A physically motivated galaxy size definition across different state-of-the-art hydrodynamical simulations

TL;DR

This paper introduces a physically motivated galaxy size definition based on the stellar mass density contour at 1 Msun pc^-2, demonstrating its robustness and consistency across different simulations and redshifts, and its potential as a reliable observational tracer.

Contribution

It proposes and validates a new galaxy size metric using hydrodynamical simulations, showing its consistency and independence from galaxy formation model variations.

Findings

The R_1-M_star relation is consistent across galaxy masses and redshifts.

The size-stellar mass relation has low scatter and is robust across different simulation suites.

R_1 correlates strongly with the galaxy's total mass, indicating its potential as a dynamical mass tracer.

Abstract

Galaxy sizes are a key parameter to distinguishing between different galaxy types and morphologies, reflecting their formation and assembly histories. Several methods define galaxy boundaries, often relying on light concentration or isophotal densities. However, these approaches were often constrained by observational limitations and did not necessarily provide a clear physical boundary for galaxy outskirts. With modern deep imaging surveys, a new physically motivated definition has emerged using the radial position of the star formation threshold as the galaxy size, approximated by the stellar mass density contour at 1 Msun pc^-2 (R_1). We test this definition using three state-of-the-art hydrodynamical simulation suites, analyzing stellar surface density profiles across a wide range of stellar masses and redshifts. We measure the galaxy sizes according to this new definition and compare them with the most traditional size metric, the stellar half-mass radius. Our analysis demonstrates that the R_1-M_star relation exhibits consistent behaviour across both low and high-stellar mass galaxies, with remarkably low scatter. This relation is independent of redshift and holds across the three different cosmological hydrodynamical simulation suites, highlighting its robustness to variations in galaxy formation models. Furthermore, we explore the connection between a galaxy's total mass within R1 and its stellar mass, finding very little scatter in this relation. This suggests that R1 could serve as a reliable observational tracer for the galaxy's dynamical mass. The size-stellar mass relation proposed provides a reliable and physically motivated method for defining the outskirts of galaxies. This method remains consistent not only at z=0 but also throughout the evolutionary history of galaxies, offering a robust and meaningful framework for galaxy evolution studies.