Year: 2008
Type: article
Abstract: We have observed emission lines of [S IV] 10.51, H(7-6) 12.37, [Ne II] 12.81, [Ne III] 15.56 and [S III] 18.71 μm in a number of extragalactic H II 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 + , S 3+ /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 ++ , S 3+ ) for H II 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 : d log (Ne/H)/dR G = -0.058± 0.014 and d log (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 H II 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 H II 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 H II 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 ++ / S 3+ , and Ne ++ / Ne + versus S 3+ / 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
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W1997392048