Quenched, bulge-dominated, but dynamically cold galaxies in IllustrisTNG
  and their real-world counterparts
  Galaxy morphologies, kinematics, and stellar populations are thought to be
linked to each other. However, both simulations and observations have pointed
out mismatches therein. In this work, we study the nature and origin of the
present-day quenched, bulge-dominated, but dynamically cold galaxies within a
stellar mass range of
$10.3\,\leqslant\,\log\,M_{\ast}/\mathrm{M_{\odot}}\,\leqslant\,11.2$ in the
IllustrisTNG-100 Simulation. We compare these galaxies with a population of
normal star-forming dynamically cold disc galaxies and a population of normal
quenched dynamically hot elliptical galaxies within the same mass range. The
populations of the present-day quenched and bulge-dominated galaxies (both
being dynamically cold and hot) used to have significantly higher
star-formation rates and flatter morphologies at redshift of $z\sim 2$. They
have experienced more frequent larger mass-ratio mergers below $z \sim 0.7$ in
comparison to their star-forming disc counterparts, which is responsible for
the formation of their bulge-dominated morphologies. The dynamically cold
populations (both being star-forming and quenched) have experienced more
frequent prograde and tangential mergers especially below $z \sim 1$, in
contrast to the dynamically hot ellipticals, which have had more retrograde and
radial mergers. Such different merging histories can well explain the
differences on the cold and hot dynamical status among these galaxies. We point
out that the real-world counterparts of these dynamically cold and hot
bulge-dominated quenched populations are the fast- and slow-rotating early-type
galaxies, respectively, as seen in observations and hence reveal the different
evolution paths of these two distinct populations of early-type galaxies.
