An analytical model to kinematically identify thin discs in MaNGA
  galaxies
  We present an analytical model to identify thin discs in galaxies, and apply
this model to a sample of SDSS MaNGA galaxies. This model fits the velocity and
velocity dispersion fields of galaxies with regular kinematics. By introducing
two parameters $\zeta$ related to the comparison of the model's asymmetric
drift correction to the observed gas kinematics and $\eta$ related to the
dominant component of a galaxy, we classify the galaxies in the sample as
"disc-dominated", "non-disc-dominated", or "disc-free" indicating galaxies with
a dominating thin disc, a non-dominating thin disc, or no thin disc detection
with our method, respectively. The dynamical mass resulting from our model
correlates with stellar mass, and we investigate discrepancies by including gas
mass and variation of the initial mass function. As expected, most spiral
galaxies in the sample are disc-dominated, while ellipticals are predominantly
disc-free. Lenticular galaxies show a dichotomy in their kinematic
classification, which is related to their different star formation rates and
gas fractions. We propose two possible scenarios to explain these results. In
the first scenario, disc-free lenticulars formed in more violent processes than
disc-dominated ones, while in the second scenario, the quenching processes in
lenticulars lead to a change in their kinematic structures as disc-dominated
lenticulars evolve to disc-free ones.
