The Two Micron All Sky Survey at IPAC
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About 2MASS
- A Brief Explanation of 2MASS
- 2MASS in the News
- 2MASS Science
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- 2MASS Personnel at IPAC
- Funding Opportunities for 2MASS Archival Research:
A Brief Explanation of 2MASS
Introduction
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 is providing 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 has uniformly scanned 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 has achieved an 80,000-fold improvement in sensitivity relative to earlier surveys.
2MASS used two highly-automated 1.3-m telescopes, one at Mt. Hopkins, AZ, and one at CTIO, 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 (1.25 microns), H (1.65 microns), and Ks (2.17 microns).
The 2MASS arrays imaged the sky while the telescopes scan smoothly in declination at a rate of ~1´ per second. The 2MASS data "tiles" are 6° long in the declination direction and one camera frame (8.5´) wide. While the entire telescope scanned in the declination direction, the telescope's secondary mirror tilted opposite the scan direction to momentarily freeze the focal plane image. At the end of a 1.3-s exposure the secondary flew back to its start position and froze a new piece slightly displaced from the previous frame. The dead time between frames used for secondary flyback and array reset was less than 0.1-s. The camera field-of-view shifted by ~1/6 of a frame in declination from frame-to-frame. The camera images each consisted of six pointings on the sky for a total integration time of 7.8 s, with sub-pixel "dithering", which improves the ultimate spatial resolution of the final Atlas Images. When accounting for dead time and the time to point the telescope and initiate a scan, the 2MASS observing system integrated on the sky approximately 84% of each night.
The northern 2MASS facility began routine operations in 1997 June, and the southern facility in 1998 March. Survey operations were complete for both hemispheres on 2001 February 15. . Analyses of the data from the entire sky which has been fully processed show that they easily meet and often exceed the Level 1 Science Requirements for the Survey.
The University of Massachusetts (UMass) was responsible for the overall management of the project, and for developing the infrared cameras and on-site computing systems at both facilities. The Infrared Processing and Analysis Center (IPAC) is responsible for all data processing through the Production Pipeline, and construction and distribution of the data products. The 2MASS project involves the participation of members of the Science Team from several different institutions. The 2MASS project is funding by the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF).
Scientific Objectives and Benefits
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
Ks=13.5 mag, including galaxies in the 60°-wide
``Zone of Avoidance,'' where dust within the Milky Way renders optical
galaxy surveys incomplete. The
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).
2MASS Data Products
2MASS is currently producing the following data products for the entire sky:
- A digital atlas of the sky comprising approximately
4 million 8´ × 16´ Atlas Images, having about 4´´
spatial resolution in each of the three wavelength bands.
- A point source catalog containing accurate positions
and fluxes for ~300 million stars and other unresolved objects.
- An extended source catalog containing positions and total magnitudes for more than 1,000,000 galaxies and other nebulae.
2MASS Data Releases
2MASS has released data products on the approximate schedule:
- The All-Sky Data Release took place in March 2003. This releases covers 99.998% and supercedes all earlier incremental data releases.
- The Second Incremental
Release of northern and southern hemisphere data in March 2000.
This release covers more than 19,600 square degrees, i.e., ~47% of the sky.
- The First Incremental
Release of northern hemisphere data occurred in May 1999.
This release covered about 2483 square degrees, i.e., ~6% of the sky.
- The 2MASS Sampler of one northern night's of data (63 square degrees of sky) was made in December 1998.
Each night of released data consists of about 250,000 point sources, 2000 galaxies, and 5000 images, or, equivalently, about 13.8 GB of data per facility. The Catalogs alone for the Second Release consist of more than 65 GB of uncompressed data. The completed and fully-processed Survey will consist of about 2 TB of catalogs and compressed images.
2MASS Level 1 Requirements
The following are the Survey's Level 1 requirements. These are nominal levels only. The actual performance achieved in many cases surpassed these requirements as described in Section VI.1 of the on-line Explanatory Supplement.
- Magnitude Limits
(For unconfused sources outside of the Galactic Plane (|b|>10°), and outside of any confusion-limited areas of the sky outside of the Galactic Plane.)
Magnitude Limits Band Wavelength (µm) Point Sources (SNR=10) Extended Sources J 1.25 15.8 15.0 H 1.65 15.1 14.3 Ks 2.17 14.3 13.5
Note: At SNR=10, sigma(mag) = 2.5 / ln 10 = 0.109. Also, the feasibility of the point source requirement at H is uncertain, due to lack of knowledge of background variations. - Completeness and Reliability
(For unconfused sources outside of the Galactic Plane (|b|>10°), and outside of any confusion-limited areas of the sky outside of the Galactic Plane.)
Galactic Latitude Range Parameter >|30|° |20|-|30|° |10|-|20|° <|10|° Differential Completeness Point Sources 0.99 -- -- -- Extended Sources 0.90 -- -- -- Differential Reliability Point Sources 0.9995 0.9995 0.9995 0.9995 Extended Sources 0.99 0.99 0.80 --
Note: Differential Completeness (Reliability) is the completeness (reliability) of sources falling in the last half-magnitude bin above the completeness limit. Also, the completeness for extended sources applies only to galaxies that can be reliably distinguished from point sources, with "shape" parameter values 0.5" greater than those of the mean point sources. - Photometric and Positional Accuracy
Photometric precision unconfused point sources
(for sources with SNR >> 20)5% unconfused extended sources
(for isophotal magnitude at 20 mag/sq. arcsec.)10% (H < 13.8) Photometric spatial uniformity point sources 4% extended sources 10% Brightest measurable stars photometric bias (for Ks>4) <2% repeatability 5% for Ks=8 10% for 4<Ks<8 Position Reconstruction Error 0.5" - Sky Coverage
The sky coverage will be >>95% for galactic latitude |b|>10° and ~95% for |b|<10°. The overall coverage will have no gaps > 200 square degrees.
For example three-color composite Atlas Images showing many beautiful and interesting objects observed so far, see the 2MASS Picture of the Week and the 2MASS Image Gallery.
Last Update: 2006 February 1
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