The Two-Micron All Sky Survey (2MASS) has had a good month in the news. I have already talked some about 2MASS on these pages. As the first comprehensive all-sky survey carried out in the near infrared (NIR), somewhat comparable to the two Palomar Sky Surveys done in the optical in the mid-1950's and repeated in the early 1990's, it is an exciting project, of key interest to all astronomers. Last March, 2MASS made its Second Incremental Data Release, with 47% of the sky included. Finally, when NASA issued a major press release about 2MASS last week (on 7/14/00), the web server at the Infrared Processing and Analysis Center (IPAC), where 2MASS data are available, shortly thereafter began running at up to 1.9 million hits per day, over 20 per second -- many times its designed maximum capacity. So perhaps this is a good time to catch up on the subject.
A near-infrared sky survey is important for a number of reasons. It is an interesting fact that in astronomy a change of wavelength or frequency of only a factor of two (2MASS observes at from two to four times the wavelength of visible light) typically results in dramatic changes in the scene which one obtains. For example, because the Galaxy is so heavily obscured by dust, in visible light we really cannot see much or far in the galactic plane, compared to what is really there. In most directions in the plane, dust obscuration limits us to a few thousand light years or less, compared to the 25,000 light-year distance to the center. The Galactic Center itself is obscured by about 30 mag in the visible, a factor of a trillion (1012).
Infrared light, however, can penetrate the dust to a great degree; the longer the wavelength, the better. So it is that to my mind at least, the 2MASS image of the Galactic Center (see the URL below) and its environs -- the central star cluster glowing red, centered on the invisible black hole, surrounded by a vast beach of galactic-bulge red giant stars like grains of sand, with the plane stretching out on both sides, laced with dark filamentary clouds deep in dust -- ranks with the best of the HST images for impact and astronomical interest.
Because it can see through the dust so well, 2MASS can observe practically all stars in the Galaxy that are bright enough, in particular almost all the cool red giants, which account for most of the NIR light. Hot objects, recall, tend to emit mainly at shorter wavelengths toward the blue or even ultraviolet, and cooler ones at longer wavelengths towards the red and infrared. In addition, although we have been conditioned to think of the Sun as a typical star, in fact it is more massive, and thus hotter and brighter, than all but roughly 5% of the main-sequence stars in the Galaxy. A vast number of stars have masses in the 0.1 to 0.5 solar-mass range, and because the luminosity or absolute magnitude of a main-sequence star is an extremely strong function of its mass, these are typically cool red dwarfs.
While 2MASS does not have the sensitivity or angular resolution to observe them, in their uncounted billions, at very great distances, clearly these truly "typical" dwarf stars are best observed in the NIR, where their emission is strongest. Thus 2MASS is perfect for exploring the near environment of the Sun. In fact, if interstellar travel is ever possible at all, it may well be that the first objects visited will be discovered by 2MASS. Among such local objects are the "brown dwarfs", until about five years ago mythical beasts of theory only. These are "stars" too low in mass, and thus too cool, to ever successfully ignite the nuclear fire of hydrogen burning. Stars with masses below about 8% of the Sun's are fated to shine only for a little while by the heat of their gravitational contraction, and then cool into dark obscurity. Jupiter, with 0.1% the mass of the Sun, is 80× below the 8% limit, yet it still emits (mainly in the far infrared) about twice as much energy, due to its slow cooling contraction, as it receives from the Sun. Thus it is an example, from the extreme low-mass end, of what brown dwarfs might be like. Not one brown dwarf was known less than ten years ago; as of this writing, 2MASS has identified many hundreds, and thousands are expected when analysis of the survey is complete. In fact, it appears that the number of brown dwarfs exceeds the number of all normal stars combined, although they contribute only a small fraction of the total mass. Thus the nearest object to the Sun may well be a yet-undiscovered brown dwarf.
Deep in the cold dark of dense molecular clouds new stars are forming, usually invisible to optical telescopes, but often apparent in the NIR. Thus the details of star formation and star-forming regions is another subject of great interest available for investigation with 2MASS data. Lastly, the optical obscuration of the Milky Way blocks not only the Galaxy itself, but everything beyond. A century ago the "spiral nebula" were known not to be found in the "zone of avoidance", near the galactic plane -- almost half the sky. At the time, before the extent of dust obscuration in the galactic plane was appreciated, some argued that this proved that they must be local objects, somehow associated with the Milky Way. Now it is clear that if we wish to observe galaxies at low galactic latitude, observation in the NIR is one of the best ways to do it. 2MASS has already discovered one large nearby spiral galaxy, and two previously-unknown galactic globular clusters. Thousands of fainter galaxies will be discovered in the Extended Source Catalog being prepared.
2MASS data are being obtained by by two identical, dedicated 1.3 m telescopes, one in
Arizona and one in Chile. The survey differs from the photographic Palomar surveys in that
the data are collected directly in digital form by 256×256 pixel infrared arrays, one for
each of three near IR color bands: J (1.25µm), H (1.58µm) and Ks (2.17µm).
The pixel size is 2"×2", but the image is stepped by 1/6th frame, with small
offsets, so each pixel is sampled six times, allowing images to be reconstructed with
1" pixels. Despite the small field of view, which is tiny compared to that of the 6°
FOV Palomar Schmidt survey telescope, the detectors are so sensitive that instead of
hours, each frame is exposed for only about 1.35 sec, giving a total of about 8 sec on
every star. The total mapping rate is then about 70 sq. degrees per night, not very
different than for the Palomar surveys, done almost completely automatically under
computer control. This exposure gives a point-source sensitivity of about 15.8 mag in J,
15.0 in H, and 14.3 in Ks at a signal-to-noise ratio (SNR) of 10:1. The data
are recorded on magnetic tape, and shipped to IPAC for processing; well over 10 TeraBytes
(1 TB = 1012 bytes) of raw data have been collected so far. Technical details
of telescopes, cameras, and observations may be found at:-
http://pegasus.phast.umass.edu:80/2mass/pub/overview.html.
2MASS is a joint project of IPAC and the University of Massachusetts at Amherst, under the
direction of Principal Investigator Dr. Mike Skrutskie of U.Mass. U.Mass is responsible
for the project as a whole, built the telescopes and cameras, and operates the
observatories.
Because of the amount of data involved, equivalent to about 6,000 CDROMs in the second release, the World Wide Web is the only practical method of distributing the survey. Because of the all-digital collection, processing, and archiving, it is also natural and convenient. IPAC is responsible for the processing, archiving, and distribution of the data, under direction of Project Scientist Dr. Roc Cutri. A "Sampler" release of just one night's data was issued at the beginning of 1999; it alone was larger than the complete data set from the entire year (1982-1983) of operation of IPAC's first project (IRAS, the InfraRed Astronomy Satellite). Then in May 1999, 6% of the sky was issued in the First Incremental Data Release; a reprocessing of these data is included in the 47% of the second release. The project has made enormous progress in the past year, with about 95% of the sky already observed under good conditions. The 4-year data collection for 2MASS will be complete early next year. All the data will be reprocessed to a uniform standard and re-issued about a year thereafter.
The data available at IPAC (http://www.ipac.caltech.edu/2mass/) include four main components:
In addition, the Image Gallery has a large number of beautiful color composite images, laboriously built up from the small 8'×16' images in the Archive, of many well-known astronomical objects. The color mapping is generally J = blue, H = green, and Ks = red, so that the usual intuitive relationship between obscuration and reddening, and color and temperature, is roughly preserved. Thus the dust that prevents us from seeing the Galactic Center at all in visible light can still be discerned along the galactic plane in the 2MASS image, by the reddening of the stars there. Each of these many images may be down-loaded for display, printing, or other use. Some 32 GB of data, mostly color composite images, were delivered by the server over the Net on the peak day after the NASA press release.