Orbit-based structural decomposition and stellar population recovery for edge-on barred galaxies

¹ Department of Astronomy, Westlake University, Hangzhou, Zhejiang 310030, China
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
³ Department of Astrophysics and Planetary Science, Villanova University, 800 E Lancaster Ave, Villanova, PA 19085, USA
⁴ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁵ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria
⁶ Cardiff Hub for Astrophysics Research & Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK

^★ Corresponding authors: jyp199333@163.com; lzhu@shao.ac.cn

Received: 13 September 2025
Accepted: 21 October 2025

Abstract

In our previous paper, we developed an orbit-superposition method for edge-on barred galaxies and constructed a set of dynamical models based on different mock observations of three galaxies from the Auriga simulations. In this study, we adopted 12 cases with side-on bars (three simulated galaxies, each with four different projections). We decomposed these galaxies into different structures combining the kinematic and morphological properties of stellar orbits. We then compared the model-predicted components to their true counterparts in the simulations. Our models can identify (BP/X-shaped) bars, spheroidal bulges, thin discs, and spatially diffuse stellar halos. The mass fractions of bars and discs are well constrained with absolute biases of |f_model − f_true|≤0.15. We recovered the mass fractions of halos with |f_model − f_true|≤0.03. For the bulge components, 10 out of 12 cases exhibit |f_model − f_true|≤0.05, while the other two cases exhibit |f_model − f_true|≤0.10. Then, by tagging the stellar orbits with ages and metallicities, we derived the chemical properties of each structure. For the stellar ages, our models recovered the negative gradients in the bars and discs, but exhibited relatively larger uncertainties for age gradients in the bulges and halos. The mean stellar ages of all components were constrained with absolute biases |t_model − t_true|≲1 Gyr. For stellar metallicities, our models reproduced the steep negative gradients of the bars and bulges, as well as all different kinds of metallicity gradients in the discs and halos. Apart from the bulge in the simulated galaxy Au-18, the mean stellar metallicities of all other components were constrained with absolute biases of |Z_model − Z_true|≤0.5 Z_⊙.