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