On the Classification of UGC1382 as a Giant Low Surface Brightness Galaxy

TL;DR

This paper reclassifies UGC1382 as a giant low surface brightness galaxy with extensive disks, challenging previous beliefs and providing detailed analysis of its structure, mass, and formation history.

Contribution

It presents the first detailed characterization of UGC1382 as a GLSB galaxy, including its structure, mass, and formation, highlighting its significance among large isolated disk galaxies.

Findings

UGC1382 has a giant low surface brightness disk with a radius of ~38 kpc.

The galaxy's stellar mass is approximately 8x10^10 solar masses.

It is embedded in a massive HI disk with a radius of 110 kpc.

Abstract

We provide evidence that UGC1382, long believed to be a passive elliptical galaxy, is actually a giant low surface brightness (GLSB) galaxy which rivals the archetypical GLSB Malin 1 in size. Like other GLSB galaxies, it has two components: a high surface brightness disk galaxy surrounded by an extended low surface brightness (LSB) disk. For UGC1382, the central component is a lenticular system with an effective radius of 6 kpc. Beyond this, the LSB disk has an effective radius of ~38 kpc and an extrapolated central surface brightness of ~26 mag/arcsec^2. Both components have a combined stellar mass of ~8x10^10 M_sun, and are embedded in a massive (10^10 M_sun) low-density (<3 M_sun/pc^2) HI disk with a radius of 110 kpc, making this one of the largest isolated disk galaxies known. The system resides in a massive dark matter halo of at least 2x10^12 M_sun. Although possibly part of a small group, its low density environment likely plays a role in the formation and retention of the giant LSB and HI disks. We model the spectral energy distributions and find that the LSB disk is likely older than the lenticular component. UGC1382 has UV-optical colors typical of galaxies transitioning through the green valley. Within the LSB disk are spiral arms forming stars at extremely low efficiencies. The gas depletion time scale of ~10^11 yr suggests that UGC1382 may be a very long term resident of the green valley. We find that the formation and evolution of the LSB disk is best explained by the accretion of gas-rich LSB dwarf galaxies.