Spitzer IRAC/MUSYC Public Legacy Survey in the Extended-CDFS (SIMPLE) Source dection and photometry Document version: 1 June 2007 Data Release 1 (DR1) - 2007/06/01 Data reduction version: v1.1 =============================================================================== This document describes the procedures for source detection and photometry. A more detailed account of the data reduction and products will appear in Damen et al. (in prep), please reference this paper when using these data products in published research. TABLE OF CONTENTS: 1. SOURCE DETECTION 2.1 Detection 2.2 Depth 2. PHOTOMETRY 2.1 PSF matching 2.2 Aperture fluxes 2.3 Total fluxes and aperture corrections 2.4 Caution 3. CATALOG CONTENTS =============================================================================== 1. SOURCE DETECTION =============================================================================== 2.1 Detection Sources were detected and extracted using the SExtractor software (Bertin & Arnouts 1996) on a detection image. The detection image is an inverse-variance weighted average of the 3.6 and 4.5 micron images. The 3.6 and 4.5 micron band are the most sensitive IRAC bands and the combination of the two leads to a very deep detection image. To enable detection to a similar signal-to-noise limit over the entire field, we multiply the 3.6+4.5 image by the square root of the combined exposure map. This produces a "noise-equalized" image with approximately constant signal-to-noise, but different depth, over the entire field. Subsequently we ran SExtractor on the detection map with an ~10 sigma detection threshold. In the detection process SExtractor first convolves the detection map with a detection kernel optimized for point sources. 2.2 Depth The depth of the SIMPLE IRAC mosaic is a function of area, as some parts are more exposed than others. We determine the depth using the rms maps (see 00README_images), scaled to apertures of 3" diameter -- which provides close to optimal signal-to-noise for point sources --, and corrected for missing flux outside the aperture using growth curves of points sources. The following lists the depths (5 sigma, AB magnitude, total, points sources) and areas over which this depth is achieved: percentile 25% 50% 75% (percentile of pixels) exptime 0.9 1.5 2.5 (hours) area ~1200 800 400 (area in arcmin^2 with least this exposure time) 3.6mu 24.09 24.43 24.64 (depth at 3.6 micron) 4.5mu 23.66 24.00 24.22 (depth at 4.5 micron) 5.8mu 21.90 22.24 22.45 (depth at 5.8 micron) 8.0mu 21.85 22.19 22.41 (depth at 8.0 micron) =============================================================================== 2. PHOTOMETRY =============================================================================== 2.1 PSF matching The goal of the release catalog is to provide fluxes that allow one to derive reasonably accurate colors and total fluxes of the IRAC-detected sources. To achieve this the images were smoothed with a gaussian so that the FWHM of the 3.6, 4.5, and 5.8 images is close to that of the image with the largest FWHM, which is the 8.0 micron image. The Gaussian sigma values used for convolution were: 3.6 micron : 0.84 4.5 micron : 0.93 5.8 micron : 0.80 The result images have a more similar FWHM and more similar aperture correction in all bands. Hence aperture flux measurements in same-size apertures, while not measuring total fluxes, can be combined directly to form integrated colors, also for extended sources. 2.2 Aperture fluxes Then SExtractor was run in "dual" image mode, meaning that the program determines the location of sources in the detection image, and then measures the fluxes in the smoothed science images in the exact same apertures. We provide circular aperture measurements of 1.5", 2.0", and 3.0" arcsecond radius, which should provide reasonable colors for small sources. Analysis of the growth curves of point sources indicates that the matching of the PSFs for these apertures is better than 5% between the short wavelength (3.6/4.5) channels and the long wavelength channels (5.8/8.0) separately, and better than 15% across all channels. More advanced techniques can be applied to achieve better PSF-matching. Future catalog releases will make use of such techniques. 2.3 Total fluxes and aperture corrections To correct the aperture measurements in the catalog to "total" fluxes (assuming a point-source profile outside the aperture), multiply the fluxes by the following aperture corrections (to within 15% the same for all bands): aperture radius 1.5" 2.0" 3.0" aperture correction 2.1 1.5 1.2 As an example, the catalog contains a f36_tot column which is the aperture corrected 2.0" radius flux (f36_ap2). The aperture corrections for the original mosaics (before smoothing, assuming point sources) are: aperture radius channel 1.5" 2.0" 3.0" 3.6 1.61 1.31 1.15 4.5 1.66 1.35 1.14 5.8 1.98 1.56 1.21 8.0 2.20 1.75 1.40 2.4 Caution We caution however that these catalogs are very crude. The IRAC point spread functions (PSFs) are quite large and many sources are blended or confused. In addition, while the aperture measurements have been corrected for differences in PSFs across the bands, the correction is still rather crude. Careful attention is needed before interpreting the fluxes in this catalog. =============================================================================== 3. Catalog contents =============================================================================== Note all flux units in the catalog are converted to the same zeropoint on the AB system: ABMAG = 25. - 2.5*log10(fluxunits) x_pos, y_pos Pixel positions (in mosaic reference frame) id Source number ra, dec Celestial coordinates (J2000.0) of IRAC 3.6+4.5 micron detected source fXX_apN Flux density in IRAC band XX (XX = 3.6, 4.5, 5.8, 8.0) in circular aperture N (N = 1,2,3 = 1.5",2",3" radius). All fluxes are normalized to an AB magnitude zeropoint of 25. feXX_apN Uncertainty in flux density in band XX in aperture N f36_tot Total flux density in 3.6 micron from 2" radius aperture, corrected to total using point source growth curves fe36_tot Uncertainty in total flux density expXX The exposure time in ksec averaged over 1.5" radius aperture in channel XX. flag_muxbleed set to 1 of source was corrected for muxbleed flag_blended set to 1 if SExtractor detection isophote (the "segmentation map") blends with a neighboring source REFERENCES Bertin & Arnouts 1996, A&AS, 117, 393 Damen et al. in preparation