Formation of galaxies in {\Lambda}CDM cosmologies. I. The fine structure of disc galaxies

TL;DR

This paper analyzes the detailed structure of simulated disc galaxies in a b5CDM universe, identifying their components, properties, and formation processes, and compares them with observational data.

Contribution

It provides a comprehensive analysis of the formation and structure of disc galaxies in b5CDM simulations, including the identification of spheroid, thin, and thick discs, and their properties.

Findings

Simulated galaxies have photometric and kinematic properties consistent with observations.

The spheroid is mainly old, metal-poor, and b1 ext{-}enhanced.

The thick disc contains about 16% of stellar mass and shows intermediate properties.

Abstract

We present a detailed analysis of the global and fine structure of four middle-mass disc galaxies obtained from simulations in a $\Lambda$CDM scenario. These objects have photometric D/T ratios in good agreement with those observed for late-type spirals, as well as kinematic properties in agreement with the observational Tully-Fisher relation. We identify the different dynamical components at z=0 on the basis of both orbital parameters and the binding energy of stars in the galaxy. In this way, we recognize a slowly rotating centrally concentrated spheroid, and two disc components supported by rotation: a thin disc with stars in nearly circular orbits, and a thick disc with orbital parameters transitional between the thin disc and the spheroid. The spheroidal component is composed mainly by old, metal-poor and {\alpha}-enhanced stars. The distribution of metals in this component shows, however, a clear bimodality with a low-metallicity peak, which could be related to a classical bulge, and a high-metallicity peak, which could be related to a pseudo-bulge. The thin disc appears in our simulations as the youngest and most metal-rich component. The radial distribution of ages and colours in this component are U-shaped: the new stars are forming in the inner regions, and then migrate through secular processes. Finally, we also find a thick disc containing about 16% of the total stellar mass and with properties that are intermediate between those of the thin disc and the spheroid. Its low-metallicity stars are {\alpha}-enhanced when compared to thin disc stars of the same metallicity. The structural parameters (e.g., the scale height) of the simulated thick discs suggest that such a component could result from the combination of different thickening mechanisms that include merger-driven processes, but also long-lived internal perturbations of the thin disc. [Abridged]