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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.
| Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 μm Calibration
We present the stellar calibrator sample and the conversion frominstrumental to physical units for the 24 μm channel of the MultibandImaging Photometer for Spitzer (MIPS). The primary calibrators are Astars, and the calibration factor based on those stars is4.54×10-2 MJy sr-1 (DNs-1)-1, with a nominal uncertainty of 2%. Wediscuss the data reduction procedures required to attain this accuracy;without these procedures, the calibration factor obtained using theautomated pipeline at the Spitzer Science Center is 1.6%+/-0.6% lower.We extend this work to predict 24 μm flux densities for a sample of238 stars that covers a larger range of flux densities and spectraltypes. We present a total of 348 measurements of 141 stars at 24 μm.This sample covers a factor of ~460 in 24 μm flux density, from 8.6mJy up to 4.0 Jy. We show that the calibration is linear over that rangewith respect to target flux and background level. The calibration isbased on observations made using 3 s exposures; a preliminary analysisshows that the calibration factor may be 1% and 2% lower for 10 and 30 sexposures, respectively. We also demonstrate that the calibration isvery stable: over the course of the mission, repeated measurements ofour routine calibrator, HD 159330, show a rms scatter of only 0.4%.Finally, we show that the point-spread function (PSF) is well measuredand allows us to calibrate extended sources accurately; InfraredAstronomy Satellite (IRAS) and MIPS measurements of a sample of nearbygalaxies are identical within the uncertainties.
| The visual and photographic ranges and the provisional orbits of Y PISCIUM and RR Draconis.
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