Identification of Bubble Nebulae around NGC 55 ULX-1 with MUSE Observations

Abstract

Using the Multi Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope, we identified three bubble nebulae (denoted as A, B, and C) around an ultraluminous X-ray source (ULX) in NGC 55. Bubble A shows a regular elliptical shape surrounding the ULX, with a morphology similar to the canonical ULX bubble around NGC 1313 X-2. It is most likely inflated by the ULX disk wind with a mechanical power close to 1039 erg s−1. Bubble B lies 11″ away from the ULX on the sky plane and is not contiguous to Bubble A. It displays a bow-shock-like morphology, and is likely driven by a collimated dark jet from the ULX with a mechanical power of about 3 × 1038 erg s−1. If this scenario is correct, we predict that Bubble B should present radio emission with a flux of about 1–102 μJy at 5 GHz. Bubble C appears within Bubble A, with a velocity and velocity dispersion distinct from the rest of Bubble A. Its nature is unclear and could be part of Bubble A as a result of low local density. The optical counterpart of ULX-1 exhibits broad Hα, consistent with emission from a hot disk wind.