The nature of high-redshift massive quiescent galaxies – searching for RUBIES-UDS-QG-z7 in Flares

ABSTRACT RUBIES-UDS-QG-z7 (RQG) is the earliest massive quiescent galaxy identified to date, inferred to have formed its abundant stellar mass in a single burst that ceases rapidly before $z\sim 8$. An object of such extreme nature challenges our understanding of galaxy formation, requiring rapid growth and quenching mechanisms only $0.6 \ \rm {Gyr}$ after the Big Bang and implying number densities $2 \ \rm {dex}$ higher than currently predicted by simulations. We use synthetic observables to identify analogous systems within the First Light and Reionization Epoch Simulations (Flares) and find two massive galaxies ($\mathrm{M}_{\ast }\gt 10^{9} \ \mathrm{{\rm M}_{\odot }}$) dominated by rapidly quenched bursts. One of these demonstrates excellent agreement with the inferred physical properties of RQG and implies a number density of analogous systems $\log _{10}(\mathrm{N_{Q}} \ / \ \mathrm{Mpc}^{-3}) = -7.92^{+0.52}_{-0.76}$. Beyond demonstrating that the current Flares model is capable of producing RQG-like systems, these analogues provide a laboratory within which to study the underlying physics. Their active galactic nuclei (AGNs) heat and expel gas, inducing rapid quenching and preventing timely rejuvenation. This causes above-average chemical enrichment at a given stellar mass, with supersolar levels predicted for RQG. These metallicities are underestimated by spectral energy distribution fitting and we show that $\alpha$-enhancement cannot be solely responsible. Degeneracies with age and dust attenuation appear to be the more likely causes. Tensions between observed and simulated number densities can be alleviated in part by considering systematics, but adjustments to AGN feedback, such as allowing super-Eddington accretion rates, may be required for full agreement.