For Release: 9:20 a.m. EST, January 12, 2000
STREAMERS OF GAS FEEDING THE BEAST AT THE CENTER OF OUR GALAXY
ATLANTA, GA-- The voracious black hole at the center of our galaxy maybe dining on a steady stream of dust and gas pouring into the MilkyWay's inner regions. Robin S. McGary and Paul T. P. Ho of theHarvard-Smithsonian Center for Astrophysics report that narrow"streamers" of ammonia gas appear to be flowing from giant clouds ofgas toward the center of the galaxy. These findings, presented todayat the American Astronomical Society meeting, could pinpoint thesource of the black hole's gluttonous diet.
Astronomers have long known that a supermassive black hole, more than2 million times more massive than our Sun, lies at the center of thegalaxy some 27,000 light-years from Earth. A point-like source ofradio emission called Sagittarius A* (pronounced "A-star") marks thelocation of this black hole.
The black hole is surrounded by a ring of dust and gas orbitingSagittarius A* (Sgr A*) at a radius of about 6.5 light-years from theblack hole. This "circum-nuclear disk" revolves around the black holeat a velocity of 110 km/s. Gas and dust are stripped from the disk bythe strong gravitational pull of the black hole and spiral towards SgrA*.
The formation and dynamics of this circum-nuclear disk are unclear, so these astronomers have been looking for clues to the origin of the dustand gas seen in the circum-nuclear disk. Their most recent resultscome from the observation of ammonia molecules in the region aroundthe disk. Emission from the ammonia molecules traces the dense, hotgas known to exist near the center of the galaxy and can be used tounderstand the location and motion of gas near the circum-nucleardisk.
Since the ammonia emission has a wavelength of 1.3 cm, the NationalScience Foundation's Very Large Array radio telescope at Socorro, NM,was used for the observations. The array is composed of 27 radioantennas, each 25 meters (82 ft) in diameter. For these observations, the antennas are arranged in a "Y" shape 3.4 km (2.1 mi) across. For observations at 1.3 cm, this allows the resolution ofdetails as small as 0.13 light-years in diameter at the galactic center.
Narrow "streamers" of ammonia emission were observed connecting giantclouds of molecular gas to the circum-nuclear disk. These "giantmolecular clouds" are located from 25 to 50 light-years from thecenter of the galaxy. The streamers are apparently pulled from theclouds by the strong gravitational force of the supermassive blackhole. This finding agrees with earlier detections by Coil & Ho (1999)of ammonia emission at lower temperatures.
Because astronomers can only obtain 2-dimensional images, there couldbe a chance that the observed "streamer" is simply a free-floatingcloud that only appears to be connected to the circum-nuclear disk.However, the velocity of the gas as well as the amount of turbulencein the gas suggests that the connection is real.
Emission from stationary ammonia molecules occurs at an exactfrequency of 23.87 GHz. However, the frequency where it is picked up bythe radio telescope can be shifted if the gas is moving. This shiftis similar to the change in pitch of a train whistle as it passes by.Similarly, if the gas is moving in the direction of the Earth, thenthe emission is shifted to higher frequencies. If the gas is movingaway, then it is shifted to lower frequencies.
The astronomers can use the frequency shift in the detectedemission to study the velocity of the gas. Observations show that thevelocities of gas increase as the streamers approach thecircum-nuclear disk. A steady increase in the velocity of the gas asit moves inward indicates that gas is accelerating along the streamertowards the disk.
The frequency of the ammonia emission also allows theastronomers to study turbulence (amount of random motion) in the gas.This random motion means that the ammonia emission will be observed ata range of frequencies (corresponding to all of the differentvelocities present in the gas). The larger the range of frequencies,the more turbulence is present in the gas. The astronomers see thatthe range of frequencies increases as the streamers approach thecircum-nuclear disk. As the gas approaches the disk, it responds tothe stronger gravitational pull of the supermassive black hole. Thepull of gravity affects the motions of the ammonia molecules andincreases the turbulence in the gas. This observation also indicatesthat the streamers are approaching the circum-nuclear disk and are nota chance projection of another cloud.
These results imply that giant molecular clouds provide thecircum-nuclear disk with gas and dust via "streamers" observable inammonia emission. This dust and gas is then "fed" into thesupermassive black hole. The detection of these streamers may answermany of the questions about the formation of the circum-nuclear diskand the interactions that take place at the center of the Milky Way.
More observations are necessary to fully understand theinteraction of these "streamers" with the circum-nuclear disk.Observations of emission produced by cooler ammonia molecules willenable a direct calculation of the temperature of the gas. Inaddition, the astronomers hope to look for shocked gas in the regionwhere the streamers appear to intersect the circum-nuclear disk. Thepresence of shocked gas in the region where the streamers intersectthe disk would more strictly rule out the possibility of a chanceprojection of an intervening cloud onto the image.
This research was funded in part by the Harvard-SmithsonianCenter for Astrophysics. The National Radio Astronomy Observatory isa facility of the National Science Foundation operated undercooperative agreement by Associated Universities, Inc.
A color image and caption of this result can be found at:
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