Appendix 1. Introduction to the 2MASS Extended Mission and Ancillary Data Products

4. Cautionary Notes

c. Survey Extended Source Reject Table, 6x XSWDB and XSC, Calibration and LMC/SMC Calibration XSWDB

1. Additional Information Available
2. Using the Extended Mission Extended Source Extraction Tables
3. Unreliable Extractions
4. Multiple Source Detections
5. Extended Sources Near Scan Edges
6. Large Galaxies

i. Please see Cautionary Notes for the 2MASS All-Sky Release Extended Source Catalog (I.6.c)

This section contains brief summaries of the key limitations common to the Survey XSRT, 6x XSWDB/XSC, Calibration and LMC/SMC Calibration XSWDBs. It is not a complete collection of cautionary notes for each of the data sets. Please review the more complete descriptions for each product at:

• A2.1.c.iii - Cautionary Notes for the 2MASS Survey Extended Source Reject Table
• A3.1.d.iii - Cautionary Notes for the 6x Extended Source Working Database and Catalog
• A4.1.c.iii - Cautionary Notes for the Calibration Extended Source Working Database

ii. Using the Extended Mission Extended Source Tables

The Survey XSRT and 6x and Calibration XSWDBs are not a "Catalogs" in the same sense as the highly reliable, uniform All-Sky XSC. As a result, they are not suitable for statistical investigations of complete samples.

The 2MASS Survey XSRT and 6x and Calibration XSWDBs contain all extractions of sources believed to be extended relative to the instantaneous point-spread-function (PSF) in the respective 2MASS survey observations. These extractions contain a complex mixture of measurements of real extended astrophysical sources such as galaxies and galactic nebulae, and detections of single and multiple single stars and spurious image artifacts that have extended source signatures. Because of possible redundant source detections and the unreliable extended source extractions, the various Extended Mission extended source tables do not provide valid statistical information about near infrared source populations such as those given by the the 2MASS All-Sky XSC. The exception to this is the 6x-XSC which was generated using criteria for reliability and uniformity similar to the All-Sky XSC, but over limited areas.

The Survey XSRT and 6x and Calibration XSWDBs are resources for supplementing source information found in the All-Sky and 6x-XSC, or other external sources. For example, the Survey XSRT contains independent measurements of 349,184 sources found in the All-Sky XSC that can be used to improve the photometric, astrometric and in some cases morphological knowledge. The Calibration XSWDB contains the individual epoch measurements of objects detected hundreds to thousands of times, enabling the generation of very high precision photometry in order to search for flux variability. The detection statistics of sources observed multiple times found in the the Survey XSRT and 6x and Calibration XSWDBs also provide an additional source reliability information. A convenient way to utilize the multi-epoch information contained in the Extended Mission extended source tables is via the 2MASS Survey, 6x and Calibration Merged Extended Source Information Tables. In these tables, the independent measurements of objects detected more than once in each data set have been combined to provide improved photometry, astrometry and detection statistics.

iii. Unreliable Extractions

2MASS extended source processing (see IV.5.d) attempts to identify all source detections that are not well-fit by a single PSF. Because the classification algorithms are not perfect, the extended source working databases contains detections of both true extended sources such as as galaxies and nebulae, as well as detections of close multiple and single stars and spurious detections of artifacts produced by background gradients around bright stars. All candidate extended source detections were loaded into the Survey, 6x and Calibration XSWDBs. The All-Sky and 6x XSCs were drawn from the superset of entries in the respectively XSWDBs by selecting sources that satisfy various reliability criteria (see V.3 and A3.6.c).

The Reliability Flag (rel)

To help discriminate between real extended source detections and single and multiple stars and spurious extractions, each Survey XSRT, 6x and Calibration XSWDB entry has been assigned a reliability flag value, rel, that is related to the probability that it corresponds to a detection of real astrophysical object that is truly resolved at the time of the 2MASS observation. The reliability flag is a single character in the range "A" to "F", with "A" representing sources with the highest reliability and "F" the lowest. Appendix 5 contains a description of the algorithm used to assign the reliability flag values and limitations of the estimator. Table 1 contains a breakdown of the Survey XSRT, 6x and Calibration XSWDBs according to reliability flag value, and the corresponding probability of reliability associated with each value of rel.

Select Survey XSRT and 6x and Calibration XSWDB entries that have a reliability flag value of rel="A" to minimize the number of real but non-extended sources and spurious extractions. Caution should be exercised when using any extraction with a lower probability of reliability. However, even the rel="A" sources in the XSRT and XSWDB's have not received the same degree of scrutiny as the highly reliable All-Sky XSC. When in doubt about the reliability of an extended source table entry, we strongly recommend examining the image of the source using the Survey, 6x and Calibration Image Atlases.

The Visual Classification Flag (vc)

Images of large numbers of candidate extended source extractions in the Survey XSRT and 6x and Calibration XSWDBs have been visually inspected and classified as part of the reliability scoring. The results of these examinations are encoded in the visual classification score (vc) The possible values of vc and frequency of occurrence in the Extended Mission extended source tables are listed in Table 2.

The Confusion and Contamination Flag cc_flg

Users should select extended sources with cc_flg="0" if they need samples that have the lowest probability of contamination.

The "contamination and confusion flag," cc_flg is used to denote Survey XSRT and 6x and Calibration XSWDB entries that have some probability of being corrupted by bright stars or nearby large galaxies, or that are outright spurious detections of image artifacts produced by bright stars. Spurious artifact detections were generally identified by visual inspection of their images.

The possible values of cc_flg and the number of occurrences in the Extended Mission extended source tables are summarized in Table 3. Unlike the All-Sky XSC, the Survey XSRT and 6x and Calibration XSWDBs do not contain entries whose positions, photometry and shape information is from the 2MASS Large Galaxy Atlas (cc_flg="Z"). Also note that in the Extended Mission extended source tables artifacts are denoted with by capital "A" rather than lower case "a", as they are in the All-Sky XSC.

iv. Multiple Source Detections

The 2MASS Survey XSRT and 6x and Calibration XSWDBs may contain multiple measurements of the same object. Some of these multiple detections are independent measurements of objects in regions observed multiple times during 2MASS survey, 6x and calibration observations, and some are duplicate source entries from the overlapping Atlas Image declination boundaries within the same scan and are not independent. Because of this "over-completeness," the Extended Mission extended source tables are not suitable for studies based on source count statistics. Use the 2MASS All-Sky XSC and 6x XSC for this type of inquiry.

Because the Survey, 6x and Calibration XSWDBs contain every candidate extended source detection extracted from all 2MASS observations made under photometric conditions, they may contain more than one independent measurement of objects in regions that were scanned multiple times. The number of duplicate source detections within each data set varies considerably across the sky, depending on the observation depth-of-coverage, and it is different between the data sets. See A2.2, A3.2 and A4.2 for descriptions of the coverage of the survey, 6x and calibration observations, respectively.

The Survey, 6x and Calibration Merged Extended Source Information Table provide combined position, brightness and shape information for multiply-detected extended sources in the respective observations, and statistics on source brightness that are useful for identifying variables. Source confirmation statistics from the Merged Source Information table is also contained in the Survey XSRT: the spos and sdet columns in the XSRT give the number of scans that covered the position of a source and the number of unique scans in which the source was detected as an extended source, respectively.

In-Scan Duplicates

Extended sources were detected and characterized on the Atlas Images during 2MASS data processing. Because the Atlas Images within each survey, 6x and calibration scan were constructed with 54 pixels (54") of declination overlap between adjacent images, objects that fall in the overlap region between images, or that span more than one image in a scan may have been extracted more than once. All extended source detections in each scan, including the "in-scan duplicates," were loaded into the Survey, 6x and Calibration XSWDBs. Because they are based on the same image data, the in-scan duplicates are not independent measurements and thus they are different than the multiple detections of extended sources that were scanned repeated times.

In-scan duplicate detections can be identified in the Survey XSRT and 6x and Calibration XSWDBs as two extended source entries from the same observation (e.g. with the same scan_key value) that also have nearly the same position and possibly magnitudes, but that have different values of the coadd_key which identifies the Atlas Image from which the source was extracted.

The Multiple Source Detection Resolution processing used to prepare the All-Sky Release XSC selected only one unique apparition of an extended source from the Survey XSWDB for the Catalog, including proper accounting for in-scan duplicate source entries. Because the Survey XSRT contains all Survey XSWDB entries not in the All-Sky XSC, objects listed in the All-Sky XSC may have an in-scan duplicate in the Survey XSRT.

v. Extended Sources Near Scan Edges

The 2MASS Survey XSRT and 6x and Calibration XSWDBs contain extended sources that are truncated by Atlas Image edges, and consequently have distorted position, flux and shape measurements. Use the measurements of any extended source that lies within approximately 1' of a scan or image edge with caution. Refer to the All-Sky XSC for the best available measurements of any objects on scan or image boundaries.

The standard 2MASS extended source processing detected and characterized objects on a single Atlas Image. Sources that are truncated by image edges were not accurately measured. Although detections of truncated objects are contained in the Survey XSRT and 6x and Calibration XSWDBs, they typically have underestimate fluxes, positions that are biased away from the image edge, and incorrect size and shape attributes.

The overlap size between 2MASS survey tiles and between the declination boundaries of Atlas Images was designed in part so that objects up to approximately one arcminute in diameter would be fully covered by at least one survey scan. Therefore, small extended objects in the Survey XSRT that lie on Atlas Image edges usually have a duplicate entry in the All-Sky XSC or the Survey XSRT from the scan or image that fully contains the object. However, extended sources in the 6x and Calibration XSWDBs that are cut by image edges may not have another measurement available because some 6x and all calibration tiles do not have adjacent overlapping tiles.

Extended sources larger than one arcminute in diameter that are truncated by scan or image edges may not always be fully contained on a single Atlas Image, and may therefore not have an accurate measurement available in any 2MASS extended source table. The largest objects detected in 2MASS survey observations were characterized using special post-processing for the Large Galaxy Atlas (LGA; Jarrett et al. AJ, 125, 525), and the LGA entries are contained in the All-Sky XSC. However, the LGA is not complete for objects smaller than a few arcminutes, so care should be taken when using any extended source measurement in close proximity to a scan edge.

vi. Large Galaxies

The 2MASS Survey XSRT and 6x and Calibration XSWDBs do not contain accurate flux, position or size measurements for objects that are large relative to the size of an Atlas Image, or that span more than one Atlas Image. Consult the 2MASS All-Sky XSC or Large Galaxy Atlas for measurements of galaxies that are larger than a few arminutes in diameter, or that are truncated by the boundaries of an Atlas Image.

The standard 2MASS extended source processing detected and characterized objects on a single Atlas Image. Extended sources that are large with respect to the size of an Atlas Image were often artificially "shredded" into multiple components which generally have misleading flux, position and shape measurements. Even when very large objects were correctly detected as a single source, their positions and photometry were usually biased due to incorrect background subtraction and/or because sections of the objects are missing from the image. Because the 2MASS Survey XSRT and 6x and Calibration XSWDBs contain the default processing output for these objects, they should not be used to obtain information about objects with very large size or that are partially truncated by scan edges. Many large galaxy/nebula segments in the Extended Mission extended source tables are flagged by the confusion and contamination flag and have values of cc_flg='z' or 'X'.

Special post-processing was used to to characterize very large objects detected in 2MASS survey observations. This processing utilized measurements on image mosaics that fully contained the large sources, and from which accurate background fitting could be performed. The resulting 2MASS Large Galaxy Atlas (LGA; Jarrett et al. 2003 AJ, 125, 525) entries were added to the 2MASS All-Sky XSC to insure the catalog's bright source completeness. Comparable LGA post-processing was not carried out for the 6x or Calibration data. Therefore, the All-Sky XSC should be consulted for the best available measurements for any large extended source rather than any of the Extended Mission extended source tables.

Figure 1 is a mosaic of 2MASS J-band Survey Atlas Images that contains M31, M32 and M110. The position of Survey XSRT entries in this field are shown by the green circles - virtually all are "chaff" extracted in an around the large galaxies and have cc_flg='X'. The red circles are the positions of the 2MASS Large Galaxy Atlas entries for these galaxies that are found in the All-Sky XSC.

Figure 1 - 2MASS J-band image mosaic showing M31, M32 and M110. Survey XSRT extractions are shown by the green circles. 2MASS Large Galaxy Atlas entries for the three galaxies are shown by the red circles.

[Last Updated: 2008 February 12; by R. Cutri]