We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; $7 ; 7) using the IRAC (3.6, 4.5, 5.8, and 8 m) and MIPS (24, 70, and 160 m) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1:2 ; 10 21 H cm À2 permits detailed studies of dust processes in the ISM. SAGE's point-source sensitivity enables a complete census of newly formed stars with masses >3 M that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass-loss rates >1 ; 10 À8 M yr À1 will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 m images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H i 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 m band and decrease dramatically toward longer wavelengths, A

The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 m and measurements of spectral energy distributions between 52 and 100 m at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 m array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.

We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands, broadband images in the J, H, and bands, narrowband images centered at the wavelengths of Brc and 0) S(1), and K s H 2 (1, imaging spectroscopy centered on [Ne II] (12.8 km). We have subtracted a composite stellar spectrum from the galaxy spectrum to measure faint emission lines that otherwise would be buried in the complicated continuum structure. We use these data and data from the literature in a comprehensive reassessment of the starburst in this galaxy. We conÐrm that the [Fe II] emission is predominantly excited by supernova explosions and show that the rate of these events can be derived from the strength of the infrared [Fe II] lines. Although the emission superÐcially resembles a thermally excited spectrum, H 2 most of the infrared luminosity is excited by Ñuorescence in low-density gas. We conÐrm the presence H 2 of a bar and also show that this galaxy has a circumnuclear ring. The relation of these features to the gaseous bar seen in CO is in agreement with the general theoretical picture of how gas can be concentrated into galaxy centers by bars. We derive a strong upper limit of D37,000 K for the stars exciting the emission lines. We use velocity-resolved infrared spectra to determine the mass in the starburst region. Most of this mass appears to be locked up in the old, preexisting stellar population. Using these constraints and others to build an evolutionary synthesis model, we Ðnd that the initial mass function (IMF) originally derived to Ðt the starburst in M82 also accounts for the properties of NGC 253 ; this IMF is similar to a modiÐed Salpeter IMF. The models indicate that rapid massive star formation has been ongoing for 20È30 million yr in NGC 253 ; that is, it is in a late phase of its starburst. Its optical spectrum has characteristics of a transitional H II/weak-[O I] LINER. We model the emission-line spectrum expected from a late phase starburst and demonstrate that it reproduces these characteristics.

Color-magnitude diagrams (CMDs) are presented for the Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the Large Magellanic Cloud (LMC). IRAC and MIPS 24 µm epoch one data are presented. These data represent the deepest, widest mid-infrared CMDs of their kind ever produced in the LMC. Combined with the 2MASS survey, the diagrams are used to delineate the evolved stellar populations in the Large Magellanic Cloud as well as Galactic foreground and extragalactic background populations. Some 32000 evolved stars brighter than the tip of the red giant branch are identified. Of these, approximately 17500 are classified as oxygen-rich, 7000 carbon-rich, and another 1200 as "extreme" asymptotic giant branch (AGB) stars. Brighter members of the latter group have been called "obscured" AGB stars in the literature owing to their dusty circumstellar envelopes. A large number (1200) of luminous oxygen-rich AGB stars/M supergiants are also identified. Finally, there is strong evidence from the 24 µm MIPS channel that previously unexplored, lower luminosity oxygen-rich AGB stars contribute significantly to the mass loss budget of the LMC (1200 such sources are identified).

We present the stellar calibrator sample and the conversion from instrumental to physical units for the 24 µm channel of the Multiband Imaging Photometer for Spitzer (MIPS). The primary calibrators are A stars, and the calibration factor based on those stars is 4.54 × 10 −2 MJy sr −1 (DN/s) −1 , with a nominal uncertainty of 2%. We discuss the data-reduction procedures required to attain this accuracy; without these procdures, the calibration factor obtained using the automated pipeline at the Spitzer Science Center is 1.6%±0.6% lower. We extend this work to predict 24 µm flux densities for a sample of 238 stars which covers a larger range of flux densities and spectral types. We present a total of 348 measurements of 141 stars at 24 µm. This sample covers a factor of ∼ 460 in 24 µm flux density, from 8.6 mJy up to 4.0 Jy. We show that the calibration is linear over that range with respect to target flux and background level. The calibration is based on observations made using 3-second exposures; a preliminary analysis shows that the calibration factor may be 1% and 2% lower for 10-and 30-second exposures, respectively. We also demonstrate that the calibration is very stable: over the course of the mission, repeated measurements of our routine calibrator, HD 159330, show a root-mean-square scatter of only 0.4%. Finally, we show that the point spread function (PSF) is well measured and allows us to calibrate extended sources accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a sample of nearby galaxies are identical within the uncertainties.