2MASS extended source catalog: Overview and algorithms

The Two Micron All Sky Survey (2MASS) will observe over 1,000,000 galaxies and extended Galactic sources covering the entire sky at wavelengths between 1 and 2 mu m. Most of these galaxies will be newly cataloged objects. The survey catalog will have both high completeness and reliability down to J = 15.0 mag and K-s = 13.5 mag, equivalent to 1.6 and 2.9 mJy, respectively. Galaxies as small as 10 " are resolved, and those as large as similar to 2.'5 are fully imaged. 2MASS will discover galaxies never seen before in the "zone of avoidance" caused by the obscuring effects of Galactic dust and gas, limited only by the extreme number of stars at very low Galactic latitude, especially toward the Galactic center. This paper describes the basic algorithms used to detect and characterize extended sources in the 2MASS database and catalog. Critical procedures include tracking the point-spread function, image background removal, artifact removal, photometry and basic parameterization, star-galaxy discrimination, and object classification using a decision tree technique. We introduce and provide examples of the types of extended sources that 2MASS detects across the sky, including galaxies, Galactic nebulae and resolved stellar objects, multiple stars and clusters, and, finally, artifacts arising from bright stars and transient events. A future paper will provide a full statistical analysis and verification of the completeness, reliability, and integrity of the first release catalog, as well as some of the basic scientific results of the catalog, including galaxy colors, number counts, and redshift distribution.