| Abstract: Galaxies evolve in structure & morphology across cosmic time. Typically, the most massive galaxies (>=10^11 solar mass) are believed to undergo a morphological transformation from disk to ellipticals across cosmic time, and thus the morphology of such galaxies in the nearby universe should be dominated by ellipticals. Yet, observations as well as simulations show disk galaxies in the above mass range. One class of such galaxies are the giant low surface brightness galaxies that are characterised by a central typical high surface brightness (HSB) stellar disk surrounded by an extended (radial scale length > 10 kpc), photometrically decoupled, low surface brightness (LSB) stellar disk component (e.g, Malin 1, UGC 1378, UGC 1382). Our understanding on the number density of such galaxies in the local universe, the formation of such hybrid morphology , their morphological evolution across redshift remain unexplored due to the limitation of the detection of the faint outskirt. Thus our concept on massive galaxy formation and evolution remain highly biased by the bright galaxies. To fill this gap, we use IllustrisTNG50, a cosmological hydrodynamical simulations to study such double-exponential disk galaxies with extended LSB disk at z=0. We find ~12 % of the >=10^11 M_solar disk galaxies at z=0, similar to this, a first theoretical estimate obtained from such simulations. We show that they lie within the mass range of 11.0 -11.5 (log (M_solar)). They lie in the green-valley region on the specific star formation vs. stellar mass plane. They also satisfy Baryonic Tully-Fisher relation, one of the fundamental relations of disk galaxies. Upcoming data from LSST as well as future surveys with large optical telescopes will be able to trace the LSB envelope of such galaxies with unprecedented details and thus unravel the evolution of one of the most massive galaxies in the local universe. |
