The physical origin of the mass–size relation and its scatter for disk galaxies

¹ Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, PR China
e-mail: dumin@xmu.edu.cn
² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
³ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
⁴ Key Laboratory for Computational Astrophysics, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

Received: 18 December 2023
Accepted: 25 March 2024

Abstract

Aims. In this study, we investigated the intricate interplay between internal (natural) and external (nurture) processes in shaping the scaling relationships between specific angular momentum (j_⋆), stellar mass (M_⋆), and the size of disk galaxies within the IllustrisTNG simulation.

Methods. Using a kinematic decomposition of simulated galaxies, we focus on galaxies with tiny kinematically inferred stellar halos indicative of weak external influences. We examined the correlation between the mass, size, and angular momentum of galaxies by comparing simulations with observations and the theoretical predictions of the exponential hypothesis.

Results. Galaxies with tiny stellar halos exhibit a large scatter in the j_⋆ − M_⋆ relation, which suggests that this scatter is inherently present in their initial conditions. Our analysis reveals that the disks of these galaxies adhere to the exponential hypothesis, resulting in a tight fiducial j_⋆ − M_⋆-scale length (size) relation that is qualitatively consistent with observations. The inherent scatter in j_⋆ provides a robust explanation for the mass–size relation and its substantial variability. Notably, galaxies that are moderately influenced by external processes closely adhere to a scaling relation akin to that of galaxies with tiny stellar halos. This result underscores the dominant role of internal processes in shaping the overall j_⋆ − M_⋆ and mass–size relations, with external effects playing a relatively minor role in disk galaxies. Furthermore, the correlation between galaxy size and the virial radius of the dark matter halo exists but fails to provide strong evidence for a connection between galaxies and their parent dark matter halos.