An In-Depth View of the Mid-Infrared Properties of Point Sources and the Diffuse ISM in the SMC Giant HII Region, N66

Abstract

(abridged)The focus of this work is to study mid-infrared point sources and the diffuse interstellar medium (ISM) in the low-metallicity (~0.2 solar) giant HII region N66 using the Spitzer Space Telescope's Infrared Spectrograph. We study 14 targeted infrared point sources as well as spectra of the diffuse ISM that is representative of both the photodissociation regions (PDRs) and the HII regions. Among the point source spectra, we spectroscopically confirm that the brightest mid-infrared point source is a massive embedded young stellar object, we detect silicates in emission associated with two young stellar clusters, and we observe spectral features of a known B[e] supergiant that are more commonly associated with Herbig Be stars. In the diffuse ISM, we provide additional evidence that the very small grain population is being photodestroyed in the hard radiation field. The 11.3 um PAH complex emission exhibits an unexplained centroid shift in both the point source and ISM spectra that should be investigated at higher signal-to-noise and resolution. Unlike studies of other regions, the 6.2 um and 7.7 um band fluxes are decoupled; the data points cover a large range of I7.7/I11.3 PAH ratio values within a narrow band of I6.2/I11.3 ratio values. Furthermore, there is a spread in PAH ionization, being more neutral in the dense PDR where the radiation field is relatively soft, but ionized in the diffuse ISM/PDR. By contrast, the PAH size distribution appears to be independent of local ionization state. Important to unresolved studies of extragalactic low-metallicity star-forming regions, we find that emission from the infrared-bright point sources accounts for only 20-35% of the PAH emission from the entire region. These results make a comparative dataset to other star-forming regions with similarly hard and strong radiation fields.

ApJ, in press. ~60 pages, 18 figures

References in corpus (10)

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• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• Past and present star formation in the SMC: NGC 346 and its neighborhood
• Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66
• The Stellar Mass Distribution in the Giant Star Forming Region NGC 346
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• Aromatic emission from the ionised mane of the Horsehead nebula
• PAH Strength and the Interstellar Radiation Field around the Massive Young Cluster NGC3603
• The Star-Forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations I. Photometry
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Cited by in corpus (14)

• The Interstellar dust properties of nearby galaxies
• The Herschel Stripe 82 Survey (HerS): Maps and Early Catalog
• The PAH emission characteristics of the reflection nebula NGC2023
• Spitzer Infrared Spectrographic point source classification in the Small Magellanic Cloud
• PAHs and star formation in the HII regions of nearby galaxies M83 and M33
• Carbon gas in SMC low-metallicity star-forming regions
• The mid-infrared appearance of the Galactic Mini-Starburst W49A
• Radiative transfer of ionizing radiation through gas and dust: stellar source case
• A Principal Component Analysis of polycyclic aromatic hydrocarbon emission in NGC 2023
• Search for Polycyclic Aromatic Hydrocarbons in the Outflows from Dust-Producing Wolf-Rayet Stars
• The study of unclassified B[e] stars and candidates in the Galaxy and Magellanic Clouds
• Massive young stellar objects in the N66/NGC346 region of the SMC
• Near-infrared spectroscopy of embedded protostars in the massive metal-poor star-forming region NGC 346
• Characterizing the PAH Emission in the Orion Bar