VI. Analysis of the 2MASS Second Incremental Release Catalogs

4. Completeness and Reliability

d. Extended Sources in the Galactic Center

The Galactic Center (GC) represents the most challenging region of the entire sky for finding extended sources. The stellar source density overwhelms any automated star-galaxy discriminators, primarily due to the presence of multiple groupings of stars (as projected on the sky). There may be real galaxies behind this "wall" of chaos, but we will never know about most of them. The GALWORKS reliability is less than 1% for the region ~6° within the center of the Milky Way. For this reason, the Extended Source Catalog (XSC) does not include "candidates" from this area. The exact area is an ellipse with semi-major axis length of 12.8° (along galactic longitude) and a semi-minor axis of 6° (along galactic latitude). Please read about it at Elimination of False GC Sources from XSC.

Nevertheless, GALWORKS does find extended sources. To wit: the reliability may be poor, but on the flip side, the completeness is probably quite good. The images below show the "extended" sources found in the GC area (defined by the "kill ellipse", see above link) and a small area outside of the kill ellipse. What kind of extended sources do we find? Nebulosity, usually associated with HII regions, globular clusters, planetary nebula, fuzzy stars and galaxies.

The elliptical kill zone of Galactic Center encompasses some 150 - 200 sq. degrees. The total number of "candidates" is 16786, nearly all of which are multiple stars. We find 77 confirmed extended sources, a mere 0.5% of the total. We find another 88 or so objects that we cannot confidently call "extended" (and, so, we call them "unknown"; sometimes they are pieces of real extended objects); hence, about 1% of the objects are worthy of follow-up observations. The GC is literally a "wall of stars" and finding anything through this wall is nearly impossible (at the 1% level!).

See Notes below for figure explanation and how the "confirmation" was carried out.

i. Confirmed Extended Sources in the GC "kill area"

Figure 1Figure 2Figure 3Figure 4Figure 5
(16 objects)(16 objects)(16 objects)(16 objects)(12 objects)

ii. Confirmed Extended Sources just outside of GC "kill area"

Figure 6Figure 7
(16 objects)(11 objects)

iii. "Unknown" or "Other" Objects in the GC "kill area"

Figure 8Figure 9Figure 10Figure 11Figure 12
(16 objects)(16 objects)(16 objects)(16 objects)(16 objects)

Figure 13
(4 objects)

iv. Notes

The images show the Ks-band "postage stamp" image. Here the grey-scale is shown with a log stretch, with the maximum (grey) tone based on the peak flux of the object at the center of the image, and the minimum (white) tone set at the background zero level. Overlaid are J-band contours: The contours start with the intensity level equal to 3 (thrice the background sky noise) or (if larger) the level equal to 5% of the J-band peak. Each successive contour is multiplied by a factor of two (i.e., 20%, 30%,...). Note that a "red" galaxy will show strong grey-tones but little J-band contouring. Nearly all of the extended sources are very red (J-Ks > 1.3).

The size of the image (in arcsec) is noted in the lower right-hand corner of the image (and has a value between 21´´ and 101´´).

The galaxy radius (r), Ks flux and J-Ks color shown in the figure caption correspond to the "Ks-band fiducial" 20 mag arcsec-2 elliptical isophotal aperture. Here, the elliptical orientation (position angle and ellipticity) of the galaxy is determined from the Ks image (at roughly the isophote corresponding to 3 times the background RMS "noise"). The radius, or semi-major axis, is derived from the Ks= 20 mag arcsec-2 elliptical isophote (which is approximately equal to 1 times the background RMS "noise"). It is this "radius" that represents the size of the galaxy. The JHKs photometry is then performed in an aperture given by the Ks position angle and ellipticity (derived from the 3 isophote) and the semi-major axis (derived from ~1 isophote); hence, the term "Ks fiducial", allowing comparison between bands and color computations.

The galactic coordinates are shown in the parenthesis in the figure caption. Finally, the NED name is given if it exists.

Although the total number of "candidates" in the GC is large (~17,000) it represents a finite number that can be viewed with interactive tools. We designed these tools to display the postage stamp image of the object in three ways: J-band grey-scale, Ks-band grey-scale, and JHKs three-color composite. The three-color image is by far the most powerful way of viewing the images. With visual examination and followup with NED and SIMBAD, we classified each object as either (1) false (triple+ star), (2) extended, or (3) unknown. (note: the DSS is useless in the GC, due to severe extinction). This process took over four weeks of dedicated examination (re: sore eyeballs). Caveat: As with any "expert"-type of classification scheme, the risk of imparting a bias is always present. On the other hand, multiple stars (doubles, triples, ... , etc) are very easy to identify with the human "eye". It is the lower surface brightness fuzzballs that are difficult to identify (hence, we liberally use the "unknown" classification when stumped). Also, we usually call an object "unknown" or equivalently, "other", if it is a piece of a real extended object (e.g., some outer part of a large galaxy; nebulosity; HII regions; etc).

There were a few scans outside of the GC "kill area" that we examined. The reason is that I wanted to see what extended sources, if any, were located in the GC mosaic constructed by E. Kopan. This mosaic covers an area slightly offset from the GC "kill area". We found several galaxies in the mosaic area. See Set 6 and Set 7 above.

[Last Updated: 2000 Jan 11 by T. Jarrett. Modified 2000 Jul 31 by S. Van Dyk.]

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