Formation of massive disk galaxies in the IllustrisTNG simulation

TL;DR

This study uses the IllustrisTNG simulation to explore how massive disk galaxies form and survive through cosmic time, highlighting the importance of merger orbit types in determining galaxy morphology.

Contribution

It reveals the significant role of merger orbit types in shaping galaxy morphology and identifies conditions under which massive disk galaxies maintain their structure.

Findings

8.4% of massive disks have quiet merger histories.

Major merger orbit type strongly influences remnant morphology.

Major mergers with spiral-in orbits tend to produce disk remnants.

Abstract

We investigate the formation history of massive disk galaxies in hydro-dynamical simulation--the IllustrisTNG, to study why massive disk galaxies survive through cosmic time. 83 galaxies in the simulation are selected with M$_{*,z=0}$ $>8\times10^{10}$ M$_\odot$ and kinematic bulge-to-total ratio less than $0.3$. We find that 8.4 percent of these massive disk galaxies have quiet merger histories and preserve disk morphology since formed. 54.2 percent have a significant increase in bulge components in history, then become disks again till present time. The rest 37.3 percent experience prominent mergers but survive to remain disky. While mergers and even major mergers do not always turn disk galaxies into ellipticals, we study the relations between various properties of mergers and the morphology of merger remnants. We find a strong dependence of remnant morphology on the orbit type of major mergers. Specifically, major mergers with a spiral-in falling orbit mostly lead to disk-dominant remnants, and major mergers of head-on galaxy-galaxy collision mostly form ellipticals. This dependence of remnant morphology on orbit type is much stronger than the dependence on cold gas fraction or orbital configuration of merger system as previously studied.