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We present the discovery of a vast cloud of ionized gas 13$^{\prime}$ (32kpc) north of the interacting system M51. We detected this cloud via deepnarrow-band imaging with the Burrell Schmidt Telescope, where it appears as anextended, diffuse H$\alpha$-emitting feature with no embedded compact regions.The Cloud spans $\sim$10$^{\prime}\times$3$^{\prime}$ (25$\times$7.5 kpc) insize and has no stellar counterpart; comparisons with our previous deepbroadband imaging show no detected continuum light to a limit of $\mu_{\rm lim,B} \sim$30 mag arcsec$^{-2}$. WIYN SparsePak observations confirm the cloud'skinematic association with M51, and the high NII/H$\alpha$, SII/H$\alpha$, andOI/H$\alpha$ line ratios we measure imply a hard ionization source such as AGNphotoionization or shock heating rather than photoionization due to youngstars. Given the strong NII emission, we infer roughly solar metallicity forthe cloud, ruling out an origin due to infall of primordial gas. Instead wefavor models where the gas has been expelled from the inner regions of the M51system due to tidal stripping or starburst/AGN winds and has been subsequentlyionized either by shocks or a fading AGN. This latter scenario raises theintriguing possibility that M51 may be the nearest example of an AGN fossilnebula or light echo, akin to the famous "Hanny's Voorwerp" in the IC 2497system.

Discovery of a vast ionized gas cloud in the M51 system