The fate of disk galaxies in IllustrisTNG clusters
  We study the stellar morphological evolution of disc galaxies within clusters
in the TNG50 and TNG100 runs from the IllustrisTNG simulation suite. We select
satellites of masses $10^{9.7} \leq M_{*,z=0}/\text{M}_{\odot} \leq 10^{11.6}$
residing in clusters of masses $10^{14} \lesssim
M_{\text{200c,z=0}}/\text{M}_{\odot} \leq 10^{14.6}$ at $z=0$ and that were
discs at accretion according to a kinematic morphology indicator (the
circularity fraction). These are traced from the time of accretion to $z=0$ and
compared to a control sample of central galaxies mass-matched at accretion.
Most cluster discs become non-discy by $z=0$, in stark contrast with the
control discs, of which a significant fraction remains discy over the same
timescales. Cluster discs become non-discy accompanied by gas removal and star
formation quenching, loss of dark matter and little growth or a loss of stellar
mass. In contrast, control discs transform while also losing gas mass and
quenching, but growing significantly in dark matter and stellar mass. Most
cluster satellites change morphologies on similar timescales regardless of
stellar mass, in $\sim0.5-4$ Gyr after accretion. Cluster discs that
experienced more numerous and closer pericentric passages show the largest
change in morphology. Morphological change in all cases requires the presence
of a gravitational perturbation to drive stellar orbits to non-discy
configurations, along with gas removal/heating to prevent replenishment of the
disc through continued star-formation. For cluster discs, the perturbation is
impulsive tidal shocking at pericentres and not tidal stripping of outer disc
stellar material, whereas for control discs, a combination of mergers and AGN
feedback appears to be the key driving force behind morphological
transformations.
