2025ApJ...986..125S

We study massive disk galaxies (stellar mass > = 10¹¹M_⊙) at z = 0 from IllustrisTNG simulation to detect galaxies that contain two exponential stellar disks-a central high surface brightness (HSB) disk surrounded by an extended low surface brightness (LSB) envelope. This is motivated by the observation of several giant LSB galaxies (Malin 1, UGC 1378, UGC 1382 etc), reported in the literature, showing such complex morphology. Studying such systems can help us to understand the mass assembly process and growth of stellar disks in massive galaxies, and thus can shed light on massive galaxy formation and evolution scenarios. We use the high-resolution IllustrisTNG50 data and perform Sérsic plus exponential profile modeling on the idealized, synthetic Sloan Digital Sky Survey g- and r-band images of the massive disk galaxies using GALFIT. We identify seven disk galaxies (12% of the parent sample of disk galaxies) that are best represented by a central Sérsic plus a central HSB disk surrounded by an extended LSB disk. The radial scale lengths of the LSB disk lie in the range of ∼9.7-31.7 kpc, in agreement with that found in the literature. We study the star formation properties of these simulated double-disk galaxies to understand the distribution of these from blue star-forming to red-quenched region. Some of these double-disk galaxies display a characteristic minima in their (g - r) color radial profiles. The double-disk galaxies are found to lie within ∼1.5σ region of the Baryonic Tully-Fisher relation from observation.