# Across the green valley with HST grisms: colour evolution, crossing time-scales, and the growth of the red sequence at z = 1.0-1.8

Abstract : We measure the colour evolution and quenching time-scales of z = 1.0-1.8 galaxies across the green valley. We derive rest-frame NUVrK colours and select blue-cloud, green-valley, and red-sequence galaxies from the spectral energy distribution modelling of CANDELS GOODS-South and UDS multiband photometry. Separately, we constrain the star-formation history (SFH) parameters (ages, τ) of these galaxies by fitting their deep archival HST grism spectroscopy. We derive the galaxy colour-age relation and show that only rapidly evolving galaxies with characteristic delayed-τ SFH time-scales of <0.5 Gyr reach the red sequence at these redshifts, after a period of accelerated colour evolution across the green valley. These results indicate that the stellar mass build-up of these galaxies stays minimal after leaving the blue cloud and entering the green valley (i.e. it may represent $\lesssim 5{{\ \rm per\ cent}}$ of the galaxies' final, quiescent masses). Visual inspection of age-sensitive features in the stacked spectra also supports the view that these galaxies follow a quenching sequence along the blue-cloud → green-valley → red-sequence track. For this rapidly evolving population, we measure a green-valley crossing time-scale of $0.99^{+0.42}_{-0.25}$ Gyr and a crossing rate at the bottom of the green valley of $0.82^{+0.27}_{-0.25}$ mag Gyr-1. Based on these time-scales, we estimate that the number density of massive (M > 1010M) red-sequence galaxies doubles every Gyr at these redshifts, in remarkable agreement with the evolution of the quiescent galaxy stellar mass function. These results offer a new approach to measuring galaxy quenching over time and represent a pathfinder study for future JWST, Euclid, and Roman Space Telescope programs.
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Journal articles

https://hal-insu.archives-ouvertes.fr/insu-03667425
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Submitted on : Friday, May 13, 2022 - 1:52:07 PM
Last modification on : Sunday, May 15, 2022 - 3:03:26 AM

### Citation

Gaël Noirot, Marcin Sawicki, Roberto Abraham, Maruša Bradač, Kartheik Iyer, et al.. Across the green valley with HST grisms: colour evolution, crossing time-scales, and the growth of the red sequence at z = 1.0-1.8. Monthly Notices of the Royal Astronomical Society, 2022, 512, pp.3566-3588. ⟨10.1093/mnras/stac668⟩. ⟨insu-03667425⟩

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