The nature and evolution of early massive quenched galaxies in the simba-C simulation

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ABSTRACT We examine the nature, origin, and fate of early ($z\ge 2$) massive ($M_\star \gt 10^{10}\,\mathrm{ M}_\odot$) quenched galaxies (EQGs) in a new $(100\,h^{-1}\,{\rm Mpc}^3)$ run of the simba-C galaxy formation model. We define ‘quenched’ to be $\gt 4\sigma$ below an iterative polynomial fit to the star-forming sequence (SFS), and find that simba-C produces EQGs as early as $z\sim 5$ and number densities agreeing with observations at $z\lesssim 3$ (though slightly low at $z\gtrsim 4$). Using a photometric-based EQG selection or a fixed specific star formation rate cut of $10^{-10}$ yr$^{-1}$ yields similar results. EQGs predominantly arise in central galaxies with stellar mass $M_\star \sim 10^{10.5-11.3}\,\rm{M}_\odot$, not necessarily the most massive systems. A Uniform Manifold Approximation and Projection shows that quenched galaxies have notably large black hole-to-stellar mass ratios, lower rotational support, and less dust, but are not atypical versus similar-mass non-EQGs in their environments, halo mass, or halo gas temperatures at the time of quenching. However, via galaxy tracking we show that the progenitor environments of EQGs are significantly more overdense than that of non-EQGs, which drives higher black hole mass fractions and stellar-to-halo mass ratios. This results in the Eddington ratio dropping sufficiently low for simba-C’s jet mode feedback to turn on, which quickly quenches the host galaxies. EQGs thus seem to be galaxies that grow their black holes quickly within highly dense environments, but end up in moderately dense environments where black hole feedback can quench effectively. We find that $\gtrsim 30~{{\ \rm per\ cent}}$ of EQGs rejuvenate, but the rejuvenating fraction drops quickly below $z\lesssim 2$. By $z=0$, it is difficult to distinguish the descendants of EQGs versus non-EQGs.