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June 28, 2004

Seeing Double: Spitzer Captures Our Galaxy's Twin

What would our Milky Way galaxy look like if we could travel outside it and snap a picture? It might look a lot like a new image by NASA's Spitzer Space Telescope of a spiral galaxy called NGC 7331 - a virtual twin of our Milky Way.

The picture, which can be viewed at http://photojournal.jpl.nasa.gov/catalog/PIA06322 , shows our twin as never before. Its swirling arms spin outward from a central bulge of light, which is outlined by a ring of actively forming stars.

"Being inside our galaxy makes it difficult to see what's going on in the center," said Dr. J.D. Smith, a member of the team that observed NGC 7331, and an astronomer at the University of Arizona, Tucson. "By looking at a very similar galaxy, we gain a bird's eye-view of what the entire Milky Way might look like."

Such an outside perspective will teach astronomers how our own galaxy, as well as others like it, might have formed and evolved.

The latest observations are the first in a large-scale effort to observe 75 nearby galaxies with Spitzer's highly sensitive infrared eyes. Called Spitzer Infrared Nearby Galaxies Survey, the program will combine Spitzer data with that from other ground- and space-based telescopes operating at wavelengths ranging from ultraviolet to radio to create a comprehensive map of the selected galaxies.

The program's first target, NGC 7331, was chosen in part for its striking similarities to the Milky Way. While these so-called twin galaxies do not share the same parents, they have many features in common, including number of stars, mass, spiral arm pattern and star-formation rate of a few stars per year. Whether the Milky Way has an inner star-forming ring like that of NGC 7331 is not known. NGC 7331 is located about 50 million light-years away in the constellation Pegasus.

The new Spitzer image demonstrates the power of the telescope's infrared eyes to dissect galaxies into their various parts. Taken by the telescope's infrared array camera, the false-colored picture readily distinguishes NGC 7331's arms (brownish red), central bulge (blue) and star-forming ring (yellow). The composition of materials making up these regions was also revealed by the Spitzer observations: the central bulge consists primarily of older stars; the ring possesses a large amount of gas and dusty organic molecules called polycyclic aromatic hydrocarbons, which typically glow when illuminated by newborn stars; and the arms contain these same dust grains to a lesser degree. Polycyclic aromatic hydrocarbons are also found on Earth, on burnt toast and in car exhaust among other places.

Data from Spitzer's infrared spectrograph instrument were also used to show that the center of NGC 7331 harbors either an unusually high concentration of massive stars, or a moderately active black hole about the same size as the one lurking at the core of our galaxy.

These findings will appear in two papers in the September issue of a special supplement to the Astrophysical Journal. Dr. Michael W. Regan of the Space Telescope Institute, Baltimore, Md., is lead author of a paper detailing observations from the infrared array camera, and Smith is lead author of a paper on the infrared spectrograph results. The Spitzer Infrared Nearby Galaxies Survey project is conducted by a team of about 25 scientists from 12 institutions, and is led by principal investigator Dr. Robert C. Kennicutt of the University of Arizona, Tucson.
 
Launched August 25, 2003, the Spitzer Space Telescope is the fourth of NASA's Great Observatories, a program that also includes the Hubble Space Telescope, Chandra X-ray Observatory and Compton Gamma Ray Observatory. 

June 1, 2004

Spitzer Leads NASA's Great Observatories to Uncover Black Holes, Other Hidden Objects

Astronomers unveiled the deepest images from NASA's new Spitzer Space Telescope today and announced the detection of distant objects -- including several supermassive black holes -- that are nearly invisible in even the deepest images from telescopes operating at other wavelengths.

Dr. Mark Dickinson, of the National Optical Astronomy Observatory, Tucson, Ariz., principal investigator for the new observations, said, "With these ultra-deep Spitzer images, we are easily seeing objects throughout time and space, where the most distant known galaxies lie. Moreover, we see some objects that are completely invisible, but whose existence was hinted at by previous observations from the Chandra and Hubble Observatories."

Seven of the objects detected by Spitzer may be part of the long-sought population of "missing" supermassive black holes that powered the bright cores of the earliest active galaxies. The discovery completes a full accounting of all the X-ray sources seen in one of the deepest surveys of the universe ever taken.

This detective story required the combined power of NASA's three Great Observatories -- the Hubble Space Telescope, Chandra X-ray Observatory and Spitzer Space Telescope. Each observatory studies different wavelengths, from high-energy X-rays with Chandra, through visible light with Hubble, and into the infrared with Spitzer. Together, these telescopes yield far more information than any single instrument.

All three telescopes looked as far as 13 billion light-years away, toward a small patch of the southern sky containing more than 10,000 galaxies, in a coordinated project called the Great Observatories Origins Deep Survey (GOODS). Chandra images detected more than 200 X-ray sources believed to be supermassive black holes in the centers of young galaxies. Extremely hot interstellar gases falling into the black holes produce the X-rays.

Hubble's Advanced Camera for Surveys revealed optical galaxies around almost all the X-ray black holes. However, seven mysterious X-ray sources remained for which there was no optical galaxy. Dr. Anton Koekemoer of the Space Telescope Science Institute, Baltimore, Md., discovered these sources and has three intriguing possibilities for their origin: "The galaxies around these black holes may be completely hidden by thick clouds of dust absorbing all their light, or may contain very old, red stars. Or some could be the most distant black holes ever observed -- perhaps as far as 13 billion light-years." If so, all their optical light would be shifted to very long infrared wavelengths by expansion of the universe.

Scientists eagerly awaited the Spitzer images to solve this puzzle. Because Spitzer observes at infrared wavelengths up to 100 times longer than those probed by Hubble, Spitzer might be able to see the otherwise invisible objects. Indeed, the very first Spitzer images of these objects, obtained earlier this year, immediately revealed the telltale infrared glow from the host galaxies around all the missing X-ray black holes.

Three of Koekemoer's galaxies are extremely "red," or bright, in infrared. The Spitzer data, together with new images at shorter infrared wavelengths from the Very Large Telescope at the European Southern Observatory, indicate that the galaxies around these black holes could be heavily obscured by dust, and perhaps more distant than other known dust-obscured galaxies. Some of the other objects, however, have quite different colors, and are even more intriguing. "Their colors may be consistent with objects more distant than any now known," said Dickinson, who cautioned that additional Spitzer observations later this year will help confirm what kind of objects these might be.

Old Galaxies Shine in Infrared: In another study using the same Spitzer data, Dr. Haojing Yan of the California Institute of Technology, Pasadena, Calif., studied 17 unusual galaxies near the Hubble Ultra Deep Field. This small patch of sky within the GOODS area was recently the target for the deepest optical images ever taken with Hubble's Advanced Camera. The Deep Field optical images, released in March 2004, reach more than five times fainter than the GOODS Hubble data. But even with that phenomenal sensitivity, two of the 17 Spitzer-selected objects remain completely invisible in optical light, while the others are only faintly detected. Yan finds that these galaxies get steadily brighter at longer wavelengths, and seem to be more distant cousins of the so-called "Extremely Red Objects," known from previous deep surveys. Most are distant galaxies that are red because they are either old or dusty. These new Spitzer-identified objects, however, appear to lie farther away to a time when the universe was only two billion years old.

"These objects could be the remnants of the first stars -- the very first galaxies formed in the earliest stages of the universe," said Yan. Most galaxies that we see today formed their stars gradually over a long period of time. But these 17 objects seem to be "old before their time," perhaps almost as old as the universe itself at that early epoch. "If we indeed are seeing the direct, 'pure' descendants of the first stars, this would make a thrilling story," says Yan. Further Spitzer observations at longer wavelengths, planned for later this year, should help decide whether these objects are red because they are old, or because they are young and actively forming stars enveloped in dust.

Black Holes In Hiding: Using Hubble and Chandra data, Dr. Meg Urry, a GOODS astronomer at Yale University, New Haven, Conn., and her team suggest that most accreting black holes are hidden at visible wavelengths, even in the early universe. Few such hidden black holes had previously been found at such large distances, despite theoretical arguments for their existence. They were missed because their visible radiation is so dim they look like faint, ordinary galaxies. "With the new Spitzer data these very luminous, distant objects are easily visible," said Urry. "The great sensitivity of the new Spitzer infrared cameras, combined with the superb spatial resolution of Chandra, means that finding all of the black holes that are powered by infalling gas is now possible."

Urry's team is using data from the three space observatories to take a census of the supermassive black holes that formed two to five billion years after the big bang. Most of these active galactic nuclei are hidden by dust, which absorbs visible and some X-ray light but emits strongly at infrared wavelengths. "The Spitzer GOODS observations verify that large numbers - perhaps three-quarters - of the obscured active galactic nuclei were indeed present in the early universe. The longer-wavelength Spitzer data still to come will reveal even more shrouded active galactic nuclei," said Urry, "including some, missed by X-ray observations, which look like ultraluminous infrared galaxies."

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope, with science operations conducted at Caltech. The Space Telescope Science Institute, Baltimore, Md. is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with the Goddard Space Flight Center, Greenbelt, Md. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA's Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

June 1, 2004

Spitzer Spies Parallelogram-Shaped Galactic Meal

Peering into the "gut" of the galaxy Centaurus A, NASA's Spitzer Space Telescope has captured in unprecedented detail this massive galaxy's last big meal: a spiral galaxy twisted into a parallelogram-shaped structure of dust.

The findings were presented today at the American Astronomical Society annual meeting in Denver, Colo. A stunning image of the galaxy and its bizarre dust structure.

While previous observations with other telescopes have revealed this galactic remnant, it appeared as one long and irregular bar of dust. Spitzer's uniquely sensitive infrared eyes allowed the telescope to see clearly this strangely geometric structure for the first time.

"Now we can actually see the shape of this structure, which helps us explain how it arose," said Dr. Jocelyn Keene, principal investigator for the new research and an astronomer at NASA's Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena, Calif.

Located 10 million light-years away, Centaurus A is a type of galaxy known as "elliptical." It is one of the brightest sources of radio waves in the sky, which suggests the presence of a supermassive black hole at its center. About 200 million years ago, this galaxy is believed to have consumed a smaller spiral galaxy - the contents of which appear to be churning inside Centaurus A's core, triggering new generations of star birth.

Resolving this unusual parallelogram structure has helped astronomers finally put together a picture of its history. The geometric shape can be explained using a model that describes a flat spiral galaxy falling into an elliptical galaxy and becoming twisted and warped in the process. The folds in the warped disc, when viewed nearly edge on, take on the appearance of a parallelogram. The model predicts that the leftover galaxy will ultimately flatten into a plane before being entirely devoured by Centaurus A. Warped discs like this are the "smoking guns" of galactic cannibalism, providing proof that one galaxy once made a meal of another.

Such galactic feeding has long thought to be a mechanism by which giant elliptical galaxies form and grow, and likely provides the fuel that drives the strong radio activity surrounding Centaurus A's central black hole.

Other authors of this research include Dr. Alice Quillen of the University of Rochester, N.Y., and Drs. Daniel Stern, Varoujan Gorjian, Karl Stapelfeldt, Charles Lawrence, Peter Eisenhardt and Michael Werner of JPL.

 May 31, 2004

Spitzer Space Telescope Sets Infrared Eyes On Dark Matter

Ten years ago, a group of astronomers set out to find invisible, or dark, matter in the outer fringes of our galaxy. Long postulated to make up a significant chunk of our universe, dark matter may be partly made up of massive, celestial objects hiding in the halos of galaxies. The astronomers spent six years scanning a large patch of sky and sensed something, but they weren't sure if they were really seeing dark matter or a different class of nearby objects getting in the way.

Now, NASA's Spitzer Space Telescope has set its infrared eyes on this mystery matter and verified that at least one of 17 invisible objects observed years ago lies within the body of our Milky Way galaxy, thereby supporting the latter hypothesis. More observations are needed to draw definitive conclusions; nonetheless, the findings illustrate the power of Spitzer to finally put together the pieces of this decade-old puzzle.

"Historically, searches for unseen matter have been part of the justification for Spitzer," said Dr. Michael Werner, the Spitzer project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and an investigator for the new research. "We are very excited about these initial results."

Matter as we know it doesn't add up to all the matter in the universe. At least 10 times more unseen, or dark, matter exists than known matter. Most dark matter is exotic, made up of something other than everyday atoms. But the rest of it may take the form of celestial objects that are too faint to see because they're very cool. These objects, referred to as "massive compact halo objects," or "machos," are thought to be lurking in the far reaches, or halos, of galaxies. They might include black holes and failed stars called brown dwarfs.

Beginning in 1992, Dr. Charles Alcock, who was then at the Lawrence Livermore National Laboratory, Calif., and is now at of the University of Pennsylvania, Philadelphia, and his colleagues went on a hunt for machos. Rather than scan for the objects themselves, the team looked for the objects' gravitational tug on starlight emanating from behind them. In this technique, called gravitational microlensing, a lens object (the invisible matter) causes the source object (a star) to brighten for a brief period of time. Alcock and his team surveyed 12 million stars for these events in the nearby Large Magellanic Cloud, which lies on the far side of our galaxy halo. They detected 17.

But, based on the predicted numbers of faint stars in our galaxy, the astronomers had expected to see much fewer than 17. Either there is a significant amount of dark matter in the galaxy halo, or there is invisible matter in our own galaxy that cannot yet be understood. Either way, the findings challenged scientific descriptions of matter.

That's where Spitzer comes in. Because it can see objects that are too cool to be seen with other telescopes, it may be able to detect the heat from many of these invisible lenses. To test this ability, a group of astronomers, including Alcock and Werner, and led by Dr. Hien Nguyen of JPL, used Spitzer to observe the macho event referred to as MACHO-LMC-5. This event is the only one of its type that could be seen by NASA's Hubble Space Telescope. Data obtained by Alcock and others using Hubble beginning in 2001, and most recently analyzed by Dr. Andrew Gould of Ohio State University, Columbus, suggest that the lens object for MACHO-LMC-5 is a low-mass star about 1,500 light-years away within our galaxy's disc.

The new Spitzer data for this event independently confirm this finding. "By luck, Hubble was able to see the lens in one of 15 events it looked at, whereas Spitzer should be able to see many more, if these microlensing events are indeed caused by nearby cool objects," Nguyen said.

Nitya Kallivayalil, a graduate student at the University of Pennsylvania, with critical insight from Dr. Daniel Stern of JPL, carefully measured the brightness of the lens using the Spitzer data. Dr. Brian M. Patten of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., used these measurements to establish that the lens is a very low mass, faint star. "The data are fantastic," said Kallivayalil. "When Brian showed us that they confirmed the nature of the star, we were ecstatic."

Added Patten, "With this new capability, we'll be able to determine the properties of many more lenses, and determine their contribution to dark matter in our galaxy."