Quenching, Mergers and Age Profiles for z=2 Galaxies in IllustrisTNG
  Using the IllustrisTNG cosmological galaxy formation simulations, we analyze
the physical properties of young quiescent galaxies at z=2 with stellar masses
above 10^10.5 solar masses. This key population provides an unaltered probe
into the evolution of galaxies from star-forming to quiescent and has been
recently targeted by several observational studies. Young quiescent galaxies in
the simulations do not appear unusually compact, in tension with observations,
but they show unique age gradients that are qualitatively consistent with the
observed color gradients. In particular, more than half of the simulated young
quiescent galaxies show positive age profiles due to recent intense central
starbursts, which are triggered by significant mergers. Yet, there is a sizable
population of recently quenched galaxies without significant mergers and with
flat age profiles. Our results suggest that mergers play a fundamental role in
structural transformation, but are not the only available pathway to quench a
z=2 galaxy.
