii that strongly overlap with that of the central quiescent galaxy (∼70 kpc), suggesting that the overdensity region is likely the highest-redshift candidate of a dense group with a spectroscopically confirmed quiescent galaxy at the center. The group provides us with a unique opportunity to gain insights into the role of the group environment in quenching at z ∼ 5, which corresponds to the formation epoch of massive elliptical galaxies in the local Universe.

Original languageEnglish
Article number49
JournalAstrophysical Journal
Issue number1
Number of pages10
Publication statusPublished - 2024


  • Galaxy evolution
  • High-redshift galaxies
  • Galaxy quenching
  • Quenched galaxies
  • Galaxy groups
  • Galaxy environments


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