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THE SPITZER SURVEY of the SMALL MAGELLANIC CLOUD: S3MC IMAGING and PHOTOMETRY in the MID- and FAR-INFRARED WAVE BANDS Alberto D

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THE SPITZER SURVEY of the SMALL MAGELLANIC CLOUD: S3MC IMAGING and PHOTOMETRY in the MID- and FAR-INFRARED WAVE BANDS Alberto D The Astrophysical Journal, 655:212Y232, 2007 January 20 A # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE SPITZER SURVEY OF THE SMALL MAGELLANIC CLOUD: S3MC IMAGING AND PHOTOMETRY IN THE MID- AND FAR-INFRARED WAVE BANDS Alberto D. Bolatto,1 Joshua D. Simon,2 Snezˇana Stanimirovic´,1, 3 Jacco Th. van Loon,4 Ronak Y. Shah,5 Kim Venn,6 Adam K. Leroy,1 Karin Sandstrom,1 James M. Jackson,5 Frank P. Israel,7 Aigen Li,8 Lister Staveley-Smith,9 Caroline Bot,2 Francois Boulanger,10 and Mo´nica Rubio11 Received 2006 July 17; accepted 2006 August 25 ABSTRACT We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S3MC), which imaged the star-forming body of the SMC in all seven MIPS and IRAC wave bands. We find that the F8/F24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 m brightness and F24/F70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to com- pile a photometric catalog of 400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: (1) bright 5.8 m sources with very faint optical counterparts and very red mid-infrared colors (½5:8À½8:0 >1:2), which we identify as YSOs; (2) bright mid-infrared sources with mildly red colors (0:16 P ½5:8À½8:0 < 0:6), identified as carbon stars; (3) bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars; and (4) unreddened early B stars (B3YO9) with a large 24 m excess. This excess is reminiscent of debris disks and is detected in only a small fraction of these stars (P5%). The majority of the brightest infrared point sources in the SMC fall into groups 1Y3. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (7%). Subject headinggs: dust, extinction — galaxies: dwarf — galaxies: stellar content — infrared: ISM — infrared: stars — Magellanic Clouds Online material: machine-readable table 1. INTRODUCTION complex is the giant H ii region N66 (also known as DEM 103, or NGC 346; Henize 1956; Davies et al. 1976), which contains ap- The Small Magellanic Cloud (SMC), one of the closest and most proximately 60 O stars (Massey et al. 1989). The star formation prominent neighbors of the Milky Way, is a southern hemisphere activity of the SMC, as well as most of its gas, is concentrated in dwarf galaxy of low mass (M 2:4 ; 109 M ; Stanimirovic´ dyn the feature known as the ‘‘bar’’ (probably the body of the SMC et al. 2004) and small size (R 3 kpc). The SMC is actively in- à proper). A second feature, the ‘‘wing,’’ is an extension roughly teracting with its companion, the Large Magellanic Cloud (LMC; studied by the Spitzer SAGE project; Meixner et al. 2006). Both perpendicular to the bar on its eastern side that continues into the H i bridge that joins both clouds (Fig. 1). As one may expect in an galaxies are being tidally disrupted by the Milky Way and also interacting system, the three-dimensional structure of the galaxy suffer from ram pressure interactions with the halo of hot gas sur- rounding the Galaxy (e.g., Gardiner & Noguchi 1996). The results remains quite unclear. Recent analyses show that the prominent irregular features, which are apparent in the light of young stars, of this complex dynamical interplay are the dramatic features of disappear when looking at the old stellar populations (Cioni et al. the Magellanic Bridge and Magellanic Stream, which dominate 2000; Zaritsky et al. 2000; Maragoudaki et al. 2001). These are the high-velocity H i emission in the southern sky. regularly distributed, and the shape of the SMC appears sphe- Befitting its nature as a gas-rich late-type dwarf galaxy, the roidal. Observations indicate that the distances to the bar and the SMC contains a number of sites of active star formation that are wing are measurably different and that the line-of-sight depth of apparent in H images, although none are as prominent as the 30 the SMC is not negligible, amounting to 6Y12 kpc (Crowl et al. Doradus starburst in the LMC. The brightest SMC star-forming 2001 and references therein). The distance to the center of the 1 SMC is approximately 61.1 kpc, corresponding to a distance mod- Department of Astronomy and Radio Astronomy Laboratory, University of À1 California, Berkeley, CA. ulus of m À M ¼ 18:93 mag and a spatial scale of 0.3 pc arcsec 2 Department of Astronomy, California Institute of Technology, Pasadena, CA. (Hilditch et al. 2005; Keller & Wood 2006). To put this in context, 3 Current Address: Department of Astronomy, University of Wisconsin, the spatial resolution of the 3.6Y8.0 m data presented here is Madison, WI. 4 similar to the 60 m resolution of the Infrared Astronomical Astrophysics Group, Lennard Jones Laboratories, Keele University, Satellite (IRAS ) at 2 kpc. The thickness of the SMC introduces a Staffordshire, UK. 5 Department of Astronomy and Center for Astrophysical Research, Boston dispersion of 0.2 mag in the distance modulus for objects lo- University, Boston, MA. cated near either edge. The line of sight to the SMC is very clear, 6 Department of Astronomy, University of Victoria, Victoria, BC, Canada. with a foreground extinction AV 0:12, corresponding to a red- 7 Leiden Observatory, RA Leiden, Netherlands. dening of E(B À V ) 0:04 (Schlegel et al. 1998). 8 Department of Physics and Astronomy, University of Missouri, Columbia, MO. 9 Australia Telescope National Facility, Epping, Australia. The proximity of the SMC makes it the lowest metallicity gas- 10 Institut d’Astrophysique Spatiale, Universite´ Paris-Sud 11, Orsay, France. rich galaxy that can be studied in great detail. For observational 11 Departamento de Astronomı´a, Universidad de Chile, Santiago, Chile. reasons metallicity, Z, is commonly quantified using the oxygen 212 SPITZER SURVEY OF SMALL MAGELLANIC CLOUD 213 Fig. 1.—Spatial coverage of the S3MC data, overlaid on the H ‘‘parking lot camera’’ image of the SMC (Kennicutt et al. 1995). The white outline indicates the coverage of the IRAC observations, while the gray outline shows the coverage for MIPS. Note that this is only approximate, since the placement of the cameras on the Spitzer focal plane results in slightly different areas of the sky mapped for the different wave bands. The SMC bar is the region where the bulk of the star formation (indicated by H emission) takes place, extending roughly from (01h15m, À7330 0) to (00h54m, À72000). The wing region, notable in H i but with very little diffuse H emission, is approximately perpendicular to the bar and extends eastward from (01h00m, À72450) to (00h45m, À73150) and beyond. abundance relative to hydrogen. The gas-phase oxygen abun- Other lines of research also suggest a dearth of the smallest dances measured in SMC H ii regions are 12 þ log (O/H) 8:0 dust grains in the low-metallicity ISM of the SMC. The first (Dufour 1975, 1984; Dufour & Harlow 1977), only a factor of wide-field infrared maps of the SMC were acquired by IRAS and 4Y5 higher than those observed in the lowest metallicity galaxies presented by Schwering & Israel (1989). Sauvage et al. (1990) known (I Zw 18 and SBS 0335À052), and a factor of 5 lower studied the IRAS colors of the SMC, comparing them to the than solar metallicity. The same metallicity with very little dis- emission from larger, higher metallicity galaxies, and concluded persion is observed in several stellar types in the SMC, includ- that there was evidence for a paucity of the small grains that are ing K, F, and A supergiants and Cepheid variables (Russell & responsible for the bulk of the extended 12 m emission. More Bessell 1989; Hill 1997; Luck et al. 1998; Venn 1999). Its low recently, Stanimirovic´ et al. (2000) reprocessed the IRAS data to heavy-element abundance suggests that the SMC may be one of produce maps with improved angular resolution of 10 at 12, 25, the best local templates for studying the primitive interstellar me- and 60 m and 1.70 at 100 m. By comparing these data with H i dium (ISM) and providing insight into the processes at work in observations of the SMC at similar resolution, they were able to primordial galaxies at high redshift. In particular, the interaction study the large-scale properties of the ISM. Stanimirovic´ et al. between massive stars, gas, and dust is unusually accessible in (2000) used these far-infrared (FIR) observations to measure the this nearby galaxy. dust mass, the dust temperature and its spatial variations, and the Compared to the Milky Way, the dust properties of the SMC dust-to-gas ratio (DGR) across the SMC. This study found a are peculiar. For quite a while it has been recognized that the UV mean dust-to-gas ratio in the SMC that is a factor of 100 times extinction curve in the SMC lacks the 2175 8 ‘‘hump’’ (e.g., lower than in the Milky Way, providing a different environment Gordon et al. 2003), which is interpreted as signaling the lack of for star formation to take place. It also concluded that the dust a population of small carbonaceous dust grains (Li & Draine 2002).
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