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The Spitzer-WISE Survey of the Ecliptic Poles
We have carried out a survey of the north and south ecliptic poles, EP-Nand EP-S, respectively, with the Spitzer Space Telescope and theWide-field Infrared Survey Explorer (WISE). The primary objective was tocross-calibrate WISE with the Spitzer and Midcourse Space Experiment(MSX) photometric systems by developing a set of calibration stars thatare common to these infrared missions. The ecliptic poles werecontinuous 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 0.40deg2 for the EP-N and 1.28 deg2 for the EP-S. WISEobserved the whole sky in four mid-infrared bands, 3.4, 4.6, 12, and 22?m, during its eight-month cryogenic mission, including severalhundred ecliptic polar passages; here we report on the highest coveragedepths achieved by WISE, an area of ~1.5 deg2 for both poles.Located close to the center of the EP-N, the Sy-2 galaxy NGC 6552conveniently functions as a standard calibrator to measure the redresponse of the 22 ?m channel of WISE. Observations fromSpitzer-IRAC/MIPS/IRS-LL and WISE show that the galaxy has a strong redcolor in the mid-infrared due to star-formation and the presence of anactive galactic nucleus (AGN), while over a baseline >1 year themid-IR photometry of NGC 6552 is shown to vary at a level less than 2%.Combining NGC 6552 with the standard calibrator stars, the achievedphotometric accuracy of the WISE calibration, relative to the Spitzerand MSX systems, is 2.4%, 2.8%, 4.5%, and 5.7% for W1 (3.4 ?m), W2(4.6 ?m), W3 (12 ?m), and W4 (22 ?m), respectively. The WISEphotometry is internally stable to better than 0.1% over the cryogeniclifetime of the mission. The secondary objective of the Spitzer-WISESurvey was to explore the poles at greater flux-level depths, exploitingthe higher angular resolution Spitzer observations and the exceptionallydeep (in total coverage) WISE observations that potentially reach downto the confusion limit of the survey. The rich Spitzer and WISE datasets were used to study the Galactic and extragalactic populationsthrough source counts, color-magnitude and color-color diagrams. As anexample of what the data sets facilitate, we have separated stars fromgalaxies, delineated normal galaxies from power-law-dominated AGNs, andreported on the different fractions of extragalactic populations. In theEP-N, we find an AGN source density of ~260 deg-2 to a 12?m depth of 115 ?Jy, representing 15% of the total extragalacticpopulation to this depth, similar to what has been observed forlow-luminosity AGNs in other fields.
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