From giant clumps to clouds -- II. The emergence of thick disc kinematics from the conditions of star formation in high redshift gas rich galaxies

Abstract

High redshift disc galaxies are more gas rich, clumpier, and more turbulent than local Universe galaxies. This early era of galaxy formation imprints the distribution and kinematics of the stars that we observe today, but it is not yet well established how. In this work, we use simulations of isolated Milky Way-mass disc galaxies to study how kinematic properties of stars change when varying the gas fraction. This allows us to quantify the roles played by internal processes, e.g. gas turbulence and gravitational scattering off massive gas clumps, in establishing the observed stellar velocity dispersions and orbital eccentricities. We find that models with gas fractions > 20 per cent feature a turbulent and clumpy interstellar medium (ISM), leading to zero-age stellar velocity dispersions ∼ 20 − 30 km s −1 and high mean orbital eccentricities. Low eccentricities cannot arise from these physical conditions. For gas fractions below 20 per cent, the ISM becomes less turbulent, with stellar velocity dispersions < 10 km s −1 , and nearly circular orbits for young stars. The turbulence present in gas-rich high redshift galaxies hence acts as a 'barrier' against the formation of thin discs. We compare our findings to the Milky Way's age-velocity dispersion relation and argue that velocity dispersions imprinted already at star formation by the ISM contribute significantly at all times. Finally, we show that observed orbital eccentricities in the Milky Way's thick and thin discs can be explained entirely as imprints by the star-forming ISM, rather than by mergers or secular processes.

From giant clumps to clouds -- III. The connection between star formation and turbulence in the ISM

“…star formation) of a galaxy is its gas fraction. By varying the initial gas fraction, at a fixed total disc mass, in the initial conditions, we model a 'low redshift' galaxy, with 𝑓 g = 10%, and the 'high redshift' counterpart, with 𝑓 g = 50% (see van Donkelaar et al 2021, for an explicit approach). These models are not necessarily analogues of the same galaxy, but they still capture the environment inside galaxies at different evolutionary stages.…”

“…Furthermore, for the galaxies without feedback the gas dominated the disc instability far longer. This makes sense from the perspective that stellar feedback heats up the gas, while the stars remain kinematically cold (see van Donkelaar et al 2021). Note that the decision to consider values corresponding to a marginally stable disc is motivated by Figure 10 (see also the discussion in Section 5.1), in which it is evident that the higher gas fraction galaxy does not go far below this threshold.…”

Dark matter cores in massive high-$z$ galaxies formed by baryonic clumps

“…For this scenario to work, some fraction of high-𝑧 galaxies with a DM core must be accompanied with their dense satellite galaxies. Giant clumps are also a key to understand the formation of rotating bulges in disk galaxies and thick disks (Noguchi 1999;Ceverino et al 2010;Inoue & Saitoh 2011, 2014van Donkelaar et al 2021). Dynamical friction drives the orbital decay of giant clumps and they can transform into the bulge and thick disk.…”