Massive double-exponential galaxies with extended LSB emission: An IllustrisTNG exploration II -are they unique?

Abstract

The massive disk galaxies (stellar mass >= 10^11 M_solar ) in the local universe (z < 0.1) encode challenging aspects of galaxy formation physics. We have studied one class of such galaxies - the giant low surface brightness galaxies (gLSBs) that show a hybrid morphology - a central Sersic, an inner high surface brightness, exponential stellar disk surrounded by an outer, extended low surface brightness disk, using large volume IllustrisTNG-100 simulation. This is motivated by the recent observation of several such systems and an earlier work by Sarkar & Saha (2025) from IllustrisTNG-50. We find that despite using a ~ 10 times larger volume simulation, only ~11% of massive disk galaxies show such morphology, consistent with findings from TNG50. This shows robustly that such galaxies are not common in the local universe. We compared them with the typical disk galaxies from observations, and single-exponential gLSBs from TNG100. We show that while the inner disk components lie well within the central surface brightness vs. radial scale length distribution of the typical disks, the outer LSB disks lie significantly at the outskirt, occupying a unique space. Their median bulge-to-total ratio is ~ 0.2 making them disk dominated, similar to the typical disks and the single-exponential gLSBs. We find their median dark matter mass fraction to be high (~ 0.9), typical of gLSBs. The outer LSB disks reach fainter central surface brightness and larger scale length values than the single-exponential gLSBs. Our exploration will be useful in understanding their formation mechanism across cosmic time.