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Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy's Evolution (SAGE). I. Overview and Initial Results
We are performing a uniform and unbiased imaging survey of the LargeMagellanic Cloud (LMC; ~7deg×7deg) using theIRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm)instruments on board the Spitzer Space Telescope in the Surveying theAgents of a Galaxy's Evolution (SAGE) survey, these agents being theinterstellar medium (ISM) and stars in the LMC. This paper provides anoverview of the SAGE Legacy project, including observing strategy, dataprocessing, and initial results. Three key science goals determined thecoverage and depth of the survey. The detection of diffuse ISM withcolumn densities >1.2×1021 H cm-2 permits detailed studies of dust processes in the ISM. SAGE'spoint-source sensitivity enables a complete census of newly formed starswith masses >3 Msolar that will determine the current starformation rate in the LMC. SAGE's detection of evolved stars withmass-loss rates >1×10-8 Msolaryr-1 will quantify the rate at which evolved stars injectmass into the ISM of the LMC. The observing strategy includes two epochsin 2005, separated by 3 months, that both mitigate instrumentalartifacts and constrain source variability. The SAGE data arenonproprietary. The data processing includes IRAC and MIPS pipelines anda database for mining the point-source catalogs, which will be releasedto the community in support of Spitzer proposal cycles 4 and 5. Wepresent initial results on the epoch 1 data for a region near N79 andN83. The MIPS 70 and 160 μm images of the diffuse dust emission ofthe N79/N83 region reveal a similar distribution to the gas emissions,especially the H I 21 cm emission. The measured point-source sensitivityfor the epoch 1 data is consistent with expectations for the survey. Thepoint-source counts are highest for the IRAC 3.6 μm band and decreasedramatically toward longer wavelengths, consistent with the fact thatstars dominate the point-source catalogs and the dusty objects detectedat the longer wavelengths are rare in comparison. The SAGE epoch 1point-source catalog has ~4×106 sources, and more areanticipated when the epoch 1 and 2 data are combined. Using Milky Way(MW) templates as a guide, we adopt a simplified point-sourceclassification to identify three candidate groups-stars without dust,dusty evolved stars, and young stellar objects-that offer a startingpoint for this work. We outline a strategy for identifying foreground MWstars, which may comprise as much as 18% of the source list, andbackground galaxies, which may comprise ~12% of the source list.
| MSX, 2MASS, and the LARGE MAGELLANIC CLOUD: A Combined Near- and Mid-Infrared View
The Large Magellanic Cloud (LMC) has been observed by the MidcourseSpace Experiment (MSX) in the mid-infrared and the Two Micron All SkySurvey (2MASS) in the near-infrared. We have performed across-correlation of the 1806 MSX catalog sources and nearly 1.4 million2MASS cataloged point and extended sources and find 1664 matches. Usingthe available color information, we identify a number of stellarpopulations and nebulae, including main-sequence stars, giant stars, redsupergiants, carbon- and oxygen-rich asymptotic giant branch (AGB)stars, planetary nebulae, H II regions, and other dusty objects likelyassociated with early-type stars. A total of 731 of these sources haveno previous identification. We compile a listing of all objects, whichincludes photometry and astrometry. The 8.3 μm MSX sensitivity is thelimiting factor for object detection: only the brighter red objects,specifically the red supergiants, AGB stars, planetary nebulae, and H IIregions, are detected in the LMC. The remaining objects are likely inthe Galactic foreground. The spatial distribution of the infrared LMCsources may contribute to understanding stellar formation and evolutionand the overall galactic evolution. We demonstrate that a combined mid-and near-infrared photometric baseline provides a powerful means ofidentifying new objects in the LMC for future ground-based andspace-based follow-up observations.
| UBV photometry of galactic foreground and LMC member stars. I - Galactic foreground stars
UBV photometry of 955 galactic foreground stars in the direction to theLarge Magellanic Cloud is presented. The stars have been chosen fromforeground star catalogs and have been measured to complete a new database containing entries of more than 5000 stars in the direction of theLMC. First and second order extinction coefficients at La Silla/Chileare given, which differ from the standard values because of the 1991eruption of the volcano Mt. Pinatubo.
| Positional reference stars in the Magellanic Clouds
The equatorial coordinates are determined of 926 stars (mainly ofgalactic origin) in the direction of the Magellanic Clouds at the meanepoch T = 1978.4 with an overall accuracy characterized by the meanvalues of the O-C coordinates, Sa = 0.35 arcsec and Sd = 0.38 arcsec,calculated from the coordinates of the Perth reference stars. Thesevalues are larger than the accuracy expected for primary standard stars.They allow the new positions to be considered as those of reliablesecondary standard stars. The published positions correspond to anunquestionable improvement of the quality of the coordinates provided inthe current catalogs. This study represents an 'astrometric step' in thestarting of a 'Durchmusterung' of the Magellanic Clouds organized by deBoer (1988, 1989).
| Radial velocities from objective-prism plates in the direction of the Large Magellanic Cloud
A catalog is presented of 711 Large Magellanic Cloud (LMC) stars, withattention to the radial velocities of 418 of these. Also given are theradial velocities of 1127 galactic stars in the direction of the LMC, aswell as discussions of the precision of these measurements and of radialvelocity dispersion in different fields.
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