Tidal Tails of Minor Mergers: Star Formation Efficiency in the Western Tail of NGC 2782

TL;DR

This study investigates star formation efficiency in the western tidal tail of NGC 2782, revealing low global efficiency but normal molecular gas-based efficiency, indicating localized dense or low-metallicity star-forming regions.

Contribution

It provides new insights into star formation in minor merger tidal tails, highlighting the discrepancy between gas density and star formation rate, and suggests the presence of localized dense star-forming regions.

Findings

Low global star formation efficiency in the western tail.

Star formation rate per unit area is much lower than expected from gas density.

Local H II region shows normal molecular gas-based star formation efficiency.

Abstract

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 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 H{\alpha} 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 ({\Sigma}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) {\Sigma}SFR from H{\alpha} 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 NGC 2782 or of a low-metallicity and/or low-pressure star-forming region.