2006ApJ...644L..29V

We present spectroscopic observations of diffuse ionized gas (DIG) in M33 near the H II region NGC 604. We present the first detection of [O I] λ6300 in the DIG of M33, one of the critical lines for distinguishing photoionization from shock ionization models. We measure [O I]/Hα in the range of 0.04-0.10 and an increase in this ratio with decreasing emission measure. Our measurements of [S II]/Hα and [N II]/Hα also rise with decreasing emission measure, while our [O III]/Hβ measurements remain fairly constant. We have one tentative detection of He I in the region of brightest emission measure, with a ratio of He I/Hα = 0.033±0.019, indicating that the helium is at least partially ionized. We compare our observed emission-line ratios to photoionization models and find that field star ionization models do not fit our data well. Leaky H II region models are consistent with our data, without the need to invoke additional ionization mechanisms to fit our [O I] or [O III] measurements. The closest large H II region is NGC 604 and is therefore a likely candidate for the source of the ionizing photons for the gas in this region.