NIHAO VI. The hidden discs of simulated galaxies
  Detailed studies of galaxy formation require clear definitions of the
structural components of galaxies. Precisely defined components also enable
better comparisons between observations and simulations. We use a subsample of
eighteen cosmological zoom-in simulations from the NIHAO project to derive a
robust method for defining stellar kinematic discs in galaxies. Our method uses
Gaussian Mixture Models in a 3D space of dynamical variables. The NIHAO
galaxies have the right stellar mass for their halo mass, and their angular
momenta and S\'ersic indices match observations. While the photometric
disc-to-total ratios are close to 1 for all the simulated galaxies, the
kinematic ratios are around ~0.5. Thus, exponential structure does not imply a
cold kinematic disc. Above log(M*)~9.5, the decomposition leads to thin discs
and spheroids that have clearly different properties, in terms of angular
momentum, rotational support, ellipticity, [Fe/H] and [O/Fe]. At log(M*)<9.5,
the decomposition selects discs and spheroids with less distinct properties. At
these low masses, both the discs and spheroids have exponential profiles with
high minor-to-major axes ratios, i.e. thickened discs.
