The first quiescent galaxies in TNG300.

We identify the first quiescent galaxies in TNG300, the largest volume of the IllustrisTNG cosmological simulation suite, and explore their quenching processes and time evolution to z  = 0. We find that the first quiescent galaxies with stellar masses M _* > 3 × 10¹⁰ M_⊙ and specific star formation rates sSFR < 10⁻¹¹ yr⁻¹ emerge at z ∼ 4.2 in TNG300. Suppression of star formation in these galaxies begins with a thermal mode of active galactic nucleus feedback at z ∼ 6, and a kinetic feedback mode acts in each galaxy by z ∼ 4.7 to complete the quenching process, which occurs on a time-scale of ∼0.35 Gyr. Surprisingly, we find that the majority of these galaxies are not the main progenitors of their z  = 0 descendants; instead, four of the five galaxies fall into more massive galaxies in subsequent mergers at a range of redshifts 2.5 < z < 0.2. By z  = 0, these descendants are the centres of galaxy clusters with average stellar masses of 8 × 10¹¹ M_⊙. We make predictions for the first quenched galaxies to be located by the JWST. [ABSTRACT FROM AUTHOR]