I. Introduction

1. 2MASS Overview

The first large-area near-infrared survey of the sky was published by Neugebauer & Leighton 1969: The Two Micron Sky Survey ( TMSS: A Preliminary Catalogue). The TMSS covered 70% of the sky and detected ~5,700 celestial sources of infrared radiation. The TMSS showed astronomers that the infrared view of the Universe was not the same as the view based on traditional optical observations. Because the Universe as a whole, and the Milky Way in particular, is much more transparent at infrared wavelengths, it was found that optically selected samples of objects which radiate most of their energy in the IR, such as cool stars, are biased. The TMSS of the 1960's barely scratched the surface of the near-infrared sky. Several near-infrared systematic surveys were conducted subsequent to the TMSS at higher sensitivity and spatial resolution, most of which probed Galactic structure such as the limited area surveys of Karawa et al. (1982 PASJ, 34, 389), Eaton et al. (1984 MNRAS, 208, 241) and Jones et al. (1984 AsAp, 138, 297), and the more extensive Galactic plane survey of Garzon et al. (1993 MNRAS, 264, 773).

The first mid-infrared sky surveys carried out by the Air Force Cambridge Research Laboratory (AFCRL) and its successor the Air Force Geophysics Laboratory (AFGL) mapped nearly 90% of the sky at 4, 11 and 20 µm (Walker and Price 1975: The AFCRL Infrared Sky Survey, AFCRL-TR-75-0373, ADA 016397 and Price and Murdock 1983: The Revised AFGL Infrared Sky Survey Catalog, AFGL-TR-83-0161, ADA 134007). These surveys measured over 2000 sources that were even redder and cooler than those detected by the TMSS, such as the most extreme AGB stars and proto-planetary nebulae like AFGL 2688 (the "Egg") and AFGL 618. The thermal emission from zodiacal dust and the diffuse emission along the Galactic Plane were also first mapped by the AFCRL and AFGL observations.

In 1982 a collaboration of the US (NASA), the Netherlands (NIVR), and the UK (SERC), launched the Infrared Astronomical Satellite (IRAS). IRAS surveyed the sky at four infrared wavelengths, 12, 25, 60 and 100 µm. The photometric survey of the whole sky revealed entirely new classes of objects. IRAS initiated a revolution in the understanding of the formation of stars and planetary systems and provided extragalactic astronomers with a first tantalizing view of the complete nearby Universe. Although the IRAS maps were useful for identifying warm and hot dust and star-forming regions near and far, they were much less useful for identifying the most well-known sources of baryons in the nearby Universe, ordinary stars. Major questions, such as the exact source of the Milky Way's motion with respect to the cosmic microwave background radiation and the nature of any possible difference in the spatial distributions of dark matter and ordinary baryonic matter, remained unanswered.

By the late 1980's, the combination of the early near-IR results on stars from the TMSS and extragalactic results from IRAS convinced astronomers that a much deeper near-infrared survey was needed. The development of sensitive near-IR array detectors and advances in computing technology made such a survey feasible. The Optical and Infrared Panel of the astronomy and astrophysics Decadal Survey of 1991 made a Near-Infrared All Sky Survey its number one ranked priority and the first among four small projects highlighted as of special scientific importance in the final overall summary (Bahcall, J. 1991, "A Decade of Discovery in Astronomy and Astrophysics", National Research Council). The proposal for the Two Micron All Sky Survey submitted by the University of Massachusetts and the Infrared Processing and Analysis Center, with Principal Investigator Susan Kleinmann, was designed in response to this recommendation. Also instrumental in the early development of the Survey were Frank Low of the University of Arizona and Fred Gillett of the National Optical Astronomy Observatory.

The resulting 2MASS project made uniformly-calibrated observations of the entire sky in the J (1.24 µm), H (1.66 µm) and Ks (2.16 µm) near-infrared bands with a pixel size of 2.0´´. Sources brighter than about 1 mJy in each band were detected with a signal-to-noise ratio (SNR) greater than 10. This represents an 80,000-fold improvement in sensitivity relative to the TMSS.

The scientific value of 2MASS is documented in over 400 publications based on early data releases made while the Survey was still underway (The 2MASS Sampler, First Incremental Data Release and Second Incremental Data Release). 2MASS provides:

2MASS used two highly-automated 1.3 m telescopes, one at Mt. Hopkins, Arizona, and one at Cerro Tololo, Chile. Each telescope was equipped with a three-channel camera, each channel consisting of a 256×256 array of HgCdTe detectors, capable of observing the sky simultaneously at J (1.24 µm), H (1.66 µm), and Ks (2.16 µm). The 2MASS facility at Mt. Hopkins began routine Survey operations in 1997 June and completed scanning the northern sky in 2000 December. The southern facility began collecting Survey data in 1998 March and conducted its final scan in 2001 February. Approximately 24.5 TB of raw imaging data were collected during that period, covering 99.998% of the sky. Data that met and often surpassed the Level 1 Science Requirements for the Survey were acquired on 691 northern and 750 southern nights. At the end of operations, the University of Massachusetts donated the telescopes to the host observatories.

a. The 2MASS All-Sky Data Release

The 2MASS All-Sky Data Release contains Image and Catalog data covering 99.998% of the sky, derived from all northern and southern Survey observations. The All-Sky Release products include a Point Source Catalog (PSC), containing positions and photometry for 470,992,970 objects, an Extended Source Catalog (XSC), containing positions, photometry and basic shape information for 1,647,599 resolved sources, most of which are galaxies, and the Image Atlas, containing over 4,121,439 J, H and Ks FITS images covering the sky. The construction, contents and formats of these products are described in detail in the following sections of this Explanatory Supplement.

The 2MASS All-Sky Catalogs and Atlas are the result of a full reprocessing of all Survey data following the completion of observatory operations. This final processing enabled all data to be run through a single, stable pipeline system that incorporated knowledge gained over the course of the Survey about the telescopes and cameras, the atmosphere and the near infrared sky. The final processing made use of the best, uniform instrumental, photometric and astrometric calibrations derived using all of the Survey data, and the best astrometric reference catalog (Tycho 2) that was not available earlier. Owing to these improvements, the All-Sky Catalogs and Atlas are superior products and supersede the incremental data release products.

The All-Sky Release Catalogs extend to the faintest possible flux limit, while still maintaining the high standards for reliability that have come to be expected for 2MASS products. For example, the All-Sky Release PSC is a composite catalog that contains a primary subset of sources which meet or surpass all of the specifications of the 2MASS Level 1 Science Requirements, as well as an extension including sources with lower signal-to-noise ratio measurements, but that still have a high degree of reliability. The two components that make up the single database which is called the PSC are discussed in more detail in I.6.b.i. This dual purpose catalog can be used both by astronomers wishing to conduct statistically robust studies that require a well-characterized, highly uniform data set, as well as those who want as sensitive as possible a database to study known sources or to search for new classes of rare objects.

b. The 2MASS Extended Mission

Following the release of the All-Sky Data Products, 2MASS entered an Extended Mission phase. No new data were acquired during this phase. Instead, specialized processing and validation were carried out to produce several Ancillary Data Products that exploit fully the scientific potential of all data gathered during the Survey operations. These products complement the highly reliable and uniform All-Sky Catalogs, by pushing to the faintest possible flux limits and by probing the time domain in specific regions of the sky. The Ancillary Products produced during the 2MASS Extended Mission include:

In addition to the extracted source databases and calibrated image products from the main survey, long exposure (6x) and calibration observations, the 2MASS Ancillary Products include two "value-added" products that are designed to assist users in exploiting the multi-epoch 2MASS observations:

c. About this Document

This Explanatory Supplement provides users of 2MASS products with a description of the Survey and its facilities (III.1), the production and contents of data products comprising the All-Sky Data Release (I.3). Information describing the generation and properties of the Extended Mission Ancillary Products is contained in Appendices 1-7. This document is also intended to provide detailed information about the quality of the data, including known limitations (I.6 and A1.4), to facilitate their most accurate scientific use (II.2 and II.3). The document was drawn in part from the 2MASS Second Incremental Release Explanatory Supplement, and has additional information specific to this data release and the Survey as a whole. This Explanatory Supplement will be updated periodically to provide users with the most current analyses and advice regarding 2MASS data products. Please see the Document Change History for the latest updates.

[Last Updated: 2006 December 20; by R. Cutri, M. Skrutskie, R. Stiening & J. Huchra]

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