Tidal Tales of Minor Mergers II: Comparing Star Formation in the Tidal Tails of NGC 2782

While major mergers and their tidal debris are well studied, they are less common than minor mergers (mass ratios <0.3). The peculiar spiral NGC 2782 is the result of a merger between two disk galaxies with a mass ratio of ~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. 2000 suggested that star formation had not yet begun to occur in that tidal tail. However, deep UBVR and H alpha narrowband images show evidence of recent star formation in the Western tail. Comparing the two tails, we find that Western tail lacks massive star clusters. Using Herschel PACS spectrsocopy, 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. Due to the lack of both CO and [CII] emission, the Western tail HII region may have a low carbon abundance and be undergoing its first round of star formation. The Western tail is more efficient at forming stars, but lacks massive star clusters. We propose that the low SFE in the Eastern tail may be due to its formation as a splash region in the merger where gas heating is important even though it has sufficient molecular and neutral gas to make massive star forming regions. The Western tail, which has lower gas surface density and so does not form higher mass star clusters, is a tidally formed region where gravitational compression dominates and enhances the star formation.