Spitzer Observations of H II Regions in NGC 6822 and the Hot Star - H II Region Connection

We observed several H II regions in the dwarf irregular galaxy NGC 6822 using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. Measurements of [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71 micron emission lines were made in each of the H II regions. The lines were observed cospatially using the IRS in the short wavelength, high resolution mode, which permits a reliable comparison of the line fluxes. From the measured line fluxes we determine ionic abundance ratios including Ne++/Ne+ and S3+/S++. These ionic abundance ratios allow an analysis of the Ne/S ratio by taking the ratio of the dominant ionization states of the respective elements, Ne (Ne++, Ne+) and S (S3+, S++). Our aim here is twofold:

(1) to examine the Neon to Sulfur abundance ratio in order to determine whether or not it is fairly universal and

(2) to discriminate and test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing our observational data

with H II region models that use these SEDs as input. This work extends our previous similar studies of H II regions in M83 and M33 to lower metallicities (and higher ionization), where we can attain a more reliable estimate of the Ne/S ratio. For the first time, we employ the new grid of SEDs from Pauldrach and Weber that have been computed with different metallicities: solar, 0.4 solar, and 0.1 solar. We demonstrate the effect on our analysis of changing just the stellar atmosphere model abundances. As expected, these changes to the SED show similar trends as does changing just the metallicities in the nebular gas abundances in our plasma simulations. Lower metallicity results in higher ionization.

Support from 09-ADP09-0169 and Spitzer 40910 are gratefully acknowledged.