Large-scale Shock-ionized and Photoionized Gas in M83: The Impact of Star Formation

We investigate the ionization structure of the nebular gas in M83 using the line diagnostic diagram, [O III](5007 A)/H Delta *b versus [S II](6716 A+6731 A)/H Delta *a, with the newly available narrowband images from the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope (HST). We produce the diagnostic diagram on a pixel-by-pixel (02 X 02) basis and compare it with several photo- and shock-ionization models. We select four regions from the center to the outer spiral arm and compare them in the diagnostic diagram. For the photoionized gas, we observe a gradual increase of the log ([O III]/H Delta *b) ratios from the center to the spiral arm, consistent with the metallicity gradient, as the H II regions go from super-solar abundance to roughly solar abundance from the center out. Using the diagnostic diagram, we separate the photoionized from the shock-ionized component of the gas. We find that the shock-ionized H Delta *a emission ranges from ~2% to about 15%-33% of the total, depending on the separation criteria used. An interesting feature in the diagnostic diagram is a horizontal distribution around log ([O III]/H Delta *b) 0. This feature is well fit by a shock-ionization model with 2.0 Z metallicity and shock velocities in the range of 250-350 km s--1. A low-velocity shock component,