The roiling cores of many active galaxies are difficult to see in detail because of surrounding gas and interstellar dust. Smithsonian astronomers announced today, however, a first-time measurement that may help to better trace the structure of these unusual regions. Elizabeth M. L. Humphreys and other Harvard-Smithsonian Center for Astrophysics (CfA) research team members presented the first detection at millimeter and submillimeter wavelengths of extragalactic water maser emission in the core of active galaxy NGC 3079 in their paper at the 207th meeting of the American Astronomical Society in Washington, D. C.
"Detections of water masers at these wavelengths will provide a unique new means of determining the physical conditions near the center of active galactic nuclei (AGN), where supermassive black holes are believed to lie," said Humphreys.
The team measured radiation from compact radio sources known as H2O (water) masers using the Smithsonian Astrophysical Observatory's Submillimeter Array and the James Clerk Maxwell Telescope, both on Mauna Kea, Hawaii. Masers amplify and beam radio-wave emission similar to the way lasers emit light. Masers can occur in nature in interstellar space. In our own galaxy, water molecules near hot, newly formed stars can absorb energy and then emit radio waves with centimeter wavelengths, creating the brightest spectral lines in the radio universe. In active galaxies, it is processes related to black holes rather than stars that heat the molecules.
Over millions of years, an immense hourglass-like bubble of hot gas has emerged from the core of NGC 3079, a spiral galaxy 50 million light-years away in the direction of the constellation Ursa Major. Astronomers have vigorously debated whether the bubble is being shaped by radiation and streams of particles released during a central burst of star formation or processes directly related to the many-million solar-mass black hole at the neck of the hourglass.
"If these masers are present in NGC 3079, they are doubtless present in other active galaxies. Discovery of millimeter and submillimeter water maser emission in an active galactic nucleus, in addition to the long-known centimeter signal, opens new opportunities to map the location of molecular gas in accretion disks and outflows perhaps just a few light-years from a massive black hole," said Lincoln J. Greenhill of CfA.
Previously, Dr. Greenhill and student Paul Kondratko led a team that mapped molecular material that lies in a roughly five-light-year diameter disk around the black hole in NGC 3079, using just the centimeter wavelength maser emission generated by this gas. Detailed comparison of images from the Hubble Space Telescope, Chandra X-ray Observatory, and Very Long Baseline Array of the National Radio Astronomy Observatory demonstrated that the accretion disk and black hole lie at the neck of the hourglass outflow.
"Although we do not yet have the angular resolution to tell if the millimeter water emission originates from the disk or outflow or both, we find that the peak in the emission is quite close to the centimeter emission, and overlap in velocity is key," said Humphreys. It reinforces the suggestion that individual maser components at different wavelengths may exist in overlapping physical regions.
"If we can find H2O masers at different wavelengths in the same volumes of gas as those emitting at centimeter wavelengths, then we can define conditions there such as gas temperature and density much better," said Mark J. Reid of CfA. "If we find them in different volumes as well, then we can trace uncharted regions of the active galactic nucleus, including those closer to the central black hole. This could shed new light on the AGN accretion process that leads to formation of supermassive black holes."
While centimeter emission is the signature for H2O, it has long been known that H2O masers emit strongly at least 10 other wavelengths. These are less well studied because relatively few telescopes can detect the radiation, much of which is blocked by the atmosphere.
"We made the observation using the first imaging interferometer working at submillimeter wavelengths ever, the Submillimeter Array of the Smithsonian Astrophysical Observatory," said James M. Moran of CfA. (The SMA is an 8-element radio interferometer located atop Mauna Kea in Hawaii that also operates at millimeter wavelengths.) "The SMA's imaging capability showed that the millimeter water emission is associated with the central region of NGC 3079, as is the warm dust emission that we also detected."
The Submillimeter Array is a joint venture of the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.