I. Introduction

1. 2MASS Overview

It has been nearly 30 years since the last large-area near-infrared survey of the sky was carried out. The Two Micron Sky Survey (TMSS; Neugebauer & Leighton 1969) scanned 70% of the sky and detected ~5,700 celestial sources of infrared radiation. Since that time there has been a revolution in the development of infrared detector technology. New, large format, sensitive array detectors can now detect astronomical objects over 100 million times fainter than those detected in the TMSS.

The Two Micron All Sky Survey (2MASS) project is designed to close the gap between our current technical capability and our knowledge of the near-infrared sky. In addition to providing a context for the interpretation of results obtained at infrared and other wavelengths, 2MASS will provide direct answers to immediate questions on the large-scale structure of the Milky Way and the Local Universe. The optimal use of the next generation of infrared space missions, such as HST/NICMOS, the Space Infrared Telescope Facility (SIRTF), and the Next Generation Space Telescope (NGST), as well as powerful ground-based facilities, such as Keck I, Keck II, and Gemini, require a new census with vastly improved sensitivity and astrometric accuracy than that previously available.

To achieve these goals, 2MASS is uniformly scanning the entire sky in three near-infrared bands to detect and characterize point sources brighter than about 1 mJy in each band, with signal-to-noise ratio (SNR) greater than 10, using a pixel size of 2.0´´. This will achieve an 80,000-fold improvement in sensitivity relative to earlier surveys.

2MASS uses two new, highly-automated 1.3-m telescopes, one at Mt. Hopkins, AZ, and one at CTIO, Chile. Each telescope is 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 (1.25 µm), H (1.65 µm), and K_s (2.17 µm).

The immediate scientific benefits from the 2MASS survey include:

An unprecedented view of the Milky Way nearly free of the obscuring effects of interstellar dust, which will reveal the true distribution of luminous mass, and thus the largest structures, over the entire length of the Galaxy.
The first all-sky photometric census of galaxies brighter than K_s=13.5 mag, including galaxies in the 60°-wide ``Zone of Avoidance,'' where dust within the Milky Way renders optical galaxy surveys incomplete. The final catalog of >1,000,000 galaxies will provide a rich statistical database, including photometric measurements in three wavelengths and a few structural parameters for large samples of galaxies in differing environments, measured at wavelengths which are sensitive to the stellar populations dominating the luminous mass.
The statistical basis to search for rare but astrophysically important objects, which are either cool, and thus extremely red (e.g., extremely low-luminosity stars and brown dwarfs), or heavily obscured at optical wavelengths (e.g., dust-obscured AGNs and globular clusters located in the Galactic plane).

The northern 2MASS facility began routine operations in 1997 June, and the southern facility in 1998 March. As of 1999 May, the time of the 2MASS Spring 1999 Incremental Data release, over 65% of the sky has been observed (perhaps 10% of that area will be reobserved because of non-optimal survey conditions). Analyses of the data from the ~50% of the sky that has been processed show that they meet and often exceed the Level 1 Science Requirements for the Survey.

The first release of 2MASS data took place in 1998 December. This release, referred to as the 2MASS Sampler, included the derived Point and Extended Source Catalogs and Atlas Images from one night of data from the northern 2MASS facility. That release was designed to introduce the astronomical community to the content and formats of the 2MASS data products.

The 2MASS Spring 1999 Incremental Data Release

The 2MASS Spring 1999 data release is the first large, incremental data products from the Survey. This release contains data obtained from the northern 2MASS facility on Mt. Hopkins, AZ between the dates of 1997 June 7 and 1998 January 30. Approximately 2483 sq.degrees of sky are covered by the Catalogs and Image Products, spanning a wide range of galactic environments. The The Spring 1999 Incremental data release includes a Point Source Catalog containing positions and brightness information for over 20.2 million objects, and Extended Source Catalog containing positions, magnitudes and basic shape information for over 73,000 resolved sources, and an Image Atlas containing over 233,000 J, H and K_simages covering the survey area.

Further Incremental releases will follow this issue approximately every six months until the end of survey operations. The Incremental releases are being made while survey operations continue in order to provide as early as possible access by the community to 2MASS products, even though they will not necessarily represent the optimum processing of 2MASS data. All of the 2MASS survey data will be reprocessed at the end of survey operations and be rereleased in a single large distribution.

About this Document

This Explanatory Supplement provides users of 2MASS products a description of the Survey and its facilities, and the production and contents of data products comprising the Spring 1999 Incremental Data Release. The document was drawn in part from the 2MASS Sampler Explanatory Supplement, and has additional information specific to this data release as well as more general information concerning the Survey. This Explanatory Supplement is designed to be a living document, in that it will be updated regularly to provide users with the most current analyses and advice regarding 2MASS data products. Consequently, the distribution of this Supplement and those accompanying later large Incremental data releases will be only as WWW hypertext document. Please review them regularly for updates.

[Last Updated: 1999 May 6; by R. Cutri]

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