The 10 kpc collar of early-type galaxies: Probing evolution by focusing on the inner stellar density profile

¹ Department of Astronomy, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
² Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai Jiao Tong University, Shanghai 200240, China
³ Key Laboratory for Particle Physics, Astrophysics and Cosmology, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
⁴ Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum – Universitá di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁵ Institute for Astrophysics, School of Physics, Zhengzhou University, Zhengzhou 450001, China

^⋆ Corresponding authors: liurongfu@sjtu.edu.cn, sonnenfeld@sjtu.edu.cn

Received: 17 January 2025
Accepted: 17 July 2025

Abstract

Context. The post-quenching evolution process of early-type galaxies (ETGs), which is typically driven by mergers, is still not fully understood. The amount of growth in stellar mass and size incurred after quenching is still under debate.

Aims. In this work, we investigated the late evolution of ETGs, both observationally and theoretically, by focusing on the stellar mass density profile inside a fixed aperture, within 10 kpc from the galaxy center.

Methods. We first studied the stellar mass and the mass-weighted density slope within 10 kpc, respectively M_*, 10 and Γ_*, 10, of a sample of ETGs from the GAMA survey. We measured the Γ_*, 10 − M_*, 10 relation and its evolution over the redshift range 0.17 ≤ z ≤ 0.37. We then built a toy model for the merger evolution of galaxies, based on N-body simulations, to explore to what extent the observed growth in Γ_*, 10 − M_*, 10 relation is consistent with a dry-merger evolution scenario.

Results. From the observations, we do not detect evidence for an evolution of the Γ_*, 10 − M_*, 10 relation relation. We put an upper limit on the redshift derivative of the normalization (μ) and slope (β) of the Γ_*, 10 − M_*, 10 relation: |∂μ/∂log(1 + z)| ≤ 0.13 and |∂β/∂log(1+z)| ≤ 1.10, respectively. Simulations show that most mergers induce a decrease in Γ_*, 10 and an increase in M_*.10, although some show a decrease in M_*, 10, particularly for the most extended galaxies and smaller merger mass ratios. By combining the observations with our merger toy model, we place an upper limit on the fractional stellar mass growth of f_M = 11.2% in the redshift range 0.17 ≤ z ≤ 0.37.

Conclusions. While our measurement is limited by systematics, the application of our approach to samples with a larger redshift baseline, particularly with a time interval Δt ≥ 3.2 Gyr, should enable detection of a signal and improves our understanding of the late growth of ETGs.