# New Horizon: On the origin of the stellar disk and spheroid of field galaxies at $z=0.7$

## TL;DR

This study uses high-resolution cosmological simulations to investigate the origins and formation processes of stellar disks and spheroids in field galaxies at redshift 0.7, revealing how mass, accretion, and mergers influence galaxy morphology.

## Contribution

It provides new insights into the formation timeline of disks, the role of accretion and mergers, and the origin of spheroidal components, based on detailed kinematic decomposition.

## Key findings

- Massive galaxies start forming disks earlier, around redshift 1-2.
- Mergers and accretion influence disk formation and misalignment.
- Counter-rotating disks can form from misaligned gas infall, lasting over a Gyr.

## 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 $\rm 10^9\,M_{\odot}$ 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 $z\sim1-2$, while low-mass galaxies do after $z\sim1$. 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 ($\sim12.5\%$) 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.