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Published July 10, 2011 | Published
Journal Article Open

The Spitzer-WISE Survey of the Ecliptic Poles


We have carried out a survey of the north and south ecliptic poles, EP-N and EP-S, respectively, with the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE). The primary objective was to cross-calibrate WISE with the Spitzer and Midcourse Space Experiment (MSX) photometric systems by developing a set of calibration stars that are common to these infrared missions. The ecliptic poles were continuous viewing zones for WISE due to its polar-crossing orbit, making these areas ideal for both absolute and internal calibrations. The Spitzer IRAC and MIPS imaging survey covers a complete area of 0.40 deg^2 for the EP-N and 1.28 deg^2 for the EP-S. WISE observed the whole sky in four mid-infrared bands, 3.4, 4.6, 12, and 22 μm, during its eight-month cryogenic mission, including several hundred ecliptic polar passages; here we report on the highest coverage depths achieved by WISE, an area of ~1.5 deg^2 for both poles. Located close to the center of the EP-N, the Sy-2 galaxy NGC 6552 conveniently functions as a standard calibrator to measure the red response of the 22 μm channel of WISE. Observations from Spitzer-IRAC/MIPS/IRS-LL and WISE show that the galaxy has a strong red color in the mid-infrared due to star-formation and the presence of an active galactic nucleus (AGN), while over a baseline >1 year the mid-IR photometry of NGC 6552 is shown to vary at a level less than 2%. Combining NGC 6552 with the standard calibrator stars, the achieved photometric accuracy of the WISE calibration, relative to the Spitzer and MSX systems, is 2.4%, 2.8%, 4.5%, and 5.7% for W1 (3.4 μm), W2 (4.6 μm), W3 (12 μm), and W4 (22 μm), respectively. The WISE photometry is internally stable to better than 0.1% over the cryogenic lifetime of the mission. The secondary objective of the Spitzer-WISE Survey was to explore the poles at greater flux-level depths, exploiting the higher angular resolution Spitzer observations and the exceptionally deep (in total coverage) WISE observations that potentially reach down to the confusion limit of the survey. The rich Spitzer and WISE data sets were used to study the Galactic and extragalactic populations through source counts, color-magnitude and color-color diagrams. As an example of what the data sets facilitate, we have separated stars from galaxies, delineated normal galaxies from power-law-dominated AGNs, and reported on the different fractions of extragalactic populations. In the EP-N, we find an AGN source density of ~260 deg^(–2) to a 12 μm depth of 115 μJy, representing 15% of the total extragalactic population to this depth, similar to what has been observed for low-luminosity AGNs in other fields.

Additional Information

© 2011 The American Astronomical Society. Received 2011 February 28; accepted 2011 April 26; published 2011 June 23. We thank the SSC instrument teams for guidance in constructing AORs, and are especially grateful to B. T. Soifer for providing Directors Discretionary Time to carry out the bulk of the calibration observations with Spitzer. We thank M. Bessell for providing the optical spectra of the SEP calibrators, D. Kilkenny, R. Sefako, F. van Wyk, and D. Cooper of SAAO for securing and reducing their optical photometry. We thank L. Armus for providing Spitzer IRS spectra of ULIRGs to help disentangle spectral RSR response differences between stars and galaxies. We thank G. Sloan and the SAGESPEC team for providing newly reduced IRS spectra of our SEP and off-pole calibrators. We thank J. Krick for providing the IRAC Dark Field catalog. We are grateful to M. Cluver for helpful discussions and review of the manuscript. This work is based (in part) on observations made with Spitzer, which is operated by JPL, Caltech under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This work is also based (in part) on observations made with 2MASS, a joint collaboration between the University of Massachusetts and the Infrared Processing and Analysis Center (JPL/Caltech), with funding provided primarily by NASA and the NSF. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.

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