2026ApJ...999..218L

Giant low surface brightness (gLSB) galaxies are galaxies with extremely extended, faint, optical disks over 50 kpc in radius and have high total masses, which can reach 10¹²M_⊙. The existence of such galaxies is problematic for current models of galaxy formation, since the major mergers responsible for the large total mass would likely have destroyed the extended optical disk. Examining the gas content of these galaxies is an important step in determining their formation mechanism, whether it be through slow gas accretion or the large disk (re)forming after a major merger. We present neutral atomic hydrogen (H I) observations of 19 gLSB galaxies identified with the Hyper Suprime-Cam Subaru Strategic Program survey. Although most have high H I masses, they are generally lower than expected based on their large optical sizes, and we do identify some gLSB galaxies with unusually low gas content. The H I spectra of these galaxies show evidence for a rotational disk, though these disks are more asymmetric than those of other galaxies with comparable mass. Four galaxies with surface brightness profiles similar to those of the gLSB galaxies have also been selected from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) simulation for comparison. There is evidence for significant galaxy mergers in the past for three of these NIHAO galaxies, and these three galaxies show similar asymmetry in their H I spectra. Together, these results could indicate that the large optical disk of a gLSB galaxy is the result of a recent merger.