We present the discovery of a vast cloud of ionized gas 13′ ( 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′ \times 3′ ( 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 _ { lim,B } \sim 30 mag arcsec ^ { -2 } .
WIYN The WIYN Observatory is a joint facility of the University of Wisconsin-Madison , Indiana University , the National Optical Astronomy Observatory and the University of Missouri . SparsePak observations confirm the cloud ’ s kinematic association with M51 , and the high [ N ii ] / H \alpha , [ S ii ] / H \alpha , and [ O i ] / 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 [ N ii ] 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 .