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We investigate the ionization structure of the nebular gas in M83 using theline diagnostic diagram, [O III](5007 \degA)/H{\beta} vs. [S II](6716 \degA+6731 \deg A)/H{\alpha} with the newly available narrowband images from theWide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST). We produce thediagnostic diagram on a pixel-by-pixel (0.2" x 0.2") basis and compare it withseveral photo- and shock-ionization models. For the photo-ionized gas, weobserve a gradual increase of the log([O III]/H{\beta}) ratios from the centerto the spiral arm, consistent with the metallicity gradient, as the H IIregions go from super solar abundance to roughly solar abundance from thecenter out. Using the diagnostic diagram, we separate the photo-ionized fromthe shock-ionized component of the gas. We find that the shock-ionizedH{\alpha} emission ranges from ~2% to about 15-33% of the total, depending onthe separation criteria used. An interesting feature in the diagnostic diagramis an horizontal distribution around log([O III]/H{\beta}) ~ 0. This feature iswell fit by a shock-ionization model with 2.0 Z\odot metallicity and shockvelocities in the range of 250 km/s to 350 km/s. A low velocity shockcomponent, < 200 km/s, is also detected, and is spatially located at theboundary between the outer ring and the spiral arm. The low velocity shockcomponent can be due to : 1) supernova remnants located nearby, 2) dynamicalinteraction between the outer ring and the spiral arm, 3) abnormal line ratiosfrom extreme local dust extinction. The current data do not enable us todistinguish among those three possible interpretations. Our main conclusion isthat, even at the HST resolution, the shocked gas represents a small fractionof the total ionized gas emission at less than 33% of the total. However, itaccounts for virtually all of the mechanical energy produced by the centralstarburst in M83.

Large-scale shock-ionized and photo-ionized gas in M83: the impact of star formation