IV. 2MASS Data Processing

5. Extended Source Identification and Photometry

b. Summary of Modifications to the Extended Source Processor

The final version of GALWORKS includes a number of minor modifications and a few bug fixes, compared to the preliminary processing for the Second Incremental Data Release. The most important modications are itemized below. Some new information is generated and output by the modified GALWORKS, including "coverage" maps, "total" flux measures, a concentration index, half-light (effective) radii and surface brightness measures.

  1. CALMON zero points from preliminary processing are input.

  2. Improved large galaxy processing.
    Modification allowed processing of all large galaxies, including M31. The end result was the formation of the Large Galaxy Atlas (Jarrett et al. 2003, AJ, 125, 525), comprising the largest galaxies observed.

  3. Coverage masks and off-scan bright star improvement.

  4. Conversion to signal to noise ratio thresholds.

  5. Large galaxy bias removed.
    We now process large galaxies without any bias as to their nature or even a priori existence.

  6. Improved background determination for scans with large galaxies.

  7. Enlarged GALWORKS working "area" for large galaxies.
    That is, process galaxies with larger working areas, thus capturing more of their flux. The postage stamps are now allowed to grow as large as 5´. Large Galaxy Atlas Images have no limit (e.g., M31 is over 1.4° in size).

  8. Improved Star Subtraction.
    Modifications included improvements to both stellar "subtraction" and 2-D ellipse fitting. The basic algorithmic improvements are described in IV.5d. The primary driver is to improve photometry for galaxies seen through the Galactic plane (and, hence, confused by foreground stars). Highlights of the new method are discussed below.

  9. Re-built Oblique Decision Tree using updated training sets.

  10. The Oblique Decision Tree classifier is input into GALWORKS.
    GALWORKS computes the e- and g-scores "on the fly." It rejects sources whose e- and g-scores exceed a value of 1.75 (i.e., this is a rather generous scoring, erring on the side of completeness, but still eliminating unwanted stars).

  11. Star-galaxy classifier added to the Oblique Decision Tree; see "Getting Rid of Stars", by J. Huchra (CfA).

  12. A better surface brightness metric has been constructed (half-light measures via the total mag extrapolation).

  13. Kron and total photometry improved; see IV.5e.

  14. Star subtraction for edge-on galaxies improved.
    The improvement is to minimize subtraction of "sources" on the major-axis of inclined galaxies (i.e., avoid subtracting pieces of the galaxy from itself)

  15. The postage stamp FITS cube images are improved.
    The postage stamps are now six planes deep, with the first three planes being the nominal set (J, H, Ks), and the additional three planes being the same, except with stars subtracted. These allow the user to perform direct photometric measurements on the postage stamps without the deleterious stellar contamination. The FITS header has also been updated (and improved). Here is an example of a new postage stamp FITS header.

Highlights of the new star subtraction method

[Last Updated: 2002 Jul 15; by T. Jarrett]

Return to Section IV.5.