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

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 $10^{39}$ erg s$^{-1}$. Bubble B lies 11\arcsec\ 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 $3 \times 10^{38}$ erg s$^{-1}$. If this scenario is correct, we predict that Bubble B should present radio emission with a flux of about $1 - 10^2$ $\mu$Jy 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 a broad H$\alpha$, consistent with emission from a hot disk wind.