Decoupling the rotation of stars and gas -- I: the relationship with
  morphology and halo spin
  We use a combination of data from the MaNGA survey and MaNGA-like
observations in IllustrisTNG100 to determine the prevalence of misalignment
between the rotational axes of stars and gas. This census paper outlines the
typical characteristics of misaligned galaxies in both observations and
simulations to determine their fundamental relationship with morphology and
angular momentum. We present a sample of ~4500 galaxies from MaNGA with
kinematic classifications which we use to demonstrate that the prevalence of
misalignment is strongly dependent on morphology. The misaligned fraction
sharply increases going to earlier morphologies (28$\pm$3% of 301 early-type
galaxies, 10$\pm$1% of 677 lenticulars and 5.4$\pm$0.6% of 1634 pure late-type
galaxies). For early-types, aligned galaxies are less massive than the
misaligned sample whereas this trend reverses for lenticulars and pure
late-types. We also find that decoupling depends on group membership for
early-types with centrals more likely to be decoupled than satellites. We
demonstrate that misaligned galaxies have similar stellar angular momentum to
galaxies without gas rotation, much lower than aligned galaxies. Misaligned
galaxies also have a lower gas mass than the aligned, indicative that gas loss
is a crucial step in decoupling star-gas rotation. Through comparison to a mock
MaNGA sample, we find that the strong trends with morphology and angular
momentum hold true in IllustrisTNG100. We demonstrate that the lowered angular
momentum is, however, not a transient property and that the likelihood of
star-gas misalignment at z = 0 is correlated with the spin of the dark matter
halo going back to z = 1.
