Papers › COSMOS2020: Exploring the dawn of quenching for massive galaxies at 3 < z < 5 with a...
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
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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 (NUV-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≥3 in SHARK, the GMM/NUVU-VJ method out-performs classical rest-frame UVJ 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 UVJ and NUVrJ selected samples. We find that although the new selection matches UVJ and NUVrJ in number, the overlap between colour selections is only ∼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 NUV flux indicating recent quenching. We find the number densities of the entire quiescent population (including post-starbursts) more than doubles from 3.5±2.2×10⁻⁶ Mpc⁻³ at 4<z<5 to 1.4±0.4×10⁻⁵ Mpc⁻³ at 3<z<4, confirming that the onset of massive galaxy quenching occurs as early as 3<z<5.
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