Beyond Morphology: Challenges in Decomposing Massive Stellar Halos in Sombrero-like, Halo-Embedded Disk Galaxies

¹ Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, China
² Department of Astronomy, School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
³ Key Laboratory for Particle Astrophysics and Cosmology (MOE) / Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai 200240, China
⁴ Fujian Provincial Key Laboratory for Soft Functional Materials Research, Research Institute for Biomimetics and Soft Matter, Department of Physics, Xiamen University, Xiamen, Fujian 361005, China

^⋆ Corresponding author: dumin@xmu.edu.cn

Received: 15 March 2025
Accepted: 5 May 2025

Abstract

Aims. Sombrero-like galaxies exhibit unique structural properties that challenge traditional photometric decomposition methods. We investigate the structural differences of Sombrero-like galaxies using both conventional photometric and kinematic decomposition approaches. This study aims to explore the extent to which photometric decomposition misidentifies key structural components, particularly the stellar halo.

Methods. We selected 270 Sombrero-like galaxies at redshift z = 0 from the TNG50 run of IllustrisTNG (TNG) simulations, filtering those with stellar mass M_*>10¹⁰ M_⊙ and stellar halo mass fraction satisfying 0.3<f_halo<0.6. Synthetic images of these galaxies are generated using the GALAXEV population synthesis code, and photometric decomposition is performed on face-on and edge-on images using GALFIT. We then compare the decomposition results with kinematic decomposition based on the auto-GMM method, analyzing differences in the recovered structural parameters, including mass fractions and Sérsic indices, and discussing their implications for identifying the bulge, disk, and stellar halo.

Results. Sombrero-like galaxies are characterized by disks embedded in massive stellar halos, namely halo-embedded disk galaxies. These galaxies likely represent 30–60% of TNG50 galaxies, but identifying them is challenging due to structural degeneracies and the presence of disk features (e.g., bars, spirals, star formation) at low or moderate inclinations. Face-on photometric decomposition systematically overestimates disk fractions as stellar halos are almost absent, while edge-on analysis provides only approximate halo fractions. Radial profiles show discrepancies between photometric and kinematic decomposition, particularly in central regions. Additionally, No conclusive link exists between the Sérsic index, n, and the presence of large stellar halos, challenging the use of n as a merger history proxy. These findings underscore the need for improved decomposition methods to better understand the complex structures of Sombrero-like galaxies. The difficulty in identifying Sombrero-like galaxies, which have often undergone significant merger events, complicates our understanding of galaxy formation and evolution.