The effects of the orbital configurations of mergers on reshaping galaxy structures

¹ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
² School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
⁴ Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong S.A.R., China
⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China

^★ Corresponding authors: lzhu@shao.ac.cn; changjiang@pmo.ac.cn

Received: 19 March 2025
Accepted: 31 May 2025

Abstract

We performed a systematic analysis of how the orbital configuration of a merger can influence the structural formation of remnant galaxies using 531 merger pairs selected from IllustrisTNG-100. We comprehensively described the merger orbital configuration, considering the relative orbital motion of the merger pair and their disk orientations. We quantified the galaxy structures by dynamically defining four components: bulge, disk, warm component, and hot inner stellar halo. For mergers on spiral-in orbits, the disk planes of the two merging galaxies tend to be aligned with the orbital plane, leading to higher fractions for the disk and warm components, as well as lower fractions for the bulge and hot inner stellar halo components in the remnant galaxy. For mergers on direct collision orbits, the disk planes of the two galaxies tend to be perpendicular to the orbital plane, leading to lower fractions for disk and warm components, as well as higher fractions of the bulge and hot inner stellar halo in the remnant. Mergers can lead to either an increase or decrease in the disk and bulge mass fraction in the remnant compared to the progenitor galaxy, depending on the merger orbital configurations; however, in 93% of cases, mergers cause an increase in the hot inner stellar halo. As a result, the luminosity fraction of the hot inner stellar halo (but not the bulge) in galaxies at z = 0 is highly correlated with its total ex situ stellar mass. In addition, we find that merger on spiral-in orbits is one of the possible reasons for the formation of recently discovered red but H I-rich (RR) galaxies.