2008MNRAS.387...45R

We have observed emission lines of [Siv] 10.51, H(7-6) 12.37, [NeII] 12.81, [NeIII] 15.56 and [SIII] 18.71 µm in a number of extragalactic HII regions with the Spitzer Space Telescope. A previous paper presented our data and analysis for the substantially face-on spiral galaxy M83. Here we report our results for the Local Group spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (R_G). The observations were made with the Infrared Spectrograph with the short wavelength, high-resolution module. The above set of five lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne⁺⁺/Ne⁺, S³⁺/S⁺⁺, and S⁺⁺/Ne⁺ and find that there is a correlation of increasingly higher ionization with larger R_G. By sampling the dominant ionization states of Ne (Ne⁺, Ne⁺⁺) and S (S⁺⁺, S³⁺) for HII regions, we can estimate the Ne/H, S/H and Ne/S ratios. We find from linear least-squares fits that there is a decrease in metallicity with increasing R_G: dlog(Ne/H)/dR_G= -0.058 ±0.014 and dlog(S/H)/dR_G= -0.052±0.021 dex kpc^–1. There is no apparent variation in the Ne/S ratio with R_G. Unlike our previous similar study of M83, where we conjectured that this ratio was an upper limit, for M33 the derived ratios are likely a robust indication of Ne/S. This occurs because the HII regions have lower metallicity and higher ionization than those in M83. Both Ne and S are primary elements produced in α-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that the Ne/S ratio remains relatively constant throughout a galaxy. The median (average) Ne/S ratio derived for HII regions in M33 is 16.3 (16.9), just slightly higher than the Orion Nebula value of 14.3. The same methodology is applied to Spitzer observations recently published for three massive HII regions: NGC 3603 (Milky Way), 30 Dor (LMC) and N 66 (SMC) as well as for a group of blue compact dwarf galaxies. We find median Ne/S values of 14.6, 11.4, 10.1, and 14.0, respectively. All of these values are in sharp contrast with the much lower `canonical', but controversial, solar value of ∼5. A recent nucleosynthesis, galactic chemical evolution model predicts an Ne/S abundance of ∼9. Our observations may also be used to test the predicted ionizing spectral energy distribution of various stellar atmosphere models. We compare the ratio of fractional ionizations <Ne⁺⁺>/<S⁺⁺>, <Ne⁺⁺>/<S³⁺>, and <Ne⁺⁺>/<Ne⁺> versus <S³⁺>/<S⁺⁺> with predictions made from our photoionization models using several of the state-of-the-art stellar atmosphere model grids. The trends of the ionic ratios established from the prior M83 study are remarkably similar, but continued to higher ionization with the present M33 objects.