Dissecting the Hub of Our Galaxy
THE MILKY WAY'S innermost région is a fascinating enigma to astrono-mers. "We are still puzzled by what's there," summed up Charles Townes of the University of California, Berkeley, during a meeting last July at NASA's Ames Research Center. "It is very hard to escape the conclusion that there is a concentration of mass in the center." It could be in the form of an ultracompact star cluster or, perhaps more likely, a multimillion-solar-mass black hole. Evidence for the latter interprétation was summarized in the Au-gust, 1982, issue, page 133.
Since then our view of the galactic center has improved dramatically. The next few pages describe some of the latest observations and their implications. Accord-ing to Reinhard Genzel of UC Berkeley, "We are in a data-gathering phase." New results are coming in so fast "the pace is mindboggling." As a resuit, nobody has yet assembled ail the diverse observations into a coherent model.
THE INFRARED CORE
Dense clouds of gas and dust in Sagit-tarius intercept ail but one photon of light for every billion that set out in our direction from the galactic core, so observations at visible wavelengths can tell us little about our galaxy's hub. The secrets of the central régions must instead be unraveled from the more penetrating infrared and radio émissions.
The rapidly maturing field of infrared astronomy is providing the most detailed
information about the galactic center. Ex-tended 2-micron émission from the center was recognized some time ago and is thought to come from a large, dense cluster of late-type (red) stars. With today's sharp infrared eyes we can see individual members of this cluster.
The seven images below were obtained between 1.65 and 20 microns (wavelengths roughly three to 40 times longer than visible light) with the aid of NASA's 3-meter Infrared Telescope Facility on Mauna Kea, Hawaii. Together they provide a concise summary of the types of objects that lie near the core. The area shown is very small, only 16 arc seconds on a side, corre-sponding to only 2'/i light-years. This distance is about half that from the Sun to its nearest stellar neighbor.
Daniel Lester, who garnered these images, explained to Sky & Telescope that "Many of the sources are red supergiant stars in or near the galactic nucleus [but possibly not in the central 10 light-years]. 1RS 7, meaning infrared source seven, is such a star. Its nature is betrayed by the fact that it is strongest at 2.2 microns and becomes fainter at longer wavelengths. This bëhavior indicates a body much hot-ter than the interstellar medium. 1RS 3 is also probably a red supergiant, but it seems to be reddened by a dust cocoon that causes its émission to be strongest at 4.8 microns."
"1RS 1 and 10," writes Lester, "are examples of H II (ionized hydrogen) régions in the nucleus, which Surround hot stars
and swamp their output at infrared wavelengths. The star ionizes the gas and heats up nearby dust to several hundred degrees Kelvin, causing the dust to show up especially well at longer wavelengths, particularly in the 20-micron frame. Also, the HII régions appear relatively small at short wavelengths but become larger and more diffuse at longer ones. This effect stems from the range of dust températures in the clouds; at short wavelengths we see only the hottest dust closest to the stars, but longer wavelengths reveal the cooler, extended parts of the cloud. Some astron-omers speculate that the clouds may be the remains of stellar atmospheres stripped from their parent stars by collisions in these densely packed régions."
1RS 16 has attracted the most attention in the last few years, for it nearly coincides with an extraordinary radio source known as Sagittarius A West. Strong and compact, it has more in common with the ul-tra-energetic cores of Seyfert and radio galaxies than with anything else in the Milky Way. However, by extragalactic standards our nucleus is quiescent, producing only a few million solar luminosities as opposed to hundreds or thousands of millions common in quasars. The evidence suggests that 1RS 16 is — or is closely associated with — the true nucleus of the Milky Way.
The infrared structure of 1RS 16 "is not simple," notes Lester. "Its position seems , to shift southward with increasing wave- j length. More observations are needed to j see if the shift is due to a température gra- !