HII regions in the northern half of M33

We construct [OIII]{lambda}5007/[OII]{lambda}3727 and [OIII]{lambda}5007/[SII]{lambda}6724 ratio maps from narrowband images continuum-subtracted in this way, from which we classify the HII regions by optical depth to ionizing radiation, based on their ionization structure. This method works relatively well in the low-metallicity regime, 12+log(O/H)<=8.4, where [OIII]{lambda}{lambda}4959,5007 is strong.

However, at higher metallicities, the method breaks down due to the strong dependence of the [OIII]{lambda}{lambda}4959,5007 emission lines on the nebular temperature. Thus, although O^++^ may be present in metal-rich HII regions, these commonly used emission lines do not serve as a useful indicator of its presence, and hence the O ionization state. In addition, IPM as a diagnostic of optical depth is limited by spatial resolution.