A Fast Radio Burst Progenitor Born in a Galaxy Merger

We report a Giant Metrewave Radio Telescope 21 cm mapping study of the neutral atomic hydrogen (Hi) in the host galaxy of the fast radio burst (FRB) FRB 20180916B at z ≈ 0.03399. We find that the FRB host has an Hi mass of MHi = (2.74 ± 0.33) × 109 M⊙ and a high Hi to stellar mass ratio, ≈1.3. The FRB host is thus a gas-rich but near-quiescent galaxy that is likely to have acquired a significant mass of Hi in the recent past. The Hi distribution is disturbed, with extended Hi 21 cm emission detected in a northeastern tail, a counter-tail toward the south, an Hi hole between the galaxy center and the FRB location, and a high Hi column density measured close to the FRB position. The FRB host is part of a group with four companions detected in their Hi 21 cm emission, the nearest of which is only 22 kpc from the FRB location. The gas richness and disturbed Hi distribution indicate that the FRB host has recently undergone a minor merger, which increased its Hi mass, disturbed the Hi in the galaxy disk, and compressed the Hi near the FRB location to increase its surface density. We propose that this merger caused the burst of star formation in the outskirts of the galaxy that gave rise to the FRB progenitor. The evidence for a minor merger is consistent with scenarios in which the FRB progenitor is a massive star, formed due to the merger event.