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Warm and Not so Fuzzy

Posted on: Tuesday, 3 August 2004, 06:00 CDT

Planets, stars, nebulae, and galaxies - early returns from the Spitzer Space Telescope show the infrared observatory tackling them all with gusto. /// BY BRUCE DORMINEY

STAR-FORMING REGION D21 (at far right) harbors a young star 100,000 times brighter than the Sun. A shroud of dust hides the region in visible light, letting through only one photon of every 10,000 trillion trillion trillion, but Spitzer penetrates it with no problem. NASA/JPL-CALIECH/A. MARSTON (ESTEC/ESA); INSEE NASA

STAR-FORMING REGION D21 (at far right) harbors a young star 100,000 times brighter than the Sun. A shroud of dust hides the region in visible light, letting through only one photon of every 10,000 trillion trillion trillion, but Spitzer penetrates it with no problem. NASA/JPL-CALIECH/A. MARSTON (ESTEC/ESA); INSEE NASA

Water vapor, carbon dioxide, ozone, and methane have long wreaked havoc on astronomers' efforts to see infrared radiation coming from the cosmos. Observers can peer through only a few windows in Earth's atmosphere at the lower end of the infrared spectrum, beginning at wavelengths just longer than those at the limits of our own eyesight. The best ground-based infrared observatories are positioned above much of the opaque atmospheric curtain, in very dry, high deserts, such as Chile's Atacama, or atop Hawaii's Mauna Kea, the dormant volcano cum astronomical reserve.

Infrared astronomers seeking to circumvent our atmosphere have used balloon-borne telescopes, high-flying observatories, such as the Kuiper Airborne Observatory, and even space itself as viewing platforms. Early space missions, such as NASA's Infrared Astronomical Satellite (IRAS) and the European Space Agency's Infrared Space Observatory (ISO), tantalized astronomers, but NASA just rewrote the play book with the successful August 2003 launch of the last of its four Great Observatories. This one-ton, $765- million jewel in the crown has been renamed the Spitzer Space Telescope (SST) in honor of the late Lyman Spitzer, Jr., the astronomer who, in 1946, first proposed placing a telescope in space.

It's been a stunning success. Early observations using the telescope's 0.85-meter Ritchey-Chretien optics have left many astronomers gaping at the detail and clarity of the images, as well as the observatory's ability to pierce heretofore hidden regions of the cosmos. Although some glitches have turned up in one of the telescope's imagine arrays, Michael Bicay, assistant director of the Spitzer Science Center in Pasadena, California, says no science has been "left on the cutting-room floor" as a result of its on-orbit performance. All three of Spitzer's observing instruments are functioning normally, and the telescope's ability to point is better than anticipated. Moreover, the latest estimates give the scope a life span of more than five and a half years.

HOWLING WINDS and high-energy radiation sculpt this stellar nursery, a cloud of gas and dust embedded in the much larger nebula IC 1396 in the constellation Cepheus. NASA/JPL-CALTECH/W. REACH (SSC/ CALTECH)

SPITZER ROCKETED INTO SPACE on August 25, 2003, aboard a Delta- 11. NASA THE TELESCOPE OBSERVES INFRARED LIGHT, so it seems fitting that NASA also recorded the observatory's launch in the infrared [inset). NASA/JPL-CALTECH

ELEGANT SPIRAL ARMS populated by clumpy, star-forming clouds and warm dust curve out from the central bulge of the bright spiral galaxy M81 in northern Ursa Major. NASA/JPL-CALTECH/S. WILLNER (HARVARD-SMITHSONIAN CFA]

When the mission is done, astronomers expect that Spitzer and its three instruments - the Infrared Array Camera (IRAC), the Multiband Imaging Photometer for Spitzer (MIPS), and the Infrared Spectrograph (IRS) - will have detected at least a million new galaxies. Although that's just a drop in the bucket given the observable universe's estimated 100 billion galaxies, Spitzer's crucial sampling will provide a better understanding of galaxy formation and evolution. It also will yield new details on the disk and star-forming regions in our own Milky Way Galaxy, as well as in dozens of other well-known spiral galaxies, such as the Andromeda Galaxy (M31) and M81. "We can legitimately argue that Spitzer is ten to hundreds of times more sensitive than the ISO mission," says Bicay. "We can see things that are ten to hundreds of times fainter for a given exposure time."

In its Earth-trailing solar orbit, Spitzer drifts away from its home planet at a rate of about 9.3 million miles per year. This places the observatory in "deep space" - where the ambient temperature remains between 30 and 40 kelvins (30 to 40 Celsius above absolute zero) - and saves the precious cryogenic liquid helium that cools the telescope to its optimal operating temperature of 5.5 kelvins. If Spitzer orbited Earth instead, it would be bathed in temperatures exceeding 250 kelvins.

The infrared part of the electromagnetic spectrum ranges from wavelengths of about 1 to 200 micrometers (one micrometer is one- thousandth of a millimeter), with the shortest wavelengths belonging to the near-infrared and the longest to the far-infrared. Spitzer observes at wavelengths between 3 and 180 micrometers. At any given time, Spitzer can view at least a third of the sky, but the Sun hamstrings its pointing - closer than 80 toward the Sun would damage the instruments; more than 120 away would cause the solar panels to receive too little sunlight, the source of the observatory's power.

MOST INFRARED LIGHT, along with most other electromagnetic radiation, gets blocked by Earth's atmosphere before it reaches the ground. ASTRONOMY: ROEN KELLY

Even so, Spitzer has several clear views out of our galaxy and back into cosmic time. Such observations should allow theorists to better understand how the distant universe evolved to mesh with what we now observe in the local universe. Eighty years after Edwin Hubble recognized that many celestial nebulae were actually galaxies in their own right, we still don't know how galaxies form and evolve. Most of the money seems to be on the "bottom-up" scenario, in which fragments of galaxies formed when clouds of atomic hydrogen collapsed into rotating disks spun up by angular momentum. These gaseous disks turned into clouds of molecular hydrogen that gradually merged to form stars and create beautiful spiral galaxies like the Milky Way and Andromeda.

Spitzer will give us a better picture of how such large galaxies came together. As part of GOODS, the multi-spectrum Great Observatories Origins Deep Survey, Spitzer will do pencil-beam infrared surveys of very distant galaxies to determine their stellar populations, masses, and star-formation rates. "People once had a simple view that galaxies collapsed at some high redshift, formed all their stars, and then just sat there, passively evolving," says Mark Dickinson, an astronomer at the National Optical Astronomy Observatories in Tucson, Arizona, and leader of Spitzer's role in GOODS. "We hope to watch galaxies building up their mass with time, find out when they did it, and what drove that process. How much star formation happened during catastrophic mergers between galaxies and how much happened quiescently? We want to watch that process happen."

Observing two patches (one each in the northern and southern sky) totaling about 300 square arcminutes, Spitzer will look at an area about half the size of the Full Moon. But its gaze will head out to a redshift of 5, or more than 11 billion years ago, when the universe was only about 2.5 billion years old.

SWIRE, the Spitzer Wide-area Infrared Extragalactic survey, will map galactic structure over large volumes out to a redshift of 2.5, or about half the depth of GOODS. SWIRE will observe six separate fields adding up to 50 square degrees. The survey will trace the evolution of dusty, star-forming galaxies and active galactic nuclei as a function of their environment and plot where they lie within the universe's large-scale structure.

Efforts to determine the large-scale structure of the universe accurately remain in their infancy. The largest structures, super- clusters, are collections of many thousands of galaxies spreading hundreds of millions of light-years across the curvature of spacetime, not unlike a spider web stretching across the corner of a dark shed. The largest individual galaxy clusters, termed rich clusters, can contain thousands of galaxies, while small galaxy groups might contain only a few dozen. Our own Milky Way Galaxy belongs to the Local Group, a cluster of galaxies at the edge of the Virgo galaxy cluster, which, in turn, inhabits a supercluster of galaxies termed the Virgo supercluster.

"Large-scale structure is a fossilized imprint of how the overall texture of space-time was imprinted a fraction of a second after the Big Bang by slight fluctuations in temperature and density," says David Koo, an astronomer at the University of California, Santa Cruz's Lick Observatory. "The overall pattern doesn't change, but the threads of that pattern do change because on small scales they are affected by gravity. Over time, the threads of the cosmic web react with gravity and convert matter into stars and galaxies. The joints of where these threads meet is where the rich clusters will be."

Spitzer also will target nearer galaxies, studying 75 t\hat mostly lie within the Virgo supercluster. The Spitzer Infrared Nearby Galaxies Survey (SINGS) is a multi-spectrum survey that runs the gamut of galaxy types. Scientists expect SINGS to help them understand the physical processes that create stars out of a galaxy's interstellar medium. "The star-formation rate varies by a factor of a million from one galaxy to the next," says Robert Kennicutt, an astronomer at the University of Arizona's Steward Observatory and SINGS principal investigator. "We are going to be able to calibrate measures of star-formation activity that we can apply to the distant universe. These nearby galaxies will provide us with libraries of galaxy spectra going all the way from X rays to radio."

SPITZER PENETRATED the dusty birth cocoon of Herbig-Haro 46/47 (inset), revealing a protostar that could end up resembling the Sun.

THE THICK DUST CLOUD shows up in this Spitzer spectrum as a big dip caused by silicates. NASA/JPL-CALTECH/A. NORIEGA-CRESPO (SSC/ CALTECH), H.KLINE (JPL)

These observations, in turn, can be used to help theorists make sense of more exotic objects like ultraluminous infrared galaxies (ULIRGs), which emit 90 percent of their total energy in the infrared. Thought to be powered by intense bursts of star formation stimulated by colliding galaxies or central supermassive black holes, ULIRGs were detected first by IRAS. Spitzer should be able to find a hundred times as many of them over a wider range of redshifts.

Spitzer also will take more than a glimpse at the Milky Way Galaxy. The Galactic Legacy Infrared Mid-plane Survey Extraordinaire (GLIMPSE), a 240-square-degree mid-infrared survey of our galactic plane, will home in on roughly half (some 2,000) of the Milky Way's star-formation regions and take a census of all star types in the inner galaxy. GLIMPSE will catalog some 100 million sources lying within 2 of the galactic plane and between 10 and 70 of the galactic center. The survey excludes the galactic center, which is too bright for Spitzer's instruments to observe.

GLIMPSE even may resolve the long-debated galactic conundrum concerning how many spiral arms our galaxy possesses. While four is the most accepted number, galactic astronomers still regularly discuss the issue. "Pan across the galaxy and count the number of times you scan through a spiral arm," says Ed Churchwell, an astronomer at the University of Wisconsin in Madison and the GLIMPSE principal investigator. "Like an egg going through the body of a snake, the stellar density should go sky-high, allowing us to extrapolate how many spiral arms there are and where they lie."

The Milky Way's spiral arms remain full of active star-forming regions. Astronomers think these regions are fed largely by highvelocity clouds of intergalactic gas as well as gas from dwarf galaxies that our giant spiral cannibalizes as it plows through intergalactic space. Spitzer is surveying 30 square degrees in five different giant molecular clouds - Perseus, Ophiuchus, Lupus, Serpens, and Chamaeleon II - in addition to 120 smaller molecular clouds. The observatory already is taking the temperatures of such molecular clouds in an effort to get a better handle on their chemical compositions and understand the physical conditions that create ripe environments for stellar nurseries. The closest of these is in the much-studied Orion Nebula (M42), which clearly is visible to the naked eye in the sword of Orion.

THE TARANTULA, the largest star-forming region known, rules the infrared scene in the Large Magellanic Cloud. NASA/JPL-CALTECH/B. BRANDL (CORNELL AND LEIDEN) IRAS VIEWED THE TARANTULA (inset) in far less detail 20 years ago. IPAC, CALTECH/JPL

PERIODIC COMET Schwassmann-Wachmann 1 flares by five magnitudes during its frequent outbursts. Spitzer imaged two asteroids (circles) along with the comet. NASA/JPL-CALTECH/D.CRUIKSHANK (NASA AMES) AND J. STANSBERRY (ARIZONA)

NEW STARS WARM the dust grains in NGC 7129 in Cepheus, causing it to glow with a pinkish hue, while molecules of carbon monoxide produce the greenish color. NASA/JPL-CALTECH/T. MEGEATH (HARVARD- SMITHSONIAN CFA)

Jeremy Mould, director of the National Optical Astronomy Observatories, has been doing some follow-up ground-based observations of early Spitzer data taken from Henize 206, an immense star-forming region located outside the Milky Way in our companion galaxy, the Large Magellanic Cloud. "This complex portrays the whole story of stellar evolution, from formation and birth to supernova and death," says Mould. "These Spitzer images are fantastic and really show the embedded infrared sources, which are invisible at shorter wavelengths because they are wrapped in dust." Our own Sun likely formed in such a dust-covered cocoon.

Some 3,300 light-years away in the constellation Cepheus lies a rather small, young, star-forming region enveloped in a larger, rosebud-shaped gas nebula. Spitzer recently observed this region, NGC 7129, as part of a survey of about forty star clusters. NGC 7129 has 130 stars of almost every spectral type crowded in a region roughly 4 light-years across. All the while, the young stars in NGC 7129 and other typical clusters are pushing their way out of the dusty masses from which they were born.

"In a hundred million years, the Orion Nebula may look like the Pleiades open cluster," says Tom Megeath, an astronomer at the Harvard-Smithsonian Center for Astrophysics. "The Pleiades may have started with lots of hot stars and then those stars went supernova, causing the low-mass stars to fly out, leaving maybe only half. Chances are our Sun formed in a cluster like NGC 7129 or bigger. But you'd never be able to figure out where the Sun's siblings are. Once the mass is gone, the stars just start drifting away."

This is not unlike the Spitzer Space Telescope itself, which continues to drift slowly away from Earth. Fortunately, the Sun will survive far longer than Spitzer ever could. But if the observatory continues to perform as it has, astronomers will gain a much clearer understanding of how the Sun and Earth came to be born from a cloud of gas and dust in a modest galaxy among billions of others spread throughout a vast universe.

To witness the launch of the Spitzer Space Telescope in infrared light, visit www.astronomy.com/toc

ON THE TRAIL OF PLANETS

A DUSTY DISK HINTS that a planetary system could be forming around the nearby star Fomalhaut. NASA/JPL-CALTECH/K. STAPELFELDT (JPL)

Spitzer also will try to answer some fundamental questions closer to home. How rare is our solar system? What percentage of stars have planetary systems that might harbor planets like our own? A significant percentage of the SST's observing time will go to such studies. Spitzer will survey 336 Sun-like stars within 500 lightyears of Earth, all between 3 million and 3 billion years old, looking for both young circumstellar disks from which planets might form and the telltale signature of older planetary debris disks.

IRAS gave us the first clues that other stars form disks. It found disks around Beta Pictoris, Vega, and Fomalhaut, three fairly massive stars of spectral type A. However, A-type stars hardly would be the first place to look for habitable earthlike planets because these stars live fast and die young, quickly becoming unstable and leaving the hydrogen-burning main sequence phase of their lives.

Spitzer recently took the first-ever snapshot of Fomalhaut's inner disk, showing dusty debris left over from putative planet formation. From 25 light-years away, the planet (or planets) circling this 200-million-year-old star can't be picked out, but astronomers can infer its presence.

"Spitzer will extend the population of known disks to much fainter disks," says Charles Telesco, an astronomer at the University of Florida. "Those disks can be fainter for two reasons: because they are more distant and because they have less mass. Initially, you get this dense disk, which is the remnant dust of the formation of the star. That disk begins to form planets. Overtime, this disk goes from being a primordial disk to a debris disk, as collisions between planets and asteroids create more dust."

Astronomers think planet formation occurs within the first 100 million years of a star's life, and a billion-year-old starwith a dusty disk around it likely would indicate a continuous generation of dust. If there's a gap in the range of observed dust temperatures within the disk of such a star, the Spitzer team might infer the presence of a planet there.

"If you looked at our solar system from a great distance," says Steward Observatory astronomer Michael Meyer, "you would see a huge, inner, evacuated region between the asteroid belt and the Kuiper Belt. In 18 months, we hope to have a quick answer - and then in 3 years, a pretty good answer - about whether our solar system's disk evolution was common or rare for Sun-like stars."

Bruce Dorminey is a France-based science journalist and author of the book Distant Wanderers: The Search for Planets Beyond the Solar System (Springer-Verlag, 2001).

Copyright Kalmbach Publishing Company Aug 2004

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