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

Abstract: Sombrero-like galaxies exhibit unique structural properties that challenge traditional photometric decomposition methods. In this study, we investigate their structural differences using both photometric and kinematic approaches to assess the extent to which photometric decomposition may misidentify key components, particularly the stellar halo. We select 270 Sombrero-like galaxies at redshift z=0 from the TNG50 simulation, applying filters to include only those with stellar mass $M_{\ast} > 10^{10}M_{\odot}$ and stellar halo mass fraction $0.3 < f_{\rm halo} < 0.6$. Synthetic images are generated using the GALAXEV population synthesis code, and photometric decomposition is carried out on both face-on and edge-on views using GALFIT. We then compare these results with kinematic decomposition based on the auto-GMM method, focusing on differences in recovered structural parameters such as mass fractions and S'ersic indices.Sombrero-like galaxies typically consist of disks embedded in massive stellar halos and may account for 30-60% of galaxies in TNG50. However, their identification is complicated by structural degeneracies and the presence of disk features (e.g., bars, spirals, star formation) at low or moderate inclinations. In face-on projections, 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 S'ersic 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.