UGC 1378 -- a Milky Way-sized galaxy embedded in a giant low-surface brightness disc

TL;DR

UGC 1378 is a rare, giant galaxy with both high and low surface brightness discs, offering insights into galaxy formation processes, especially in isolated environments with abundant gas reservoirs.

Contribution

This study provides detailed observations of UGC 1378, revealing its unique structure and challenging the merger-based formation scenario for giant low-surface brightness discs.

Findings

Stellar velocity dispersion data inconsistent with recent major merger.

Low SFR surface density suggests recent gas accretion.

Large disc size likely due to rich gas reservoir and isolation.

Abstract

The dominant physical processes responsible for the formation and longevity of giant gaseous and stellar discs in galaxies remain controversial. Although they are rare (less than 10 confirmed as of now), giant low-surface brightness (gLSB) discy galaxies provide interesting insights given their extreme nature. We describe observations of UGC 1378 including deep spectroscopy with the Russian 6m telescope and multi-band imaging with Binospec at the MMT. Galaxy UGC 1378 has both high surface brightness and an extended low surface brightness discs. Our stellar velocity dispersion data for the high-surface brightness, Milky Way-sized, disc appears inconsistent with a recent major merger, a widely discussed formation scenario for the very extended low surface brightness disc. We estimate the star formation rates (SFR) from archival Wide-Field Infrared Survey Explorer data. The SFR surface density in the LSB disc is low relative to its gas density, consistent with recent gas accretion. We argue that the unusually large size of UGC 1378's disc may be the product of a rich gas reservoir (e.g. a cosmic filament) and an isolated environment that has preserved the giant disc.