IV. 2MASS Data Processing
5. Extended Source Identification and Photometry
a. Algorithms, Analysis and Results
vi. XSC -- Virgo Galaxy Cluster
The Virgo Galaxy Cluster, provides a unique testbed to validate the completeness of the XSC. Owing to its close proximity (vhelio = 1000 km s-1; d ~ 20 Mpc), cluster richness, and extreme angular size (over 50 deg2), Virgo contains a variety of galaxies, from the big (e.g., M87) to the very small or low surface brightness (e.g., IC 3475), which is within the sensitivity and resolution of the XSC. Here we use large 2MASS image mosaics to search for Virgo galaxies which are absent from the XSC.
We visually inspect the mosaics, overlaying extended sources from the 2MASS working survey database (WSDB) and the XSC, scanning the images for obvious Virgo galaxies missing from the XSC. This experiment is optimized to clearly-resolved galaxies (diameters > 10´´-20´´), and, less so, to compact (possibly background) galaxies. Our goal is to identify Virgo galaxies and to assess the completeness of the 2MASS XSC.
Two experiments are carried out:
- Inspect a low spatial-resolution Virgo mosaic, which is of large angular scale, 50 deg2
- Inspect a full-resolution smaller, 3 deg2, Virgo mosaic.
For the large mosaic, we only concentrated upon the XSC detections, and do not consider previously-cataloged galaxies (which may or may not be visible in the 2MASS images). For the full-resolution mosaic, we consider the XSC and PSC, as well as NED (previously catalogued) detections.
"Bottom Line" Results
The overall XSC completeness for the Virgo Galaxy Cluster is well above 90%, exceeding the 2MASS Level-1 Science Requirements for absolute completeness. The reliability looks solid as well (although this experiment was not optimized to assess the detailed reliability). The only galaxies that are missing from the XSC are faint, low surface brightness objects that either exceed the SNR thresholds of the XSC or are at the limit of reliable detection and characterization. All of the large, major galaxies of Virgo are included in the XSC.
The dominant population of galaxies missing from the XSC
are small and faint, at the limits of both detection
and star-galaxy separation. Figure 1
shows the brightness uncertainty as a
function of magnitude at J-band for the
Virgo galaxies (white points)
and extended sources identified through visual inspection of
the full-resolution Virgo mosaic (red points).
What is missing from the XSC?
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| Figure 1 |
Most of the sources are either fainter than the Level-1 Science requirement (J = 15 mag) or have a SNR that extends well below the requirement limits (SNR = 10). One of the brightest "missing" sources (UGC 7652) is a satellite galaxy of M87 (see the M87 Virgo Panel); hence, this source is swept into the larger field of M87 (Virgo A) through the Large Galaxy Atlas.
Consider the curious case of IC 3388, a Virgo dwarf galaxy: This object is relatively bright (J ~ 14.4 mag) in total integrated flux, but it is low in surface brightness. It was observed twice by 2MASS, on 1998 Dec 6 from the northern facility (see Figure 2) and on 2000 Apr 20 from the southern facility (see Figure 3). This galaxy was detected only in the 2000 observation and extracted into the 2MASS working extended source database.
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| Figure 2 | Figure 3 |
For the public release Catalogs, the 1998 observation (2MASS tile) was deemed superior to the 2000 observation (tile) in background sensitivity, and hence was used in the XSC. The unintended consequence is that IC3388 is not in the XSC -- it is missing from the public release catalog, yet it safely resides in the extended source database. Other notable Virgo galaxies missing from the XSC are shown in IV.5a7.
LSBs and Other "Invisible" Galaxies
The relatively shallow 2MASS survey is not optimized to detect (face-on) late-type spirals, dwarfs and other low surface brightness (LSB) galaxies. Only the nearest and physically largest LSBs are clearly delineated and characterized in the XSC (see, for example, NGC 6822). For the Virgo Cluster, dwarfs are, for the most part, invisible to 2MASS. See the example of the dE2 galaxy VCC 815 in Figure 4.
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| Figure 4 |
The PSC Contains Many (Faint) Virgo Galaxies
The 2MASS Point Source Catalog (PSC) includes many extragalactic sources, both of the extended and of the compact, or point-like, variety. The Virgo mosaics are full of faint point sources which are undoubtedly galaxies (based on their "soft" surface brightness and "red" colors). Figure 5 shows a small Virgo area with PSC sources indicated by orange circles. Three of the sources are faint galaxies, with integrated J magnitudes much fainter than the XSC limits, but well within the sensitivity limits of the PSC. For a larger view of PSC and XSC sources, view the full-resolution Virgo mosaic panels in IV.5a8.
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| Figure 5 |
2MASS Virgo Sample
Here we present further visual inspection and analysis of the equatorial region of the entire Virgo Cluster, with 183.3° < RA < 191.1° and 10.4° < Dec < 16.4°. The working extended source database (blue line) contains 4450 sources, and the XSC (green line) contains 2532 sources. The distribution of J magnitudes is given in Figure 6, and the distribution in the photometric uncertainty at J is given in Figure 7. The difference between the database sources and the XSC distribution is due to: (1) the signal to noise ratio, (2) the degree of star-galaxy separation, and (3) the elimination of "junk" sources.
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| Figure 6 | Figure 7 |
The "Heart of Virgo" region consists of the central 3 deg2 of the cluster, bordered by M87 in the south, and the Markarian Chain of galaxies to the north (including M84, M86).
Sky Distribution
Virgo lies within the supergalactic plane of nearby galaxy clusters (see, for example, Figure 3 of II.3d2). A flat "tangent" projection of the XSC (integrated flux) within the Virgo area is shown in Figure 8, which is centered at RA=12.4h, Dec=+12.7°, with an 8°×8° field-of-view (N.B.: this figure is very large, 9 Mb).
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| Figure 8 |
Visual Inspection Analysis
See details of this analysis in IV.5a7 (low spatial resolution, 50 deg2 mosaic) and IV.5a8 (full-resolution, 3 deg2 mosaic of the "heart of Virgo").
[Last Updated: 2002 Oct 10 ; by T. Jarrett]
Return to Section IV.5a.