Tidal Tails of Minor Mergers II: Comparing Star Formation in the Tidal
  Tails of NGC 2782
  The peculiar spiral NGC 2782 is the result of a minor merger with a mass
ratio ~4:1 occurring ~200 Myr ago. This merger produced a molecular and HI
rich, optically bright Eastern tail and an HI-rich, optically faint Western
tail. Non-detection of CO in the Western Tail by Braine et al. (2001) suggested
that star formation had not yet begun. However, deep UBVR and H-alpha
narrowband images show evidence of recent star formation in the Western tail,
though it lacks massive star clusters and cluster complexes. Using Herschel
PACS spectroscopy, we discover 158 micron [CII] emission at the location of the
three most luminous H-alpha sources in the Eastern tail, but not at the
location of the even brighter H-alpha source in the Western tail. The Western
tail is found to have a normal star formation efficiency (SFE), but the Eastern
tail has a low SFE. The lack of CO and [CII] emission suggests the Western tail
HII region may have a low carbon abundance and be undergoing its first star
formation. The Western tail is more efficient at forming stars, but lacks
massive clusters. We propose that the low SFE in the Eastern tail may be due to
its formation as a splash region where gas heating is important even though it
has sufficient molecular and neutral gas to make massive star clusters. The
Western tail, which has lower gas surface density and does not form high mass
star clusters, is a tidally formed region where gravitational compression
likely enhances star formation.
