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.

CALMON zero points from preliminary processing are input.
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.
Coverage masks and off-scan bright star improvement.
Conversion to signal to noise ratio thresholds.
Large galaxy bias removed.
We now process large galaxies without any bias as to their nature or even a priori existence.
Improved background determination for scans with large galaxies.
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).
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.
Re-built Oblique Decision Tree using updated training sets.
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).
Star-galaxy classifier added to the Oblique Decision Tree; see "Getting Rid of Stars", by J. Huchra (CfA).
A better surface brightness metric has been constructed (half-light measures via the total mag extrapolation).
Kron and total photometry improved; see IV.5e.
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)
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, K_s), 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

For the first "guess" at the 2-D elliptical shape of the galaxy, the rotating second-moment algorithm is used.

Before star subtraction, the first-guess shape of the galaxy is used and is subtracted from the image (i.e, the galaxy is removed). The galaxy removal is carried out by using the first-guess axis ratio and position angle. For each elliptical annulus, the lower quartile of the pixel distribution is computed. The lower quartile is robust to star contamination. Then, the lower quartile value is subtracted from each annulus pixel. The galaxy subtraction is carried out until the point at which the lower quartile is deep in the noise.

With the galaxy removed, the neighboring stars are examined. The stars are subtracted using the PSF "shape" ("SH"), to determine its size, or extent, and the local background median. Subtraction uses the local annulus median for each star: (a) the size of star is determined, (b) annulus size is determined, (c) the median is computed , (4) using the median, how much of the star should be removed is determined from the non-galaxy subtracted image. In this way, the star is removed without losing any of the galaxy. The danger is removing stars close to the galaxy nucleus. Here the galaxy removal step is not as accurate (due to 2MASS sampling and pixelization), and the local annulus median is not well-determined. Hence, for sources near the nucleus, a star is not subtracted, but instead is masked. The masked pixels are later recovered with isophotal substitution.
The following is an example of a typical galaxy, with K_s=12.8 mag, in the cluster Abell 3558. The galaxy has a contaminating star nearby (along its major axis). Figure 1 shows the raw K_s-band field. Figure 2 shows the star-subtracted image. Figure 3 shows the 3- and 1- (20 mag /arcsec²) elliptical isophotes. Figure 4 shows the shows the mean surface brightness in elliptical annuli about the galaxy. The green dashed line corresponds to the isophotal radius (20 mag/arcsec²). The red dotted line is the exponential fit to the disk, with extrapolation to eight times the disk scale length. The extrapolation results in an additional 0.35 mag (i.e., the total K_s flux of the galaxy is closer to 12.5 mag).

Figure 1 Figure 2 Figure 3

Figure 4

With stars removed, the true isophotal shape (2-D ellipse) of the galaxy is determined.
This process is iterated twice to robustly determine the ellipse shape.
The modification includes adding three new "planes" to the postage stamp images. The current planes are 1 = J, 2 = H, 3 = K_s. The new planes include 4,5, and 6 = J, H, K_s with stars subtracted or removed.
The following examples show some galaxies with their neighboring stars removed. The images are the galaxy postage stamps. The raw image of the first galaxy shown in Figure 5 is a Hercules spiral used for Tully-Fisher distance calibration. The second galaxy shown in Figure 6 is a bright galaxy in the Abell 3558 cluster (note the smaller galaxies around it; they do not smoothly subtract away, because they are not point sources.) The last galaxy shown in Figure 7 is a spiral located within the "Zone of Avoidance." Figures 8, 9, and 10 show the star-subtracted images for the three galaxies, respectively. In these latter three figures, the contours show the 3- isophote (red) from which the elliptical orientation is derived, and the 20 mag/arcsec² isophote (green), the standard photometry isophote.

Figure 5 Figure 6 Figure 7

Figure 8 Figure 9 Figure 10

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

Return to Section IV.5.


Figure 1	Figure 2	Figure 3


Figure 5	Figure 6	Figure 7

Figure 8	Figure 9	Figure 10