COSMOS2020: Exploring the dawn of quenching for massive galaxies at 3 < z < 5 with a new colour selection method

Katriona M. L. Gould; Gabriel Brammer; Francesco Valentino; Katherine E. Whitaker; John R. Weaver; Claudia del P. Lagos; Francesca Rizzo; Maximilien Franco; Bau-Ching Hseih; Olivier Ilbert; Shuowen Jin; Georgios Magdis; Henry J. McCracken; Bahram Mobasher; Marko Shuntov; Charles L. Steinhardt; Victoria Strait; Sune Toft

doi:10.3847/1538-3881/accadc

paper

COSMOS2020: Exploring the dawn of quenching for massive galaxies at 3 < z < 5 with a new colour selection method

arXiv:2302.10934 · doi:10.3847/1538-3881/accadc

Abstract

We select and characterise a sample of massive (log(M $_{*} /$ M $_{⊙}) > 10.6$ ) quiescent galaxies (QGs) at $3 < z < 5$ in the latest COSMOS2020 catalogue. QGs are selected using a new rest-frame colour selection method, based on their probability of belonging to the quiescent group defined by a Gaussian Mixture Model (GMM) trained on rest-frame colours ( $N U V - U, U - V, V - J$ ) of similarly massive galaxies at $2 < z < 3$ . We calculate the quiescent probability threshold above which a galaxy is classified as quiescent using simulated galaxies from the SHARK semi-analytical model. We find that at $z \geq 3$ in SHARK, the GMM/ $N U V U - V J$ method out-performs classical rest-frame $U V J$ selection and is a viable alternative. We select galaxies as quiescent based on their probability in COSMOS2020 at $3 < z < 5$ , and compare the selected sample to both $U V J$ and $N U V r J$ selected samples. We find that although the new selection matches $U V J$ and $N U V r J$ in number, the overlap between colour selections is only $\sim 50 - 80%$ , implying that rest-frame colour commonly used at lower redshifts selections cannot be equivalently used at $z > 3$ . We compute median rest-frame SEDs for our sample and find the median quiescent galaxy at $3 < z < 5$ has a strong Balmer/4000 Angstrom break, and residual $N U V$ flux indicating recent quenching. We find the number densities of the entire quiescent population (including post-starbursts) more than doubles from $3.5 \pm 2.2 \times 1 0^{- 6}$ Mpc $^{- 3}$ at $4 < z < 5$ to $1.4 \pm 0.4 \times 1 0^{- 5}$ Mpc $^{- 3}$ at $3 < z < 4$ , confirming that the onset of massive galaxy quenching occurs as early as $3 < z < 5$ .

19 pages, 10 figures + appendix. Accepted for publication in AJ. Both the GMM model and code to calculate quiescent probabilities from rest frame flux densities are made available online at https://github.com/kmlgould/GMM-quiescent

COSMOS2020: Exploring the dawn of quenching for massive galaxies at 3 < z < 5 with a new colour selection method

Abstract

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