6. Cautionary Notes
c. Extended Source Catalog (XSC)Frequently Asked Questions
- Please see the FAQ page "How to Use the 2MASS XSC", Section II.3b.
i. Definition of Extended Sources
2MASS pipeline processing (see IV.1) attempts to identify all sources that are not well-fit by just a single point-spread-function (PSF). Therefore the database from which the XSC was selected includes true extended sources, such as galaxies and nebulae. Since the algorithms (see V.3) are not perfect, at a much lower level (~1%), the XSC also includes objects made out of multiple stars that are close together and artifacts around bright stars due to gradients in the backgrounds around such bright stars.
Extended sources were identified and separated from point sources using a "decision tree" method. The method uses a variety of source attribute measures to separate resolved from the unresolved, including size, central surface brightness and color. In all, we use about 10 different measures (or dimensions), whose values are correlated to one degree or another. Read more about this parameter set in IV.5.a.vi.
Two different decision tree "scores" are generated: (1) G score and the (2) E score. The "G score" is geared towards recognizing galaxies. Not only does it include the standard star-galaxy parameters (e.g., shape, size & SB, ... etc) but it also includes the "color attribute". The "E score" is a simpler version of the G-score: it has only three parameters that are tested in the decision trees, including surface brightness, shape and the double/triple star discriminator. It does not include a color attribute. In this sense, it is less biased than the G-score. It was conceived to capture Milky Way objects that might have unusually blue colors, or shapes distinctly different from galaxies.
It should be understood that: (1) the E score is less reliable than the G score, and (2) although it may be less biased to color, the E score is not necessarily better at finding Milky Way objects (the G-score is more than happy to find HII regions, nebula, YSOs, etc).
These are employed as the final arbiter for star-galaxy separation. It turns out that a value of around 1.4 gives a satisfactory separation, while maintaining satisfactory completeness. Summary: extended objects have scores between 1 and 1.4, while point-like objects have values between 1.4 and 2.0.
The parameters of the XSC were tuned to meet as closely as possible the Level 1 Science requirements of 99% reliability for "G" sources above point source densities representative of galactic latitude glat ~ 20° and 80% reliability for "G" sources at glat ~ 10°. Reliability is defined here as the percentage of sources which are truly extended (e.g., galaxies and nebulae), with multiple point sources, artifacts around bright stars and meteor trails counted as "unreliable" extended sources.
For higher source densities, the emphasis shifted to completeness, at the expense of reliability. Hence the reliability of "G" sources may be as low as 50% at the highest source densities allowed in the catalog (cf. VI.4).
No requirements were placed on the reliability of Galactic Plane sources, and hence a higher percentage of them are composed of multiple point sources.
iv. Incompleteness Due to Source Confusion
The presence of foreground Galactic stars limits galaxy detection both in the sense that the background noise is enhanced (i.e., confusion noise) and the ability to distinguish extended emission from the stellar (point-like) emission is diminished. The rate at which these effects become increasingly important is dictated by the stellar number density, roughly matching the exponential distribution of stars along the the Galactic latitude axis.
The drop in completeness with the source confusion can be clearly seen in the allsky "movie" maps and more quantitatively in the XSC source count "movie" map. Note the large gaps in completeness near the Galactic Center, and note the "reddening" of the XSC sources as seen within (or projected against) the Milky Way.
The problems of bright stars mentioned above (primarily scattered light and diffraction spikes) create vast numbers of spurious extended source detections. Hence it was necessary to reject more area around bright stars for the XSC in order to meet reliability requirements. About 1 to 2% of the entire sky is excluded from the XSC due to bright star confusion (cf. IV.5).
When in doubt about a given source, consulting the Atlas Image will usually immediately reveal whether a source is an artifact or not.
The "visual classification" flag, vc, is totally separate from any aspect of the Catalog Generation, and can be used as a means of quickly assessing the reliability of any given selection criteria imposed on the catalog. The visual classification flag was assigned on the basis of examining the image of a source, by eye, comparing across bands and (when needed) with the Digital Sky Survey. The catalog contains 316107 sources with "galaxy" or "extended" visual classifications; 7383 with "artifact" or "unreliable" visual classifications; and 35270 with "unknown" visual classifications (that is, the source defies classification, usually because it is too faint in surface brightnesss).
Visual inspection of the XSC reveals a small fraction (less than 1% of the total) of sources that may be classified as image artifact -- usually associated with bright stars. These "sources" are often possessed with unusual photometry or size attributes. Be wary of sources with unusual colors or other "outlier" attributes. In addition to bright stars, other kinds of artifact inducing phenomena include "airglow" (particularly at H-band), transients (e.g., meteor streaks) and image edges. We have made every effort to minimize these contaminants to the XSC.
- 2MASX01311985-3233531: horizontal stripe from bright 2.2 micron source
2MASX01581860-0704402: this is an "edge flare" created by a moderately bright star located just off the eastern
edge of the scan
TABLE of Known or Suspected Artifacts in the XSC
vii. False Extended Objects
With projection effects, close groupings of stars come in every permutation imaginable. These "double" and "triple" stars represent a small contaminant to the XSC, mostly confined to the Plane of the Galaxy. Since these sources are stellar in nature, they tend to have blue colors, J-Ks < 1 mag, with respect to galaxies. Refer to the XSC FAQ: Reliability: What is a Star Doing in the XSC?.
2MASX18395823+1055321: double (or possibly triple) stars
- 2MASX19120930+1946194: multiple stars with a bright star to the southeast; this source is located in the Galactic Plane, with a stellar number density value of 4.1.
TABLE of Suspected Stars and Other Non-Extended Objects in the XSC
The cc_flag is used to highlight XSC entries that have some probability of being artifacts or contaminated by nearby large galaxies, or that are Large Galaxy entries that were processed specially.
Sources flagged as artifact (cc_flg="a") include those corrupted by a bright star or those that are outright false detections of filter glints or ghost images produced by bright stars. These sources were identified as such during visual inspection operations.
- See also
Why Are Large Galaxies Special Objects in the XSC?
- See also "Mis-positioned Galaxies & Foreground Contamination" below.
The following table summarizes the possible values in the cc_flg, and shows the number of sources in the All Sky XSC having each cc_flg value.
ix. Duplicate Sources
Duplicate sources are very rare, but do live within the XSC. The (usual) phenomenon arises from scan-to-scan overlap observations (some 10-15% of the scan), where the same source in two different scans may have slightly (~few arcsec) different centroids. The differing centroids are usually associated with low S/N sources (or nuclei), where noise fluctuations may induce the difference. Another source of mis-measurement of the centroid is the scan edge itself, which might (under certain circumstances) disrupt the source characterization process. Be wary of any two XSC sources that are within a few arcsec of each other in coordinate space. Refer to the XSC FAQ: Duplicity: Why Do Some Galaxies Appear to be Duplicates".
x. Null Photometry and/or Attributes
"Null" values arise from either (1) corruptions in the source characterization process, or (2) Signal-to-Noise limitations. The source characterization process is corrupted (or interrupted) by a variety of circumstances, including the deleterious presence of bright star artifacts, scan edges, high-frequency background gradients ("airglow"), and source confusion. Faint sources may (for example) prove to be too formless to adequately measure the isophotal shape or flux. Consequently you will often find "null" attributes for sources in confused regions and for faint (low S/N) sources. "Null" photometry (or corrupted photometry) may result in bizarre, unphysical associations (e.g., strange colors). Refer to the XSC FAQ: Why Do Some Galaxies Appear to Have Unphysical Colors or Brightnesses?.
2MASX16501134-2227582: object (in this case a star) is contaminated by a nearby bright star; the
resulting measurements are either nulled or unphysical
2MASX05491361+2256343: faint little galaxy, whose H-band isophote is undetermined
(note the unusual colors)
- faint red galaxy, whose J and H isophotes are undetermined (note the very red colors)
There is another class of XSC object that is rife with "null" attributes. These are "sources" associated with very large galaxies, and are believed to be (for the most part) unreliable "junk" spawned by the parent galaxy. They may be real (e.g., H II regions), or they may be bogus (e.g., noise bump on top of the large galaxy), but either way their source characterization is seriously compromised by the confused environment. For this reason, these sources have been identified and their photometry and source attributes artificially "nulled". The cc_flg value is set to "z" (see table above). The only useful information in the XSC is the coordinate position of the object itself. Read more about it in: "Sources in close proximity to large galaxies".
- 2MASX09550980+6904071: source embedded in the glare of the nearby galaxy M81; all photometry has been nulled for this source.
xi. Mis-positioned Galaxies & Foreground Contamination
The close presence of foreground stars to an XSC galaxy (i.e., confusion) will on occasion induce (1) poor central positions (i.e., astrometry) and/or (2) poor integrated fluxes (e.g., see below "Wacky Measurements"). The user should beware of stellar confusion.
- 2MASX J08152294+2843574: faint LSB galaxy, with 2MASS position peaked up on foreground star to the southeast
TABLE of source with poor positions due to stellar confusion
xii. Wacky Measurements
In the same vein as corrupted photometry and their resultant strange colors, the occasional source characterization mis-measurement may result in totally unphysical attributes, such as angular size. These measurement gaffes typically arise from image problems (e.g., capricious background variations inherent to ground-based observations) or disorder from source confusion. (See above discussion)
2MASX06340957+1834525: object (in this case probably a double/triple star) is contaminated by a nearby bright star; the
resulting measurements are unphysical
2MASX14194311+2544134: artifact source, where the circular isophotal radius is negative (i.e., corrupted)
2MASX19421669+6648445: faint galaxy whose circular isophotal radius is negative (i.e., corrupted) for
but whose other measurements are just fine
- 2MASX17405030-5342397: stars associated with a globular cluster; the circular isophotal radius is corrupted, and the other attributes are also unphysical due to confusion
They also arise from software design flaws. For example, the early 2MASS pipeline data (see the incremental releases) featured a set of objects, thought to be associated with large galaxies, but instead were either off-center pieces of large galaxies or nearby stars:
"Special processing was used to select sources spatially coincident with large galaxies (but not too large; see I.6.c.ix above) and pass them into the Extended Source Database.
Usually sources so identified capture at least part of the flux of the large galaxies. However, some large galaxies have poorly determined positions, and as a result sources that
are not in fact part of these large galaxies were picked up by the same processing."
- 2MASX01402453-2300002: this is a small galaxy with NED designation "APMUKS B013802.72-231510.3"; the measured attributes for this object are just fine, whereas the postage stamp image is much too large for the small size of the galaxy.
The 2MASS survey design did not cater to the needs of large angular extent objects. Not only are these objects clipped by the edges of scans, but the background removal process itself is compromised by the small size of the survey images with respect to nearby (zero redshift) galaxies. The Large Galaxy Atlas (LGA) was created to rectify this problem. The XSC now includes the ~500 largest galaxies in sky. The LGA is a still a work in progress, however. A number of moderate to large (non-LGA) galaxies in the XSC are clipped by scan edges, due to the unfortunate proximity within a survey scan. These galaxies can be expected to have underestimated integrated fluxes, as well as compromised large-scale attributes (e.g., isophotal radii). In time the LGA will include these XSC galaxies, and the fluxes and source characterization will be recovered. Here is an example of an XSC galaxy in close proximity to a scan edge:
xiv. Pieces of Large Galaxies or Clusters
A very small fraction of the XSC sources are pieces of larger objects, including galaxies and clusters.
TABLE of Known Large Galaxy/Cluster Pieces
xv. Little Big Man Galaxies
The Large Galaxy Atlas is, as the name suggests, comprised of large angular-extent galaxies. It was created to fill a large gap in the XSC where these sources were either unmeasureable or mis-measured in the 2MASS pipeline. However, due to rather obtuse circumstances (usually related to the author's interest in these special galaxies), a handful of small galaxies have found their way into the LGA. They were created and processed using the LGA pipeline. As such, their photometry and source characterization is perfectly valid. Their only outstanding quality is that they are much smaller than nearby galaxies, and in fact they are small in comparison to typical XSC galaxies. An example is MRK 897. Refer to the XSC FAQ: Why Are Some "Large" Galaxies in Fact Barely Resolved?"
The shape and light concentration are difficult to determine for small galaxies, whose profiles are significantly distorted by the point spread function (PSF). For galaxies smaller than 10 arcsec (representing most of the XSC), the half-light radii and concentration indexes will probably be overestimated due to the rounding and extending effects of the PSF. These attributes should be used with caution. See also Half-Light "Effective" Aperture and Concentration Index (IV.5.e) and Axis Ratio and Small Radii.
xvii. Other Miscellaneous Notes
- Beware that amorphous nebulae, such as those found in molecular clouds
(e.g., rho Oph) will have measurements that are, for the most part, unusable.
This is due to undeterminable measurements and background removal difficulties.
Example: rho Oph core showing the heavily reddened
embedded stars and complex background.
The only way to properly recover the flux of these complex objects is to treat them with an optimized pipeline (such as the Large Galaxy Atlas pipeline). This in fact has been carried out for the Tarantula Nebula in the LMC.
- Untracked Seeing
Atmospheric seeing variations cause the observed PSF to change, and the scan processing pipeline attempts to follow such PSF changes in order to properly discriminate extended sources from point sources. However, it is not possible to follow rapid seeing changes, which usually results in some point sources falsely identified as extended sources.
- Additional Photometric Uncertainty
Extended sources are sensitive to a wider spectrum of noise sources than point sources, which are affected only by high spatial frequency noise. Some of the known noise sources are mentioned below.
- Atmospheric Airglow Emission
The extended source background-removal algorithm removes any background variation at J and Ks such that the residual noise in the Images is usually consistent with the measurement error. However, atmospheric OH airglow emission variations contribute extra noise in the H-band roughly equal to the measurement error.
- example of an image with significant airglow background. The left image shows a case of severe airglow emission as seen in the H-band Atlas image. The airglow fluctuates on scales of ~1". The right image shows a case of correlated "electronic" (non-astronomically related) noise ridges along the right side of a J-band image. The ridges are the result of one array frame quadrant (NICMOS arrays have four quadrants) having elevated pixel values, which induce sinusoidal waves with drift scanning and coaddition of individual frames (i.e., the drift step size and quadrant size are constructively synchronized). Note also the frame-edge features seen in the right panel.
The H-band photometric error due to airglow noise varies strongly with time and spatial position and with the total brightness and size of an extended source. A statistical analysis of galaxies with H = 13.8 mag shows that about 25% of all sources have a measurable increased uncertainty which is correlated with the measured of the background-removed pixel intensity distribution.
In the First Incremental 2MASS Data Release, scans that have large measured background sigmas were not included. For this All Sky Release, we have refined our airglow diagnostic to only reject scans that have measured noise that is significantly greater than that predicted by the measured H background. This should allow better rejection of scans truly contaminated by background structure due to airglow. Some Tiles undoubtedly still contain airglow structure that is not extensive enough to trigger our thresholds for Tile rejection. Therefore, users should still be aware that H band fluxes for a significant number of sources will have a higher photometric error than the quoted error, which reflects only the Poisson noise in the background. The best estimate we can make at this time for the magnitude of the excess noise comes from a statistical analysis of sources from Atlas Images with a residual background of just under 1.20 DN. These sources have an extra H photometric error equal to their Poisson uncertainty of ~0.10 mag, making the total photometric error ~0.15 mag.
- Electronic Noise
Electronic noise with spatial periods of 50-75´´ is sometimes present in the Atlas Images. Preliminary analysis shows that the noise can sometimes resemble a square-wave distribution in the Images, producing a bias in extended source photometry that is either full-amplitude positive or negative. These biases can be as large as 17%, 7% and 11% at J, H and Ks, respectively, for galaxies with Poisson errors of less than 10%, for perhaps ~1% of all galaxies.
- The NIR position of a source may, on occasion, greatly differ from what is published in
the literature due to a variety of reasons. The most common reason arising from poor (inaccurate) positions
as determined from photographic plates. Other reasons include the complexity of the object
(e.g., interacting galaxies) and real differences in position with wavelength (e.g., the NIR is
less sensitive to the deleterious effects of extinction and reddening). The following is a list
of galaxies that "mis-matched" with optical catalogs (e.g., UGC), but turned out to be just fine XSC sources.
TABLE of interesting XSC galaxies and other notable sources
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