Time Space Science

Scientists are using the giant Robert C. Byrd Green Bank Telescope (GBT) to go prospecting in a rich molecular cloud in our Milky Way Galaxy. They seek to discover new, complex molecules in interstellar space that may be precursors to life.

"Clouds like this one are the raw material for new stars and planets. We know that complex chemistry builds prebiotic molecules in such clouds long before the stars and planets are formed. There is a good chance that some of these interstellar molecules may find their way to the surface of young planets such as the early Earth, and provide a head start for the chemistry of life. For the first time, we now have the capability to make a very thorough and methodical search to find all the chemicals in the clouds," said Anthony Remijan, of the National Radio Astronomy Observatory (NRAO).

In the past three years, Remijan and his colleagues have used the GBT to discover ten new interstellar molecules, a feat unequalled in such a short time by any other team or telescope.

The scientists discovered those molecules by looking specifically for them. However, they now are changing their strategy and casting a wide net designed to find whatever molecules are present, without knowing in advance what they'll find. In addition, they are making their data available freely to other scientists, in hopes of speeding the discovery process. The research team presented its plan to the American Astronomical Society's meeting in St. Louis, MO.

As molecules rotate and vibrate, they emit radio waves at specific frequencies. Each molecule has a unique pattern of such frequencies, called spectral lines, that constitutes a "fingerprint" identifying that molecule. Laboratory tests can determine the pattern of spectral lines that identifies a specific molecule.

Most past discoveries came from identifying a molecule's pattern in the laboratory, then searching with a radio telescope for that set of spectral lines in a region of sky. So far, more than 140 different molecules have been found that way in interstellar space.

The new study reverses the process. The astronomers will use the GBT to study a cloud of gas and dust in detail, finding all the spectral lines first, then later trying to match them up to molecular patterns using data-mining software.

The astronomers will make a thorough survey of the interstellar cloud in the wide range of radio frequencies between 300 MHz to 50 GHz. This technique, they said, will allow them to discover molecules that would elude more narrow-range observations.

"This strategy wasn't possible at frequencies between 300 MHz and 50 GHz before the GBT. That telescope's tremendous capabilities enable us to open a whole new era of astrochemistry," said Jan M. Hollis, of NASA's Goddard Space Flight Center.

"Based on earlier studies, there are a number of complex, prebiotic molecules that we think are present in such clouds, but only this wide-net approach with the GBT will capture the evidence we need to discover them," Remijan said.

"Complex organic molecules formed in interstellar space are undoubtedly the fundamental building blocks of astrobiology. The complete inventory of such molecules in this cloud will produce a tremendous advance in our understanding of the physical conditions in that cloud and of the first chemical steps toward life," said Phil Jewell, of the NRAO.

As the survey with the GBT continues, the research team plans to release their data to the scientific community. In addition, they are providing software that will allow other scientists to efficiently "mine" the data for the telltale evidence of new molecules.

"There is a wealth of laboratory data now available about the radio fingerprints of many molecules. Data-mining software will make it possible to efficiently match up the spectral lines seen in the laboratory with ones we observe in the interstellar clouds," said Frank Lovas of the National Institute for Standards and Technology.

The scientists are observing Sagittarius B2(N), a cloud near the center of our Galaxy, some 25,000 light-years from Earth, Numerous molecules have been discovered in that cloud in the past.

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Like cold case investigators, astronomers have used NASA's Chandra X-ray Observatory to uncover evidence of a powerful outburst from the giant black hole at the Milky Way's center.
A light echo was produced when X-ray light generated by gas falling into the Milky Way's supermassive black hole, known as Sagittarius A* (pronounced "A-star"), was reflected off gas clouds near the black hole. While the primary X-rays from the outburst would have reached Earth about 50 years ago, the reflected X-rays took a longer path and arrived in time to be recorded by Chandra.

"This dramatic event happened before we had satellites in space that could detect it," said Michael Muno of the California Institute of Technology in Pasadena. "So, it's remarkable that we can use Chandra to dig into the past and see this monster black hole's capacity for destruction."

Previously, scientists have used Chandra to directly detect smaller and more recent outbursts from the black hole. This latest outburst revealed by the X-ray echo was about 1,000 times brighter and lasted well over 1,000 times longer than any of the recent outbursts observed by Chandra.

Theory predicts that an outburst from Sagittarius A* would cause X-ray emission from the clouds to vary in both intensity and shape. Muno and his team found these changes for the first time, thus ruling out other interpretations. The latest results corroborate other independent, but indirect, evidence for light echoes generated by the black hole in the more distant past.

Scientists have long known that Sagittarius A*, with a mass of about 3 million suns, lurked at the center for Milky Way. However, the black hole is incredibly faint at all wavelengths, especially in X-rays.

"This faintness implies that stars and gas rarely get close enough to the black hole to be in any danger," said co-author Frederick K. Baganoff of the Massachusetts Institute of Technology in Cambridge. "The huge appetite is there, but it's not being satisfied."

During the outburst, the area close to the black hole would have been about 100,000 times brighter than it is currently. If such an outburst had occurred more recently, it likely would have been detected by an X-ray instrument, or would have produced similar features in other nearby clouds.

"Our data show it has been 50 years or so since the black hole had its last decent meal," said Muno. "This is nothing like the feasting that black holes in other galaxies sometimes enjoy, but it gives unique knowledge about the feeding habits of our closest supermassive black hole."

The details of how Sagittarius A* feeds remain unclear. For example, one possibility is that the black hole grows by pulling in matter from the winds of nearby young stars. Also, if there is a disk of material swirling around Sagittarius A*, it might be unstable in such a way that material migrates toward the black hole's edge in clumps, emitting X-rays before disappearing from the universe forever. The theoretical work is still being developed.

Studying this light echo is also important because it illuminates and probes the poorly understood molecular clouds near the center of the galaxy. In particular, it gives information about the dense cores of these clouds where new stars may be forming.

Variability in the X-ray emission between three Chandra observations in 2002, 2004 and 2005 argues against an alternate source for the light echo, which is that it came from a neutron star or black hole pulling matter away from a binary companion. This explanation is not favored because the data show the outburst would have been unusually long and bright for such a binary.

These results were presented at the American Astronomical Society meeting in Seattle, Wash., and will appear in an upcoming issue of The Astrophysical Journal Letters. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center, Cambridge, Mass.


Adapted from materials provided by Chandra X-ray Center.