When did the first passive galaxies emerge? Passsive galaxies are common in the local Universe, but at high redshifts they are much rarer; understanding what leads to quiescence in the early Universe is key to understanding the processes leading to passivity at all times (Santini et al., 2021).
In (Lovell et al., 2023) we studied the passive population in FLARES. Due to the unique FLARES approach we can study very rare populations of galaxies at high redshift, and found passive populations of galaxies out to redshifts of 7-8. This passivity is due to AGN feedback in the EAGLE model.
Predicted number density of passive galaxies from FLARES + EAGLE compared to recent observational constraints.
A number of passive galaxies have now been detected at these early times, and FLARES is being used to understand these objects (missing reference).
References
2023
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First light and reionisation epoch simulations (FLARES) - VIII. The emergence of passive galaxies at z ≥ 5
Christopher C. Lovell, Will Roper, Aswin P. Vijayan, and 9 more authors
MNRAS, Nov 2023
ADS Bibcode: 2023MNRAS.525.5520L
Passive galaxies are ubiquitous in the local universe, and various physical channels have been proposed that lead to this passivity. To date, robust passive galaxy candidates have been detected up to z ≤ 5, but it is still unknown if they exist at higher redshifts, what their relative abundances are, and what causes them to stop forming stars. We present predictions from the first light and reionisation epoch simulations (FLARES), a series of zoom simulations of a range of overdensities using the EAGLE code. Passive galaxies occur naturally in the EAGLE model at high redshift, and are in good agreement with number density estimates from Hubble Space Telescope (HST) and early JWST results at 3 ≤ z ≤ 5. Due to the unique FLARES approach, we extend these predictions to higher redshifts, finding passive galaxy populations up to z ~ 8. Feedback from supermassive black holes is the main driver of passivity, leading to reduced gas fractions and star forming gas reservoirs. We find that passive galaxies at z ≥ 5 are not identified in the typical UVJ selection space due to their still relatively young stellar populations, and present new rest-frame selection regions. We also produce mock NIRCam and MIRI fluxes, and find that significant numbers of passive galaxies at z ≥ 5 should be detectable in upcoming wide surveys with JWST. Finally, we present JWST colour distributions, with new selection regions in the observer-frame for identifying these early passive populations.
2021
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The emergence of passive galaxies in the early Universe
P. Santini, M. Castellano, E. Merlin, and 10 more authors
A&A, Aug 2021
The emergence of passive galaxies in the early Universe results from the delicate interplay among the different physical processes responsible for their rapid assembly and the abrupt shut-down of their star formation activity. Investigating the individual properties and demographics of early passive galaxies improves our understanding of these mechanisms. In this work we present a follow-up analysis of the z > 3 passive galaxy candidates selected by Merlin et al. (2019, MNRAS, 490, 3309) in the CANDELS fields. We begin by first confirming the accuracy of their passive classification by exploiting their sub-millimetre emission to demonstrate the lack of ongoing star formation. Using archival ALMA observations we are able to confirm at least 61% of the observed candidates as passive. While the remainder lack sufficiently deep data for confirmation, we are able to validate the entire sample in a statistical sense. We then estimate the stellar mass function (SMF) of all 101 passive candidates in three redshift bins from z = 5 to z = 3. We adopt a stepwise approach that has the advantage of taking into account photometric errors, mass and selection completeness issues, as well as the Eddington bias, without any a posteriori correction. We observe a pronounced evolution in the SMF around z ∼ 4, indicating that we are witnessing the emergence of the passive population at this epoch. Massive (M > 10\textlessSUP\textgreater11\textless/SUP\textgreater M\textlessSUB\textgreater⊙\textless/SUB\textgreater) passive galaxies, only accounting for a small (< 10%) fraction of galaxies at z > 4, become dominant at later epochs. Thanks to a combination of photometric quality, sample selection, and methodology, we overall find a higher density of passive galaxies than in previous works. The comparison with theoretical predictions, despite a qualitative agreement (at least for some of the models considered), denotes a still incomplete understanding of the physical processes responsible for the formation of these galaxies. Finally, we extrapolate our results to predict the number of early passive galaxies expected in surveys carried out with future facilities.
