NIHAO VI. The hidden discs of simulated galaxies

Abstract

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