Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared (k P 5 m) continuum of stellar populations is dominated by the composite blackbody curve and peaks at approximately 1.6 m, the ultraviolet to mid-infrared continuum of active galactic nuclei (AGNs) is dominated by a power law. Consequently, with a sufficient wavelength baseline, one can easily distinguish AGNs from stellar populations. Mirroring the tendency of AGNs to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGNs tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. On the basis of this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGNs, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1 galaxies. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGNs and find evidence for a large population of optically obscured active galaxies.

The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 µm. Two nearly adjacent 5.2×5.2 arcmin fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 µm; 4.5 and 8 µm). All four detector arrays in the camera are 256×256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutely calibrated by comparing photometry of a set of A stars near the north ecliptic pole to predictions based on ground-based observations and a stellar atmosphere model. The brightness of point sources is calibrated to an accuracy of 3%, relative to models for A-star stellar atmospheres, for observations performed and analyzed in the same manner as for the calibration stars. This includes corrections for the location of the star in the array and the location of the centroid within the peak pixel. Long-term stability of the IRAC photometry was measured by monitoring the brightness of A dwarfs and K giants (near the north ecliptic pole) observed several times per month; the photometry is stable to 1.5% (rms) over a year. Intermediate-timescale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC was in nominal operations; the intermediate-term photometry is stable, with a 1% dispersion (rms). One of the secondary calibrators was found to have significantly time-variable (5%) mid-infrared emission, with a period (7.4 days) matching the optical light curve; it is possibly a Cepheid variable.

The Spitzer Space Telescope was used to study the mid-infrared to far-infrared properties of NGC 300, and to compare dust emission to Halpha to elucidate the heating of the ISM and the star formation cycle at scales < 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 micron dust emission with respect to 24 micron dust and to HII regions traced by the Halpha light. The 8 micron emission highlights the rims of HII regions, and the 24 micron emission is more strongly peaked in star forming regions than at 8 microns. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 microns, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star forming regions. When averaging over regions larger than ~ 1 kpc, the ratio of Halpha to Aromatic Feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid-to-far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.Comment: to appear in the ApJS Spitzer special issue (September 2004

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We explore the role of active galactic nuclei (AGN) in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1 Gyr) the termination of their star formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the All-wavelength Extended Groth strip International Survey (AEGIS). Stacking the X-ray photons at the positions of galaxies (0.4 < z < 0.9) not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-infrared (IR) properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that black hole (BH) accretion outlives the termination of the star formation. This is also supported by our finding that poststarburst galaxies at z ≈ 0.8 and AGN are associated, in agreement with recent results at low z. A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after quenching their star formation.

We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z > 0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 µm sources are selected from a Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Boötes field and have corresponding 8 µm data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 µm to 8 µm flux ratio. The X-ray detected sources fall within the peak corresponding to a flat spectrum in νf ν , implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 µm to 8 µm flux ratio is likely due to LIRGs whose infrared emission is powered by starbursts. The 24 µm emission is increasingly dominated by AGN at higher 24 µm flux densities (f 24 ): the AGN fraction of the z > 0.6 sources increases from 9% at f 24 ≈0.35 mJy to 74±20% at f 24 ≈ 3 mJy in good agreement with model predictions. Deep 24 µm, small area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ∼3-7% of the total 24 µm background.

Infrared source counts at wavelengths 3 < λ < 10 µm cover more than 10 magnitudes in source brightness, four orders of magnitude in surface density, and reach an integrated surface density of 10 5 sources deg −2 . At m < 14 mag, most of the sources are Galactic stars, in agreement with models. After removal of Galactic stars, galaxy counts are consistent with what few measurements exist at nearby wavelengths. At 3.6 and 4.5 µm, the galaxy counts follow the expectations of a Euclidean world model down to ∼16 mag and drop below the Euclidean curve for fainter magnitudes. Counts at these wavelengths begin to show decreasing completeness around magnitude 19.5. At 5.8 and 8 µm, the counts relative to a Euclidean world model show a large excess at bright magnitudes. This is probably because local galaxies emit strongly in the aromatic dust ("PAH") features. The counts at 3.6 µm resolve <50% of the Cosmic Infrared Background at that wavelength.