IV. 2MASS Data Processing

5. Extended Source Identification and Photometry

The extended source sensitivity (10) is ~1 mag brighter than the point source limits, or 14.7 (2.1 mJy), 13.9 (3.0 mJy), and 13.1 (4.1 mJy) mag at J, H, Ks, respectively, with the precise threshold depending on the brightness profile of the extended source. Given the ~2´´ angular resolution of the image data and the detector sensitivity, 2MASS is well suited for detecting most types of galaxies to cz < 10,000 km/s and high-luminosity, giant galaxies beyond 30,000 km/s. In addition to galaxies, 2MASS will also identify compact and diffuse Galactic objects. The 2MASS Catalogs consist of detections of about 470 million stars and 1.6 million galaxies.

The scientific objectives of the extended source portion of 2MASS include studies of large scale structure, utilization of the infrared Tully-Fisher relation, a complete survey of the Local Group of galaxies, and an unprecedented census of galaxies located behind the plane of the Milky Way, often referred to as the "Zone of Avoidance." As such, survey requirements were established in order to satisfactorily achieve these science goals. In addition to the sensitivity limits given above, the extended source Level 1 Specifications include >90% completeness and >98% reliability for most of the sky (free of stellar confusion). There are no set requirements for observations deep in the Galactic plane, but the survey maintains a high level of utility all the way down to |b| ~ 0° ( Jarrett et al. 2000a, AJ, 120, 298). The Level 1 science requirements apply to the galaxy catalog derived from the 2MASS database.

The basic 2MASS data and pipeline reduction overview, including discussion of the point spread function (PSF) - a basic component of star-galaxy separation - is given here. We also describe the key parametric measurements made on extended sources and the crucial operational step of background removal. In addition, we describe the algorithms developed to cleanly discriminate between point sources and extended sources. The Catalog reliability criterion is, in particular, a difficult goal to achieve, necessitating implementation of algorithms specifically designed to perform star-galaxy separation with 2MASS imaging data. Finally, we give some examples of the wide array of extended sources that 2MASS detects. We present more detailed scientific results from the 2MASS Catalog, including galaxy colors, source counts, completeness and reliability, and clustering in II.3d.

For a detailed list of changes and modifications that were made to the extended source pipeline during the final re-processing, see IV.5b.

  1. The Extended Source Processor
    1. Atlas Image Background Removal
    2. Source Positions
    3. Ellipse Fitting and Object Orientation
    4. Photometry
    5. Source Parameterization
    6. Star - Galaxy Discrimination
    7. Post-processing Star-Galaxy Separation
    8. Bright Extended (Fuzzy) Stars
    9. Source Extraction
    10. Extended Source Objects
  2. Summary of Modifications to the Extended Source Processor
  3. Stellar Number Density Metric: "density"
  4. Improved Ellipse Fitting and Isophotal Photometry in Crowded Fields
  5. Large Aperture Photometry: Profile Fitting, Isophotal and Total Magnitudes
  6. Galaxy Photometry Error Tree
  7. Stellar Number Density and Confusion Noise

[Last Updated: 2003 Mar 10; by T. Jarrett, T. Chester, S. Schneider, S. Van Dyk, & R. Cutri]

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