A Green Pea Starburst Arising from a Galaxy–Galaxy Merger

Green Pea galaxies are low-redshift starburst dwarf galaxies, with properties similar to those of the high-redshift galaxies that reionized the universe. We report the first mapping of the spatial distribution of atomic hydrogen (Hi) in and around a Green Pea, GP J0213+0056 at z = 0.0399, using the Giant Metrewave Radio Telescope (GMRT). Like many Green Peas, GP J0213+0056 shows strong Hi 21 cm emission in single-dish spectroscopy, strong Lyα emission, and a high [Oiii]λ5007 Å/[Oii]λ3727 Å luminosity ratio, O32 ≈ 8.8, consistent with a high leakage of Lyman-continuum radiation. Our GMRT Hi 21 cm images show that the Hi 21 cm emission in the field of GP J0213+0056 arises from an extended broken-ring structure around the Green Pea, with the strongest emission coming from a region between GP J0213+0056 and a companion galaxy lying ≈4.7 kpc away, and little Hi 21 cm emission coming from the Green Pea itself. We find that the merger between GP J0213+0056 and its companion is likely to have triggered the starburst, and led to a disturbed Hi spatial and velocity distribution, which in turn allowed Lyα (and, possibly, Lyman-continuum) emission to escape the Green Pea. Our results suggest that such mergers, and the resulting holes in the Hi distribution, are a natural way to explain the tension between the requirements of cold gas to fuel the starburst and the observed leakage of Lyα and Lyman-continuum emission in Green Pea galaxies and their high-redshift counterparts.