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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

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Despite decades of study, it remains unclear whether there are distinct radio-loud and radio-quiet populations of quasi-stellar objects (QSOs). Early studies were limited by inhomogeneous QSO samples, inadequate sensitivity to probe the radio-quiet population, and degeneracy between redshift and luminosity for flux-density-limited samples. Our new 6 GHz EVLA observations allow us for the first time to obtain nearly complete (97%) radio detections in a volume-limited color-selected sample of 179 QSOs more luminous than M i = −23 from the Sloan Digital Sky Survey (SDSS) Data Release Seven in the narrow redshift range 0.2 < z < 0.3. The dramatic improvement in radio continuum sensitivity made possible with the new EVLA allows us, in 35 minutes of integration, to detect sources as faint as 20 µJy, or log[L 6 GHz (W Hz −1 )] ≈ 21.5 at z = 0.25, well below the radio luminosity, log[L 6 (W Hz −1 )] ≈ 22.5, that separates star-forming galaxies from radio-loud active galactic nuclei (AGNs) driven by accretion onto a super-massive black hole. We calculate the radio luminosity function (RLF) for these QSOs using three constraints: (a) EVLA 6 GHz observations for log[L 6 (W Hz −1 )] < 23.5, (b) NRAO-VLA Sky Survey (NVSS) observations for log[L 6 (W Hz −1 )] > 23.5, and (c) the total number of SDSS QSOs in our volume-limited sample. We show that the RLF can be explained as a superposition of two populations, dominated by AGNs at the bright end and star formation in the QSO host galaxies at the faint end.

We present new Karl G. Jansky Very Large Array radio continuum images of the nuclei of Arp 220, the nearest ultra-luminous infrared galaxy. These new images have both the angular resolution to study the detailed morphologies of the two nuclei that power the galaxy merger and sensitivity to a wide range of spatial scales. At 33 GHz, we achieve a resolution of 0. 081 × 0. 063 (29.9 × 23.3 pc) and resolve the radio emission surrounding both nuclei. We conclude from the decomposition of the radio spectral energy distribution that a majority of the 33 GHz emission is synchrotron radiation. The spatial distributions of radio emission in both nuclei are well-described by exponential profiles. These have deconvolved half-light radii (R 50d ) of 51 and 35 pc for the eastern and western nuclei, respectively, and they match the number density profile of radio supernovae observed with very long baseline interferometry. This similarity might be due to the fast cooling of cosmic rays electrons caused by the presence of a strong (∼ mG) magnetic field in this system. We estimate extremely high molecular gas surface densities of 2.2 +2.1 −1.0 × 10 5 (east) and 4.5 +4.5 −1.9 × 10 5 (west) M pc −2 , corresponding to total hydrogen column densities of N H = 2.7 +2.7 −1.2 × 10 25 (east) and 5.6 +5.5 −2.4 × 10 25 cm −2 (west). The implied gas volume densities are similarly high, n H 2 ∼ 3.8 +3.8 −1.6 10 4 (east) and ∼ 11 +12 −4.5 × 10 4 cm −3 (west). We also estimate very high luminosity surface densities of Σ IR ∼ 4.2 +1.6 −0.7 × 10 13 (east) and Σ IR ∼ 9.7 +3.7 −2.4 × 10 13 (west) L kpc −2 , and star formation rate surface densities of Σ SFR ∼ 10 3.7±0.1 (east) and Σ SFR ∼ 10 4.1±0.1 (west) M yr −1 kpc −2 . These values, especially for the western nucleus are, to our knowledge, the highest luminosity surface densities and star formation rate surface densities measured for any star-forming system. Despite these high values, the nuclei appear to lie below the dusty Eddington limit in which radiation pressure is balanced only by self-gravity. The small measured sizes also imply that at wavelengths shorter than λ = 1 mm, dust absorption effects must play an important role in the observed light distribution while below 5 GHz free-free absorption contributes substantial opacity. According to these calculations, the nuclei of Arp 220 are only transparent in the frequency range ∼ 5 to 350 GHz. Our results offer no clear evidence that an active galactic nucleus dominates the emission from either nucleus at 33 GHz.14 Lonsdale et al. (2006) report offsets from the center of Arp 220, which we take to be α 2000 = 15 h 34 m 57.259 s , δ 2000 = 23 • 30 11 .409.