Disc instability and bar formation: a detailed view from the
  IllustrisTNG
  Assessing in a simple manner the stability of disc galaxies against bar
formation is a considerable challenge, especially for semi-analytic models of
galaxy formation. Thus far, the standard approach to address this has been the
analytical criterion originally proposed by Efstathiou, Lake, and Negroponte
(ELN-criterion), although several works have questioned its reliability. Here
we assess the ELN-criterion performance by making use of z=0 barred and
unbarred disc galaxy samples extracted from TNG100 and TNG50 cosmological
hydrodynamical simulations. We first study the evolution of these two
populations, finding that barred galaxies generally assemble earlier, are more
stellar dominated in the central region and their discs are more massive and
compact than those of unbarred galaxies. We then apply the ELN-criterion to the
two populations, finding that it is capable of correctly identifying 75% and
80% of the barred and unbarred galaxies, respectively. We find that barred
galaxies not correctly identified by the criterion are galaxies with generally
more extended discs, higher spin values and with bars assembled at later times
with respect to the average of the barred sample. We also find an indication
that the bars in these galaxies developed during the interaction with a close
neighbor (external trigger) rather than being the consequence of the secular
growth of the disc. On the other hand, we find that the unbarred galaxies that
the ELN-criterion misclassifies as barred, have stellar discs similar to those
of barred galaxies but are less stellar dominated in the central region,
possibly because of their late formation time. Taken all together, our results
indicate that the ELN-criterion can robustly describe the stability of most
disc galaxies secularly evolving, whereas additional environmental criteria
should be possibly considered to account for interaction-induced bar formation.
