New Horizon: On the Origin of the Stellar Disk and Spheroid of Field Galaxies at z = 0.7

Abstract

The origin of the disk and spheroid of galaxies has been a key open question in understanding their morphology. Using the high-resolution cosmological simulation, New Horizon, we explore kinematically decomposed disk and spheroidal components of 144 field galaxies with masses greater than 10 9 M at z = 0.7. The origins of stellar particles are classified according to their birthplace (in situ or ex situ) and their orbits at birth. Before disk settling, stars form mainly through chaotic mergers between proto-galaxies and become part of the spheroidal component. When disk settling starts, we find that more massive galaxies begin to form disk stars from earlier epochs; massive galaxies commence to develop their disks at z ∼ 1 − 2, while low-mass galaxies do after z ∼ 1. The formation of disks is affected by accretion as well, as mergers can trigger gas turbulence or induce misaligned gas infall that prevents galaxies from forming co-rotating disk stars. The importance of accreted stars is greater in more massive galaxies, especially in developing massive spheroids. A significant fraction of the spheroids comes from the disk stars that are perturbed, which becomes more important at lower redshifts. Some (∼ 12.5%) of our massive galaxies develop counter-rotating disks from the gas infall misaligned with the existing disk plane, which can last for more than a Gyr until they become the dominant component, and flip the angular momentum of the galaxy in the opposite direction. The final disk-to-total ratio of a galaxy needs to be understood in relation to its stellar mass and accretion history. We quantify the significance of the stars with different origins and provide them as guiding values.

Using angular momentum maps to detect kinematically distinct galactic components

“…At a given component stellar mass, there is a clear trend (based on the median lines) which shows that disc components are younger and more metal-rich than the spheroid components, in agreement with previous simulations (e.g. Naab et al 2014;Park et al 2019;Rosito et al 2019;Wang et al 2019)…”

“…Similar behaviour is present for the stellar and gaseous components, however with an additional weak trend as the two become more anti-aligned. The alignment (or not) between disc and spheroid, and stellar and gaseous components is linked to environmental effects (Scannapieco et al 2009;Sales et al 2012;Aumer, White & Naab 2014;Garrison-Kimmel et al 2018;Park et al 2019), and we intend to further investigate this behaviour in a future work.…”

Predicting the observability of population III stars with ELT-HARMONI via the helium 1640 Å emission line

“…Our source for mock "observable" galaxies is the NEWHORIZON simulation, a high spatial resolution (∆x ∼ 35 pc 1 ), hydrodynamical, cosmological simulation run using the hydro+N -body, Adaptive Mesh Refinement (AMR) code RAMSES (Teyssier 2002). Here we give a brief overview of the simulation but direct the reader to Dubois et al (2020) for complete details (see also Park et al 2019). NEWHORIZON is a re-simulation of a field environment spherical region with a radius of 10 Mpc comoving which has been extracted from the Horizon-AGN simulation (Dubois et al 2014b;Kaviraj et al 2017).…”

“…The spectrum of each of these sources is determined by a number of factors including local gas density and the age of the star particle supplying photons, in short they are unique and combine to give each galaxy its own unique spectrum. Furthermore, the spectrum of each galaxy is shaped by its evolutionary history in the simulated universe of NEWHORIZON, which was designed to be a realistic model of our Universe (see Park et al 2019;Dubois et al 2020). Therefore the SSDCs and the spectrum produced after observation with HSIM are cosmologically constrained for a given IMF.…”

Physical properties and scaling relations of molecular clouds: the impact of star formation

“…An alternative method is to calculate the amount of molecular gas in a given region, from which to form stars (for example, see Krumholz et al 2009b). More recently there have been several simulations that only forming stars in regions where the gas is self-gravitating or unstable against gravitational collapse (for example see Park et al 2019;Dubois et al 2020).…”