Tidal tails of minor mergers: Star formation efficiency in the Western tail of NGC2782

While major mergers and their tidal debris are well studied, they are less common than minor mergers (mass ratios ≲ 0.3). The peculiar spiral NGC2782 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 H I-rich, optically bright eastern tail and an H I-rich, optically faint western tail. Non-detection of CO in the western tail by Braine et al. suggested that star formation had not yet begun to occur in that tidal tail. However, deep Hα narrowband images show evidence of recent star formation in the western tail. Across the entire western tail, we find the global star formation rate per unit area (Σ_SFR) to be several orders of magnitude less than expected from the total gas density. Together with extended FUV+NUV emission from Galaxy Evolution Explorer along the tail, this indicates a low global star formation efficiency in the tidal tail producing lower mass star clusters. The H II region that we observed has a local (few-kiloparsec scale) Σ_SFR from Hα that is less than that expected from the total gas density, which is consistent with other observations of tidal debris. The star formation efficiency of this H II region inferred from the total gas density is low, but normal when inferred from the molecular gas density. These results suggest the presence of a very small, locally dense region in the western tail of NGC2782 or of a low-metallicity and/or low-pressure star-forming region.