The SIRTF Wide-area InfraRed Extragalactic survey (SWIRE), the largest SIRTF Legacy program, is a wide-area, imaging survey to trace the evolution of dusty, starforming galaxies, evolved stellar populations, and AGN as a function of environment, from redshifts z∼3 to the current epoch. SWIRE will survey 7 high-latitude fields, totaling 60-65 sq. deg. in all 7 SIRTF bands: IRAC 3.6, 4.5, 5.6, 8µm and MIPS 24, 70, 160µm. Extensive modeling suggests that the Legacy Extragalactic Catalog may contain in excess of 2 million IR-selected galaxies, dominated by (1) ∼150,000 luminous infrared galaxies (LIRGs: L F IR > 10 11 L ⊙) detected by MIPS (and significantly more detected by IRAC), ∼7,000 of these with z>2; (2) 1 million IRAC-detected early-type galaxies (∼ 2×10 5 with z > 1 and ∼10,000 with z > 2); and (3) ∼ 20,000 classical AGN detected with MIPS, plus significantly more dust-obscured QSO/AGN among the LIRGs. SWIRE will provide an unprecedented view of the evolution of galaxies, structure, and AGN. The key scientific goals of SWIRE are: (1) to determine the evolution of actively star-forming and passively evolving galaxies in order to understand the history of galaxy formation in the context of cosmic structure formation; (2) to determine the evolution of the spatial distribution and clustering of evolved galaxies, starbursts and AGN in the key redshift range, 0.5

We characterize the Spitzer Wide-area Infrared Extragalactic Legacy Survey (SWIRE) galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24 m Spitzer data and deep U, g 0 , r 0 , i 0 optical imaging within an area $1/3 deg 2 for $16,000 Spitzer SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6 m and almost 350,000 at 24 m; $70,000 of these will be five-band 3.6-24 m detections. The colors cover a broad range, generally well represented by redshifted spectral energy distributions of known galaxy populations; however, significant samples of unusually blue objects in the [3.6]À[4.5] color are found, as well as many objects very red in the 3.6À24 m mid-IR. Nine of these are investigated and are interpreted as star-forming systems, starbursts, and active galactic nuclei (AGNs) from z ¼ 0:37 to 2.8, with luminosities from L IR ¼ 10 10:3 to 10 13.7 L .

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