South Pole Telescope Sees Origin of Starbursts

The Earth is approximately 30,000 light years from the center of the Milky Way Galaxy. Stars in the inner 6,000 light years are organized into an Way Galaxy. Stars in the inner 6,000 light years are organized into an elongated, bar-like shape. This causes the interstellar gas near the elongated, bar-like shape. This causes the interstellar gas near the galactic center to form a ring of around the center about 950 light years, galactic center to form a ring of around the center about 950 light years, or 300 parsecs, in radius. The gas in this "300 parsec ring" consists or 300 parsecs, in radius. The gas in this "300 parsec ring" consists mostly of molecular hydrogen and other interstellar molecules such as mostly of molecular hydrogen and other interstellar molecules such as carbon monoxide. Inside the ring are gigantic clouds of interstellar gas, carbon monoxide. Inside the ring are gigantic clouds of interstellar gas, ten million times more massive than the Sun, located near the ionized ten million times more massive than the Sun, located near the ionized hydrogen region Sgr B2 and the galactic center itself at Sgr A. hydrogen region Sgr B2 and the galactic center itself at Sgr A.

The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) observed carbon monoxide emission lines from the 300 parsec molecular observed carbon monoxide emission lines from the 300 parsec molecular ring, Sgr B2, and Sgr A during July 2000. AST/RO is a 1.7 meter diameter ring, Sgr B2, and Sgr A during July 2000. AST/RO is a 1.7 meter diameter telescope operating at wavelengths between 1.3 millimeters and 0.3 telescope operating at wavelengths between 1.3 millimeters and 0.3 millimeters. AST/RO is operated by astronomers at the Smithsonian millimeters. AST/RO is operated by astronomers at the Smithsonian Astrophysical Observatory, as part of the Center for Astrophysical Astrophysical Observatory, as part of the Center for Astrophysical Research in Antarctica, under a cooperative agreement with the National Research in Antarctica, under a cooperative agreement with the National Science Foundation. It is located 800 meters from the geographic South Science Foundation. It is located 800 meters from the geographic South Pole, at the United States National Science Foundation Amundsen-Scott Pole, at the United States National Science Foundation Amundsen-Scott Station. During the Antarctic winter, from June through September, the Station. During the Antarctic winter, from June through September, the South Pole is in constant darkness and is among the coldest places on South Pole is in constant darkness and is among the coldest places on Earth, with temperatures as low as -82 C. These frigid conditions Earth, with temperatures as low as -82 C. These frigid conditions desiccate the atmosphere, allowing submillimeter-wave astronomical desiccate the atmosphere, allowing submillimeter-wave astronomical observations which are not possible at ordinary observatory sites, where observations which are not possible at ordinary observatory sites, where submillimeter waves from space are blocked by water vapor. submillimeter waves from space are blocked by water vapor.

By observing several spectral line transitions of carbon monoxide in the submillimeter-wave band, AST/RO scientists were able to show that the 300 submillimeter-wave band, AST/RO scientists were able to show that the 300 parsec ring contains, on average, several thousand molecular hydrogen parsec ring contains, on average, several thousand molecular hydrogen molecules per cubic centimeter. This density is near a critical value. molecules per cubic centimeter. This density is near a critical value. If the density of the 300 parsec ring is below this value, then it can If the density of the 300 parsec ring is below this value, then it can persist as a uniform ring of material in orbit around the galactic center. persist as a uniform ring of material in orbit around the galactic center. If additional material is added to the ring from dust lanes in the If additional material is added to the ring from dust lanes in the galactic center bar, increasing the density, then the ring will coagulate galactic center bar, increasing the density, then the ring will coagulate under its own gravity and form a giant molecular cloud like Sgr B2. under its own gravity and form a giant molecular cloud like Sgr B2. Gravitational and hydrodynamic forces will then cause this cloud to spiral Gravitational and hydrodynamic forces will then cause this cloud to spiral into the galactic center, where it will cause an energetic burst of star into the galactic center, where it will cause an energetic burst of star formation. formation.

Astronomers have observed such star bursts in other galaxies, such as the nearby irregular galaxy M82. The center of the Milky Way undergoes a star nearby irregular galaxy M82. The center of the Milky Way undergoes a star burst whenever one of the giant galactic center clouds arrives at the burst whenever one of the giant galactic center clouds arrives at the center. This happens at roughly 500 million year intervals. The interval center. This happens at roughly 500 million year intervals. The interval at which this occurs, and the size of the resulting burst, are regulated at which this occurs, and the size of the resulting burst, are regulated by the periodic formation of giant clouds by the 300 pc ring, and the rate by the periodic formation of giant clouds by the 300 pc ring, and the rate at which it is fed material by the galactic center bar. ``The instability at which it is fed material by the galactic center bar. ``The instability of the 300 pc ring shows how starbursts can occur at regular of the 300 pc ring shows how starbursts can occur at regular intervals--material accumulates in the ring, and when there is enough of intervals--material accumulates in the ring, and when there is enough of it, it falls on the galactic center all at once.'' said Dr. Antony Stark, it, it falls on the galactic center all at once.'' said Dr. Antony Stark, Principal Investigator of the AST/RO project. Principal Investigator of the AST/RO project.

A copy of the image can be found here:

[Image Caption]: Submillimeter-wave emission from highly-excited carbon monoxide (CO) molecules has been mapped toward the center of the Milky Way monoxide (CO) molecules has been mapped toward the center of the Milky Way galaxy by the Antarctic Submillimeter Telescope and Remote Observatory galaxy by the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), located at Amundsen-Scott South Pole Station. The photo shows CO (J=7-6) emission (light yellow) and CO (J=4-3) emission (blue) as a function of galactic longitude and velocity. The CO (J=4-3) emission is distributed throughout the Galactic Center region in a manner almost identical to that of CO (J=1-0). In contrast, the CO (J=7-6) emission from the Galactic Center region is confined to the SgrA and Sgr B2 complexes. (AST/RO), located at Amundsen-Scott South Pole Station. The photo shows CO (J=7-6) emission (light yellow) and CO (J=4-3) emission (blue) as a function of galactic longitude and velocity. The CO (J=4-3) emission is distributed throughout the Galactic Center region in a manner almost identical to that of CO (J=1-0). In contrast, the CO (J=7-6) emission from the Galactic Center region is confined to the SgrA and Sgr B2 complexes.

Scientific contacts:

Dr. Sungeun Kim (617-495-7020, skim@cfa.harvard.edu)
Dr. Antony A. Stark (617-496-7648, aas@cfa.harvard.edu; currently at SouthPole)
Dr. Chris L. Martin (617-496-5462, cmartin@cfa.harvard.edu; currently at South Pole)
Dr. Adair P. Lane (617-496-7654, alane@cfa.harvard.edu)