Discovery of a vast ionized gas cloud in the M51 system

Abstract: We present the discovery of a vast cloud of ionized gas 13$^{\prime}$ (32 kpc) north of the interacting system M51. We detected this cloud via deep narrow-band imaging with the Burrell Schmidt Telescope, where it appears as an extended, diffuse H$\alpha$-emitting feature with no embedded compact regions. The Cloud spans $\sim$10$^{\prime}\times$3$^{\prime}$ (25$\times$7.5 kpc) in size and has no stellar counterpart; comparisons with our previous deep broadband imaging show no detected continuum light to a limit of $\mu_{\rm lim, B} \sim$30 mag arcsec$^{-2}$. WIYN SparsePak observations confirm the cloud's kinematic association with M51, and the high NII/H$\alpha$, SII/H$\alpha$, and OI/H$\alpha$ line ratios we measure imply a hard ionization source such as AGN photoionization or shock heating rather than photoionization due to young stars. Given the strong NII emission, we infer roughly solar metallicity for the cloud, ruling out an origin due to infall of primordial gas. Instead we favor models where the gas has been expelled from the inner regions of the M51 system due to tidal stripping or starburst/AGN winds and has been subsequently ionized either by shocks or a fading AGN. This latter scenario raises the intriguing possibility that M51 may be the nearest example of an AGN fossil nebula or light echo, akin to the famous "Hanny's Voorwerp" in the IC 2497 system.