JPL and NASA News

Bill Wheaton, IPAC

1999 February

WIRE Launch Approaches

Last month in the quick survey I gave of a busy period, I included a few sentences about WIRE, the Wide Field IR Explorer. This month I want to return to my habit of trying to give a more thorough discussion, and talk about WIRE in some detail. WIRE is a good example of the NASA Small Explorer Program (SMEX) ideal: a mission narrowly focused on a question of great current scientific interest.

When we look at galaxies in the local universe, we see, at the coarsest level:

elliptical galaxies, which consist of a smooth distribution of trillions of old stars, with almost no gas and dust, and hence no new stars being formed;
spiral galaxies, which, in their disk and arm regions, are full of gas and dust and evidently forming stars at a fairly rapid rate; and
starburst galaxies, which are extremely dusty and gassy systems, in which stars are forming very rapidly.

Because starburst galaxies contain so much dust and gas, visible and ultraviolet light from the main energy-emitting, star-formation regions is heavily absorbed, to be re-reradiated mainly in the infrared. Thus starburst galaxies were first clearly identified as a class by IRAS, the Infrared Astronomical Satellite, which first mapped the sky in the 12, 25, 60, and 100 µ bands in 1983-1984. Classic examples are M82, an irregular-looking companion of M81, and the large dusty spiral NGC 253, both of which are only about 10 million light years away (M lt-y; hence seen, as usual, as they were 10,000,000 y ago, the look-back time); and also the spectacular highly-disturbed system Arp 220, at well over 100 M lt-y distance. Evidently, the ellipticals used up their store of gas long ago, whereas for typical spirals, it is estimated that the observed gas will last for several 10⁹ years (Gy) at the current estimated rates of star formation, up to as much as 10 Gy in some cases. For starburst galaxies, by contrast, star formation is proceeding so rapidly that the observed store of gas will be exhausted in less than 1 Gy, so such galaxies must be transient, short-lived phenomena.

In the more distant, older, universe, a key mystery is the way in which galaxies formed, and the role of star formation in the process. It is highly suggestive to suppose that the nearly gas-free elliptical galaxies we now observe are the remnants of ancient starburst galaxies, which used all their gas during the early history of the Universe. The possibility that the stars formed first, and then collapsed into galaxies, seems ruled out because the time for an extended cloud of stars to collapse into a galaxy is estimated by calculation to be very long, much longer than the maximum reasonable estimates of the age of the Universe. The possibility that the gas collapsed first, and then led to star formation after the gross structure of the galaxies was set, is alive and well; as is the intermediate possibility, that smaller clumps of gas first formed smaller galaxies, in which stars formed early, and that these small galaxies then collided and aggregated to form the largest galaxies we see today. Until recently, the most distant galaxies that could be observed (excluding QSO's) were at redshifts z of less than about 1, corresponding to distances of 5-10 G lt-y (depending somewhat on the cosmological model), and ages of half the age of the Universe, now very roughly estimated as 15 Gy. With the advent of the two Deep Field exposures, north and south, by the Hubble Space Telescope (HST), the twin 10 m Keck telescopes, and the new ESO Very Large Telescope (VLT) that is just coming into operation in Chile, galaxies are being imaged in the visible and near infrared (eg, 2 µ) at z's of 5 and more, very shortly (maybe 1-2 Gy) after the Big Bang. However, all these observations are hampered by the fact that, being in the optical or near-infrared, the enormous absorption typical of starburst regions, and the large red shifts due to the cosmic expansion combine to greatly reduce the available signal. The number of distant galaxies actually observed is severely limited by the small field of view that can be observed so intensively.

The mission of WIRE is to study starbursts in the distant Universe, from z of 0.5 to about 3, in galaxies and protogalaxies. Because such starbursts are enormously luminous in the infrared, they can be observed at great distances by even quite a small telescope. The observation strategy is to conduct three distinct surveys, at successively greater depth, studying the number of sources found versus brightness, and also the color evolution of sources with look-back time, or age:

The moderate-depth survey will occupy about 60% or WIRE's time, with 15-75 min exposures over each of some 50 survey "areas", each about 1.5° X 1.5° in size.
The deep survey will obtain a large (> 2000 galaxy) sample at the largest look-back time. about 30% of WIRES's time will be devoted to it, with exposures of several hours in each field. The number of sources observed will imply the rate of evolution, as a rapid increase in starbursts with look-back will be reflected in a larger number of detected sources (see tables below).
The ultra-deep survey, to which 10% of the time is allocated. will take exposures of 24 hours or more in a small number of fields to constrain the IR background, and study the number of sources by observing the "lumpiness" of the background due to unresolved, confused sources. The results of this survey should yield the fraction of the luminosity of the Universe which is due to starbursts beyond z = 0.5.

The reddest detected sources will be selected for intensive follow-up study with HST and ground-based telescopes, such as the Kecks, to determine their spectra, obtain precise z, distances, and other properties.

What these surveys will actually see depends considerably on the way in which galaxies evolved with time since the Big Bang:

Projected WIRE Sample Size

No Evolution Scenario
Survey	Sky Coverage	25 micron flux limit (5 sigma)	# of sources
Moderate Depth	170 sq deg	0.69 mJy	30,000
Deep	8.3 sq deg	0.25 mJy	5,000
Ultra-deep	0.5 sq deg	0.17 mJy	500

Moderate Density Evolution Scenario
Survey	Sky Coverage	25 micron flux limit (5 sigma)	# of sources
Moderate Depth	830 sq deg	1.5 mJy	70,000
Deep	42 sq deg	0.56 mJy	14,000
Ultra-deep	1 sq deg	0.34 mJy	700

The 30 cm WIRE telescope operates at 12µ and 25µ, and has two 128X128 pixel arsenic-doped silicon infrared detector arrays cooled by solid hydrogen to less than 7° K. Its spatial resolution will be 20 arc-sec at 12 µ and 23 arc-sec at 25 µ. It is expected that its objectives, including the observation of tens of thousands of starburst galaxies, can be fulfilled in only 4 months. Launch is still scheduled for 26 February, 1999, on an Orbital Sciences Pegasus air-launched rocket. Dr. Perry Hacking is the P.I., with colleagues at IPAC, JPL, Cornell, and NASA Goddard Space Flight Center. Full details may be found at http://www.ipac.caltech.edu/wire.