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2MASS First Incremental Data Release

(Released 1999 May 6)

This is the first large incremental data release from the Two Micron All Sky Survey. This release covers 2,483 deg2 of northern sky observed from the 2MASS facility at Mt. Hopkins, AZ. The release data products include 233,979 compressed 512×1024 pixel (1"/pixel) Atlas Images in the three survey bands, and Catalogs containing positional and photometric information for 20.2 million Point and 73,980 Extended sources.

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Also, please include this acknowledgment in any published material that makes use of the 2MASS First Incremental Release data products.

Additional general information about the 2MASS First Incremental Release

Sources in the Point Source Catalog (PSC) meet the following criteria:

  1. Source signal-to-noise ratio >7 in at least one band (to insure reliability and avoid flux overestimation bias at lower SNR)
  2. Chi-square from profile-fit photometry <10 in at least one band (to reject residual cosmic rays)
  3. Brighter sources detected on >2 individual frames, and on >40% of the possible frames
  4. Not identified with a bright star artifact or meteor trails (objects believed to be real, but affected by artifacts are also included).
  5. Source lies >10" from edge of Survey tile (to avoid <3-band coverage areas at edges of arrays)
  6. Only one apparition of multiply-detected sources in Tile overlap regions included in Catalog. Apparition farthest from respective Tile edge selected.

The distribution of sources in the First Incremental PSC with galactic latitude is shown in Figure 1. Differential point source counts are shown in Figure 2 (for 2° diameter, or 3.1 sq. deg., field at high Galactic latitude, l=301°, b=78°) and Figure 3 (same as Figure 2, except in the Galactic Plane at l=55°, b=1°), where the blue line denotes J-band; green line, H-band; and, red line, Ks-band.

At high latitude, the 2MASS PSC contains accurate detections well below the nominal Survey completeness limits of J=15.8, H=15.1 and Ks=14.3 mag. The turnover in the source counts in the Galactic Plane field occurs nearly 1 mag brighter, because of the effects of confusion noise on the detection thresholds.

Figure 1Figure 2Figure 3

The characteristic photometric precision of measurements of unconfused sources in the 2MASS PSC is illustrated by Figure 4 which shows the photometric repeatability for point sources in the P161-D calibration field.

Figure 5 shows the J-H-Ks Color-Color Diagram for 3,141 three-band-detected sources in a 2° diameter high Galactic latitude field at l=301°, b=78° (yellow=SNR(Ks)>40; blue=SNR(Ks)>20; black=SNR(Ks)<20); red and green lines indicate dwarf and giant tracks, respectively, from Bessell & Brett [1988, PASP, 100, 1134]; the diagonal black line indicates the reddening vector for AV=5 mags). There is no evidence for reddening in this field, and there are very few bright (high SNR) stars. Figure 6 shows the J-H-Ks Color-Color Diagram for 190,157 three-band-detected sources in a 2° diameter low Galactic latitude field at l=55°, b=1° (legend same as Figure 5). There is considerable reddening of the stars in this field, plus a significant population of giant stars. Confusion in high source density regions contributes to the large scatter in the colors of the low SNR sources.

Figure 4Figure 5Figure 6


Sources in the Extended Source Catalog (XSC) meet the following criteria:

  1. J < 15.1, or, Ks < 13.5 mag
  2. SNR(J) > 7, or, SNR(Ks) > 7 (SNR=signal-to-noise ratio)
  3. "E_score" < 1.4, or, "G_score" < 1.4 (see the Explanatory Supplement for details on the scores)
  4. Scan-scan overlap duplicates have been removed

Figure 7 shows the sky distribution of the XSC. The Galactic Plane is denoted with a red dashed line. Figure 8 shows the fractional distribution of 2MASS XSC sources with galactic latitude. Note that the source counts are not normalized by the areal coverage. Figure 9 shows the differential extended source counts corresponding to low stellar confusion fields, <1200 stars deg-2 brighter than Ks=14 mag. The error bars represent sqrt(n) uncertainty measures. The sources comprise a total areal coverage of ~2350 square degrees. Most of the area is free of known nearby clusters (redshift z < 0.1), but a small percentage does i nclude the Abell 262 cluster in Perseus and the outer fringes of the Virgo Cluster. For comparison, deep but narrow-field counts from Glazebrook et al. (1994, MNRAS, 266, 65), denoted with a dotted line, and Gardner et al. (1997, ApJ, 480, L99), denoted with "+" symbols, are shown.

Figure 7Figure 8Figure 9

Figure 10 is the color-color distribution for extended sources located within low stellar confusion fields, <1200 stars deg-2 brighter than mag=14 at Ks. The galaxy color-color is demarked by symbols coded according to the J-Ks SNR. The solid lines denote the main sequence locus (giants diverge at large J-H). The dashed line and triangle points denote the K-correction curve for a spiral galaxy (no evolution), where each triangle represents 0.1 in redshift (z). Figure 11 shows the angular size of extended sources as a function of Ks. The top panel shows the size distribution (in arcsec) of the semi-major axis radius corresponding to the best-fit ellipse of the 20 mag arcsec-2 Ks-band isophote. The minimum radius corresponds to 7´´ (horizontal dashed line). The middle panel shows the size distribution of the semi-major axis radius corresponding to the best fit ellipse of the 21 mag arcsec-2 J-band isophote. The bottom panel shows the semi-major axis radius corresponding to the 3 Ks-band isophote, corresponding to roughly 18.8 mag arcsec-2 surface brightness. The minimum 3 radius is ~5´´. Note: for the 3 distribution, only about half of the sources are plotted.

Figure 10Figure 11

Standard Acknowledgment for Use of 2MASS Data in Astronomical Publications

Please include the following in any published material that makes use of the 2MASS data products:

"This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation."

Thank you very much.

Last update: 1999 Dec 20,

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