The PARADIGM project I: How early merger histories shape the present-day sizes of Milky-Way-mass galaxies

TL;DR

This study uses genetically modified initial conditions in two cosmological simulations to explore how early merger histories influence the present-day sizes of Milky-Way-mass galaxies, revealing robust trends despite differences in simulation models.

Contribution

It introduces the PARADIGM project, demonstrating how varying early merger mass ratios affect galaxy sizes and morphology across two different simulation codes.

Findings

Smaller early merger mass ratios lead to larger galaxy sizes at z=0.

Size normalization differs significantly between the two simulation codes.

Both simulations reproduce observed size-mass relations despite differences.

Abstract

How mergers affect galaxy formation depends on both feedback processes, and on the geometry and strength of the mergers themselves. We introduce the PARADIGM project, where we study the response of a simulated Milky-Way-mass galaxy ($M_{\rm 200c} \sim 10^{12}M_{\odot}$ at $z=0$) forming in a cosmological setting to differing merger histories, using genetically modified initial conditions, each simulated with the VINTERGATAN and IllustrisTNG codes. While VINTERGATAN has been developed with an emphasis on resolving the cold interstellar medium, IllustrisTNG uses a subgrid two-phase model and consequently scales to large volume simulations, making them ideal to examine complementary views on how merger histories and feedback interact. Our genetic modifications alter the mass ratio of an important $z \approx 2$ merger while maintaining the halo's $z=0$ mass. Whether simulated with VINTERGATAN or IllustrisTNG, smaller mass ratios for this early merger result in larger galaxies at $z=0$, due to a greater build up of a kinematically cold disc. We conclude that such broad trends are robustly reproducible; however, the normalization of the resulting stellar sizes is substantially different in the two codes (ranging between $0.5-1.7\ \rm{kpc}$ for VINTERGATAN but $1.3-7.0\ \rm{kpc}$ for IllustrisTNG). The VINTERGATAN galaxies systematically form stars earlier, leading to a larger bulge component. Despite the difference in size normalization, both simulation suites lie on the observed size-mass relation for their respective morphological types. In light of these results, we discuss the interplay between internal processes and large scale gravitational interactions and gas accretion, and how the two galaxy models converge on similar emergent trends but along different evolutionary pathways.