WISE photometry for 400 million SDSS sources

TL;DR

This paper introduces a method for measuring WISE infrared fluxes for 400 million SDSS sources using forced photometry, enabling detection of faint signals and consistent cross-survey analysis.

Contribution

The authors develop and apply a forced photometry technique with The Tractor code to extract fluxes from WISE images based on SDSS source positions, improving detection of faint sources.

Findings

Many sources have measurable fluxes below WISE pipeline detection thresholds.

The method provides consistent flux measurements across SDSS and WISE.

Results and code are publicly available for community use.

Abstract

We present photometry of images from the Wide-Field Infrared Survey Explorer (WISE; Wright et al. 2010) of over 400 million sources detected by the Sloan Digital Sky Survey (SDSS; York et al. 2000). We use a "forced photometry" technique, using measured SDSS source positions, star-galaxy separation and galaxy profiles to define the sources whose fluxes are to be measured in the WISE images. We perform photometry with The Tractor image modeling code, working on our "unWISE" coaddds and taking account of the WISE point-spread function and a noise model. The result is a measurement of the flux of each SDSS source in each WISE band. Many sources have little flux in the WISE bands, so often the measurements we report are consistent with zero. However, for many sources we get three- or four-sigma measurements; these sources would not be reported by the WISE pipeline and will not appear in the WISE catalog, yet they can be highly informative for some scientific questions. In addition, these small-signal measurements can be used in stacking analyses at catalog level. The forced photometry approach has the advantage that we measure a consistent set of sources between SDSS and WISE, taking advantage of the resolution and depth of the SDSS images to interpret the WISE images; objects that are resolved in SDSS but blended together in WISE still have accurate measurements in our photometry. Our results, and the code used to produce them, are publicly available at http://unwise.me.