The ATLAS3D project – XXV. Two-dimensional kinematic analysis of simulated galaxies and the cosmological origin of fast and slow rotators

Abstract

We present a detailed two-dimensional stellar dynamical analysis of a sample\nof 44 cosmological hydrodynamical simulations of individual central galaxies\nand their satellites. Kinematic maps of the stellar line-of-sight velocity,\nvelocity dispersion, and higher-order Gauss-Hermite moments $h_3$ and $h_4$ are\nconstructed for each central galaxy and for the most massive satellites. The\namount of rotation is quantified using the $\\lambda_{\\mathrm{R}}$-parameter.\nThe velocity, velocity dispersion, $h_3$, and $h_4$ fields of the simulated\ngalaxies show a diversity similar to observed kinematic maps of early-type\ngalaxies in the ATLAS$^{\\rm{3D}}$ survey. This includes fast (regular), slow,\nand misaligned rotation, hot spheroids with embedded cold disk components as\nwell as galaxies with counter-rotating cores or central depressions in the\nvelocity dispersion. We link the present day kinematic properties to the\nindividual cosmological formation histories of the galaxies. In general, major\ngalaxy mergers have a significant influence on the rotation properties\nresulting in both a spin-down as well as a spin-up of the merger remnant. Lower\nmass galaxies with significant in-situ formation of stars, or with additional\ngas-rich major mergers - resulting in a spin-up - in their formation history,\nform elongated fast rotators with a clear anti-correlation of $h_3$ and\n$v/\\sigma$. An additional formation path for fast rotators includes gas poor\nmajor mergers leading to a spin-up of the remnants. This formation path does\nnot result in anti-correlated $h_3$ and $v/\\sigma$. The galaxies most\nconsistent with the rare class of non-rotating round early-type galaxies grow\nby gas-poor minor mergers alone. In general, more massive galaxies have less\nin-situ star formation since $z \\sim 2$, rotate slower and have older stellar\npopulations. (shortened)\n