Milky Way-like galaxies: stellar population properties of dynamically defined discs, bulges and stellar haloes

Abstract

The formation of galaxies can be understood in terms of the assembly patterns of each type of galactic component. To perform this kind of analysis, it is necessary to define some criteria to separate those components. Decomposition methods based on dynamical properties are more physically motivated than photometry-based ones. We use the unsupervised Gaussian Mixture model of galactic structure finder to extract the components of a sub-sample of galaxies with Milky Way-like masses from the eagle simulations. A clustering in the space of first- and second-order dynamical moments of all identified substructures reveals five types of galaxy components: thin and thick discs, stellar haloes, bulges and spheroids. We analyse the dynamical, morphological and stellar population (SP) properties of these five component types, exploring to what extent these properties correlate with each other, and how much they depend on the total galaxy stellar and dark matter halo masses. All galaxies contain a bulge, a stellar halo and a disc. In total, 60 per cent of objects host two discs (thin and thick), and 68 per cent host also a spheroid. The dynamical disc-to-total ratio does not depend on stellar mass, but the median rotational velocities of the two discs do. Thin discs are well separated in stellar ages, [Fe/H] and α-enhancement from the three dispersion-dominated components, while thick discs are in between. Except for thin discs, all components show correlations among their SP properties: older ages mean lower metallicities and larger α-enhancement. Finally, we quantify the weak dependence of SP properties on each component's dynamics.