Appendix 6. Merged Working Databases

1. Overview

The 2MASS Survey, 6x and Calibration observations each covered some regions of the sky multiple times. As a result, the respective extracted point and extended source Working Databases (WDBs) may contain multiple, independent measurements of sources in the multi-epoch coverage areas. Whereas the 2MASS All-Sky and 6x Catalogs contain only one measurement of sources that were detected more than once for uniformity of depth, the Merged Working Database Tables make use of all available measurements to provide improved possible position and brightness information for these objects.

WDB merging was carried out by spatially autocorrelating the extractions in each of the Survey, 6x and Calibration point and extended source Working Databases (WDBs) and identifying groups of independent extractions that are positionally associated. For each associated group, the mean source properties were computed, and coverage and detection statistics accumulated. Each merged group in a specific WDB table has been assigned a unique identifier, gcntr, that is used to link it with the individual WDB extractions that belong to the group. The Merged Source Cross-Reference tables contain listings of the individual extractions associated with each merged group, and group identifiers and confirmation statistics derived from the merging have been added to the WDB entries for multiply-observed extractions. Merging was carried out for each WDB table separately. There was no cross-merging between tables (i.e. the 6x point source WDB was not merged with the Survey point source WDB).

The 2MASS Merged Source Information tables contain the averaged astrometry and photometry in each survey band for multiply-observed sources. The tables also contain a variety of statistics on the observed position and flux distributions that indicate the accuracy of the combined source parameters, and that can be used to identify potentially moving or variable objects. In addition, the Merged Information tables contain source detection statistics that are useful for evaluating source reliability.

The merging process and the parameters derived for the merged sources are described in A6.2. The formats of the Merged Source Information and Cross-Reference tables are described in A6.3. Limitations of the merging process and caveats concerning the merged databases are summarized below in A6.1.b. Users are encouraged to read and familiarize themselves with these cautionary notes.

a. Usage and Scope

The merged working databases are intended primarily to be used as a source of improved astrometry, photometry and reliability information for the subset of objects observed more than once during the 2MASS Survey, 6x and Calibration observations. They are not substitutes for the Point and Extended Source Catalogs. The merged databases do not contain data for objects observed only once, so their spatial coverage is not uniform. Furthermore, because the relative depth and coverage of the merged tables is highly non-uniform, the Merged Source Information tables are not a useful resource for large-scale statistical investigations.

The number of entries in each of the Survey, 6x and Calibration point and extended Merged Source Tables is given in Table 1. The number of entries in the Merged Source Information Tables are naturally smaller than in each of the respective WDB tables (see Table 1 in A1.2) because the multi-epoch sightings of each source have been collapsed down to one entry, because only sources detected more than once are included in the Merged Source tables, and because the merging process for each WDB was restricted to extractions that have >50% probability of being real source detections (see A6.2).

Table 1 - 2MASS Merged Source Table Sizes
Merged Point Source Information165,942,3574,771,737456,48035,278
Merged Point Source Cross-Reference396,697,28812,267,173204,143,4404,250,232
Merged Extended Source Information406,63630,2522,146101
Merged Extended Source Cross-Reference960,84176,636473,9718,660

The degree of improvements in photometry and astrometry afforded by the source merging scales approximately as the square root of the number of times a source is detected. Figure 1 and 2 show the frequency distributions of the number of unique detections of a point and extended sources, respectively, within a given WDB, given by the value of sdet in the merged source information tables. The majority of sources in the Survey and 6x WDBs were detected only a few times, and will thus show only modest gains in measurement accuracy. However, there are thousands of sources in the polar regions of the Survey and in the calibration fields that were detected hundreds to thousands of times and will have correspondingly precise photometry and astrometry available.

Figure 1 - Frequency distribution of the number of unique detection of sources in the Survey (black), 6x (green) and Calibration (blue) Merged point source WDBs. Figure 2 - Frequency distribution of the number of unique detection of sources in the Survey (black), 6x (green) and Calibration (blue) Merged extended source WDBs.

The improvement in photometric accuracy with increasing numbers of detections is illustrated in Figure 3. Figure 3 shows the standard deviation in of Survey Ks-band measurements of point sources near the equatorial poles plotted versus the number of number of times each source was detected. The vertical spread of points in the diagram is caused by the distribution in intrinsic source brightnesses; brighter objects start out with smaller measurement uncertainties. The measurement accuracy for objects in a narrow range of brightness follows the expected sqrt(N) trend, as illustrated by distributions of bright and faint objects shown in green and red, respectively.

Similarly, the improvement in astrometric accuracy is illustrated in Figure 4 wherein is shown the semi-major axes of the merged position error ellipses (emj_mrg) for point sources in the 90191 calibration field plotted as a function of number of times they were detected. The merged source position error ellipses are computed from the standard deviation of the mean positions from all independent measurements. Again, there is a vertical spread in the distribution of points that is due to the distribution of source brightnesses. The majority of sources are fainter and make up the upper envelope of points that follow the sqrt(N) trend. Sources brighter than J<15.8 (SNR>10 in a single scan) are shown in red, and define the lower envelope that follows the sgrt(N) relationship. The group of outlying points circled in red are discussed below.

Figure 3 - Standard deviation of Ks measurements plotted as a function of number of times a source is detected for objects near the equatorial poles in the survey merged point source information table. Bright sources are shown in green and faint in red. Figure 4 - Merged source position error ellipse semi-major axes plotted as a function of number of times sources were detected in the 2,086 scans of the 90191 calibration field. Sources brighter than J<15.8 (SNR>10 in a single scan) are shown in red. The group of outliers circled in red are associated with the nearby M6.5V star LHS 191 whose proper motion led to confusion in the positional merging.

In Figure 5, the J-H-Ks color-color diagram for sources in the Survey Merged Point Source Information table detected 100 or more times is compared to that for the same sources in the 2MASS Point Source Catalog (single epoch). These field objects are all near the equatorial poles where the survey coverage was deepest. The improved colors from the merged tables show clearly that the majority of objects are stars on or near the main sequence, with a relatively small fraction of galaxies. The most dramatic gains in photometric precision are found in the Calibration Merged Source tables, where thousands of independent measurements are combined. Figure 6 compares color-magnitude diagrams from the Calibration Merged Point Source Information table for sources detected more than 1000 times in the 90067 calibration field (covering the M67 open cluster) and the same sources in the 2MASS PSC (single epoch). The precise CMD from the merged table shows a tightly defined main sequence with a clear turn-off near Ks=11.0 mag, as well as a clearly delineated binary main sequence.

Figure 5 - Color-color diagram from the 2MASS PSC (left) and Survey Merged Point Source Information table (right) for sources detected more than 100 times during the survey. Figure 6 - Color-magnitude diagram from the 2MASS PSC (left) and Calibration Merged Point Source Information table (right) for objects in the 90067 calibration field detected more than 1000 times.

The statistics on the flux and position distributions of the associated groups provided in the Merged Source Information tables can be used to assess the quality of the merge, and to identify potentially variable and moving sources. For example, Figure 7 shows the flux distribution chi-squared statistics computed for the merged sources in the 90272 calibration field (observed 1973 times). Most of the merged sources have chi-squared values near unity. However, one very obvious outlier, circled in red in Figure 7, has a very high chi-squared value indicating a flux distribution that is not consistent with the uncertainties in the individual observations. This source is the radio-loud QSO B31456+375 (z=0.33) which shows strong time variability as seen in the light curve shown in Figure 4 of Section A.4.1.

The position error ellipse semi-major axis distribution for the 90191 calibration field shown in Figure 4 has a group of outliers with much larger position residuals than other sources observed a similar number of times. These three points, circled in the figure, are all associated with the nearby M6.5 dwarf star LHS 191. This star has a proper motion of over 1"/year which produced the large position residual in the merged source table. Although LHS 191 was detected in all 2,086 scans of this field, over the source of the survey it moved a distance larger than the 2" correlation radius used in the point source positional merging (see A6.2) which resulted in it being "split" into three confused merged groups. The large position residuals for LHS191 are a direct result of its motion.

The source confirmation statistics supplied with each merged source, the number of times a source was detected and the number of times it was observed (sdet and spos in the merged source information tables) can be used as a reliability indicator. Figure 8 shows the detection fraction (=sdet/spos) for sources in the 3,430 scans of the 90301 calibration field. The roll-off in detection fraction at faint magnitudes is a natural consequence of the diminishing detection completeness with decreasing signal-to-noise ratio of real objects. However, the clusters of "sources" with detection fractions <0.05 at the faint end of the distributions are in fact spurious detections of noise excursions and other features such as image artifacts.

Figure 7 - Chi-squared values of the merged source flux distribution for objects detected more than 1,000 times in the 90272 calibration field. The outlier circled in red is the highly-variable radio-loud QSO B3 1456+375. Figure 8 - The ratio of the number of times a source was detected to the total number times it was observed plotted versus the source magnitude in the 3,430 scans of the 90301 calibration field.

b. Cautionary Notes

Before using the 2MASS Merged Source databases, users are strongly encouraged to familiarize themselves with the Cautionary Notes to the All-Sky Data Release, and the Cautionary Notes to the individual Extended Mission products in A2.1.b (Survey Reject Files and Full Image Atlas), A3.1.d (6x Databases, Catalogs and Image Atlas), and A4.1.c (Calibration Databases and Image Atlas). The 2MASS Merged Source Tables were constructed using Working Source Databases from each key data set, and so characteristics and limitations associated with those products will be imprinted onto the merged source databases.

The sections below describe features and caveats that are unique to the Merged Source databases. These sections are intended to supplement the Cautionary Notes for the individual Survey, 6x and Calibration databases.

i. Scope of the Merged Source Tables

ii. Limitations of the Merging Process

iii. Position Reconstruction Errors

The reconstructed astrometry of two of the 1,582 scans of the 90009 Calibration Field had errors that resulted in positional offsets of 5-15" with respect to all other scans. The affected observations are scan 106 taken on 05/15/1999 and scan 102 taken on 05/18/1999, both from the southern observatory (scan_key values of 26284 and 25081, respectively). The astrometric errors in these scans are described in AIV.1.c.i.

Because the source extractions from these scans were included in the Calibration point and extended source WDBs, they were incorporated into the merge of those tables. Most of the extractions in these two scans were not properly associated with the extractions from other scans because of the large position offsets. If they appear in the Calibration merged source tables at all, the extractions from these two scan may appear as merged sources with a small number of detections offset from the remaining merged sources. In some cases, there may have been coincidental matches between the real source detections in the two offending scans and spurious noise detections in the scans that did not have position errors.

iv. Merge Quality Flags

A variety of parameters and flags are provided with each entry in the Merged Source Information table to help users assess of the accuracy of the association of extractions and the ensuing combined positions and photometry. These flags are described in A6.2.v, and a few of the key ones are summarized below. Users are strongly encouraged to examine the merge quality flags when using the merged tables.

[Last Updated: 2008 February 18; by R. Cutri]

Previous page. Next page.
Return to Explanatory Supplement TOC Page.