Spitzer Analysis of H II Region Complexes in the Magellanic Clouds: Determining a Suitable Monochromatic Obscured Star Formation Indicator

Lawton, B., Gordon, K. D., Babler, B., Block, M., Bolatto, A. D., Bracker, S., Carlson, L. R., Engelbracht, C. W., Hora, J. L., Indebetouw, R., Madden, S. C., Meade, M., Meixner, M., Misselt, K., Oey, M. S., Oliveira, J. M., Robitaille, T., Sewilo, M., Shiao, B., Vijh, U. P., & Whitney, B.
2010, The Astrophysical Journal, 716, 453

H II regions are the birth places of stars, and as such they provide the best measure of current star formation rates (SFRs) in galaxies. The close proximity of the Magellanic Clouds allows us to probe the nature of these star forming regions at small spatial scales. To study the H II regions, we compute the bolometric infrared flux, or total infrared (TIR), by integrating the flux from 8 to 500 μm. The TIR provides a measure of the obscured star formation because the UV photons from hot young stars are absorbed by dust and re-emitted across the mid-to-far-infrared (IR) spectrum. We aim to determine the monochromatic IR band that most accurately traces the TIR and produces an accurate obscured SFR over large spatial scales. We present the spatial analysis, via aperture/annulus photometry, of 16 Large Magellanic Cloud (LMC) and 16 Small Magellanic Cloud (SMC) H II region complexes using the Spitzer Space Telescope's IRAC (3.6, 4.5, 8 μm) and MIPS (24, 70, 160 μm) bands. Ultraviolet rocket data (1500 and 1900 Å) and SHASSA Hα data are also included. All data are convolved to the MIPS 160 μm resolution (40 arcsec full width at half-maximum), and apertures have a minimum radius of 35''. The IRAC, MIPS, UV, and Hα spatial analysis are compared with the spatial analysis of the TIR. We find that nearly all of the LMC and SMC H II region spectral energy distributions (SEDs) peak around 70 μm at all radii, from ~10 to ~400 pc from the central ionizing sources. As a result, we find the following: the sizes of H II regions as probed by 70 μm are approximately equal to the sizes as probed by TIR (≈70 pc in radius); the radial profile of the 70 μm flux, normalized by TIR, is constant at all radii (70 μm ~ 0.45TIR); the 1σ standard deviation of the 70 μm fluxes, normalized by TIR, is a lower fraction of the mean (0.05-0.12 out to ~220 pc) than the normalized 8, 24, and 160 μm normalized fluxes (0.12-0.52); and these results are the same for the LMC and the SMC. From these results, we argue that 70 μm is the most suitable IR band to use as a monochromatic obscured star formation indicator because it most accurately reproduces the TIR of H II regions in the LMC and SMC and over large spatial scales. We also explore the general trends of the 8, 24, 70, and 160 μm bands in the LMC and SMC H II region SEDs, radial surface brightness profiles, sizes, and normalized (by TIR) radial flux profiles. We derive an obscured SFR equation that is modified from the literature to use 70 μm luminosity, SFR(M sun yr-1) = 9.7(0.7) × 10-44 L 70(ergs s-1), which is applicable from 10 to 300 pc distance from the center of an H II region. We include an analysis of the spatial variations around H II regions between the obscured star formation indicators given by the IR and the unobscured star formation indicators given by UV and Hα. We compute obscured and unobscured SFRs using equations from the literature and examine the spatial variations of the SFRs around H II regions.


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