Milky Way's puzzling core
Boston — A vast arc of interstellar gas, rising from the galactic disk, has posed new questions about what lies at the heart of our Milky Way galaxy. This previously unsuspected feature, discovered by a team of United States radio astronomers, has shattered the preconception of experts that the galactic center is a prolific nursery for stars. The arc itself now appears to be the source of the intense radio emission that had been attributed to star formation.
Also, the arc's structure suggests that the gas - composed of ionized (positively charged) atoms and negatively charged electrons - may be under the control of an extensive magnetic field. If confirmed, astronomers will have to account for the presence of this magnetic field, which also had not been suspected. They will have to construct a plausible theory as to what sort of natural dynamo could generate such magnetism at the galactic center.
Mark Morris of the University of California at Los Angeles and Farhad Yusef-Zadeh and Don Chance of Columbia University discovered the arc using the Very Large Array (VLA) of the US National Radio Astronomy Observatory, some 50 miles west of Socorro, N.M. In the VLA, which is funded by the National Science Foundation, 27 radiotelescopes, each with a dish 25 meters (82 feet) in diameter , are mounted on tracks and aligned in a Y-shaped pattern.
According to Dr. Morris, the arc was a surprising discovery. He explains that he and his colleagues had set out to study star formation at the galactic center , which is some 30,000 light-years from Earth. That region has enough cold interstellar gas to make several billion stars with the mass of our sun. As did other astronomers, they had assumed such star formation to be taking place. They expected the VLA, which can give a detailed radio ''view'' of the galactic center region, to show many details of star formation. They found the arc instead.
Morris describes the arc as ''quite perpendicular to the disk'' of the galaxy and ''quite spectacular.'' The research team ''looked'' within a region 15 degrees of latitude above and below the galactic plane. (Astronomers use a system of latitude and longitude for the galaxy similar to that used for Earth.) The arc extends beyond their view and is probably longer than the 150 light-years they measured. (A light year is 6 million million miles.)
The VLA showed enough detail for Morris and his colleagues to see that it is made up of long filaments. These may twist like the strands of a rope. If that turns out to be so, it would show that the arc structure is determined by a particular kind of magnetic field that astronomers had not previously associated with our galaxy, Morris says.
There has been evidence of a general magnetic field, hundreds of thousands of times weaker than that of Earth and oriented within the galactic disk. The arc suggests a field something like that of a bar magnet extending out of the disk. This would be a so-called poloidal field, with a north and south pole. It would not be as symmetrical as a bar magnet's field. But it would be a type of field like that of Earth or the sun.
Generally, such poloidal magnetic fields are generated naturally by rotation and turbulence within an electrically conducting fluid, such as Earth's liquid core or the sun's gaseous interior, both of which rotate. Morris says that, in the case of the arc, a poloidal field can't be due simply to rotation of the galaxy. Thus galactic astronomers may have a new puzzle to solve.
Also, Morris notes, discovering that the gaseous arc generates the radio noise that had been attributed to star formation leaves the question of why there isn't a lot of star birth at the galactic center. There is plenty of gas there. Thus, Morris says, there should be plenty of star formation.
The fact that such puzzles have arisen both deepens the mystery of the nature of the galactic center and emphasizes the importance of understanding it. Many so-called active galaxies and similar objects have violent centers that produce explosive outpourings of energy. Astronomers suspect that a galaxy's center plays a key role in the evolution of the galaxy as a whole. If they can understand what is going on in our own galaxy's relatively quiet center, astronomers should be better able to figure out what is happening in the violent centers of active galaxies.
Noting this, Morris says that the presence of poloidal magnetic fields in galaxy centers may be ''an important clue.'' He says that, in retrospect, there are hints in the data on galaxies that something like the arc may be associated with other spiral galaxies. Thus, he adds, he and his colleagues expect to be using the VLA again in a few weeks to study motions in the arc and see if they can find definitive evidence for a guiding magnetic field.