ASTRONOMERS FIND DYING QUASARS IN NEARBY GALAXIES
Astronomers are announcing today that they have found dying quasars in the nuclei of nearby galaxies. The discovery is being presented to the American Astronomical Society meeting in Atlanta, GA, by Neil M. Nagar, Andrew S. Wilson (University of Maryland, College Park), Heino Falcke (Max Institute fur Radioastronomie, Bonn, Germany), Luis Ho (Observatories of the Carnegie Institution of Washington, Pasadena, CA) and James S. Ulvestad (National Radio Astronomy Observatory, Socorro, NM). The result is of particular interest since quasars are believed to be supermassive (many millions to billions times the mass of the sun) black holes which are accreting (swallowing) gas from the surrounding galaxy. The new work thus indicates that such supermassive holes exist in the nuclei of nearby galaxies and are still swallowing gas, but at a much lower rate than occurs in quasars.
The new observations were made at radio wavelengths with the Very Large Array and the Very Long Baseline Array of the National Radio Astronomy Observatory, funded by the National Science Foundation. Very compact (smaller than a fraction of a light year) radio emission has long been known to be a characteristic of quasars, the active nuclei of distant galaxies, seen when the universe was only a fraction of its present age. The radio emission is generated by the synchrotron process, in which electrons (and possibly positrons) moving at speeds close to that of light gyrate around magnetic field lines. At the high electron densities in these radio sources, the electrons can reabsorb their own synchrotron radiation. This reabsorption gives a characteristic spectral signature to the radio emission, in which the intensity of radio emission stays roughly constant as a function of frequency (a ``flat spectrum''), unlike the declining intensity towards higher frequencies normally seen from synchrotron radiation (a ``steep spectrum'').
In the 1980's and 1990's, sensitive optical spectra of nearby galaxies found signatures of ionized gas in the nuclei of almost half of all nearby galaxies. Two explanations were suggested. The first has it that the gas is ionized by ultraviolet radiation from hot stars. The second proposes that the ionizing radiation comes from a black hole accreting surrounding gas. This ambiguity cannot be resolved with optical observations alone.
In the work reported today, the astronomers first observed about 100 of these nearby galaxies with nuclear ionized gas on 2 cm radio wavelengths with the Very Large Array. They found that the nuclei of at least 30% of the observed galaxies show the very compact, ``flat spectrum'' type of radio emission previously seen in quasars, though the radio power of the nearby galaxies is much lower than the quasars. The astronomers then observed the stronger radio nuclei in these galaxies on 6 cm wavelength with the Very Long Baseline Array, which provides much finer resolution than the Very Large Array. The size of the radio source in the nuclei of all the galaxies observed was found to be tiny (a fraction of a light year), but very bright, like quasars, but completely unlike the radio emission of galaxies with gas ionized by radiation from stars. Nagar and his colleagues thus adduce that these galaxies contain a supermassive black hole, slowly accreting gas from the surrounding galaxy. This process is the same as that believed to power quasars, but at only a fraction of the accretion rate.
Neil Nagar, of the University of Maryland, said that ``an accreting supermassive black hole is the only object we know of that can generate these extremely bright, but very small, radio sources''. He added that ``these nuclear radio sources were not seen before because previous radio observations did not have the resolution and sensitivity to pick them out''. Andrew Wilson, also of the University of Maryland, said ``we thought we might see a few of these bright, tiny radio sources, but instead we saw them in all the galaxies we observed with the Very Long Baseline Array - that was a big surprise!''
The exact process by which the fast-moving electrons and magnetic fields, responsible for the radio emission, are produced is still unclear. In one model, the radio source is the base of a ``jet'' of particles spewed out by the black hole - accretion disk combination. Indeed, in four galaxies, the images with the Very Long Baseline Array seem to show such ``jets'' (see Figure). In another model, the accretion disk itself is so hot in its center that the electrons move at speeds close to that of light and radiate synchrotron radio emission. Adds Heino Falcke, an astronomer at the Max Planck Institut fur Radioastronomie, ``the next step is to distinguish the two models''.
The Very Long Baseline Array, used to make these observations, consists of 10 radio dishes, each 25 meters (80 feet) in diameter. Eight of the dishes are spread over the continental United States, another is on the big island of Hawaii, and the last in the Virgin Islands. The signals from all ten dishes are combined electronically to make radio images with a resolution of about a thousandth of an arc second. This is equivalent to seeing Lincoln's head on a penny from a distance of 500 miles. More information on the Very Long Baseline Array is available on the Internet at http://www.nrao.edu.
This research was supported by the Division of Extragalactic Astronomy of the National Science Foundation.
For further information, contact:
Neil M. Nagar, (301) 405 1554, neil@astro.umd.edu, or Andrew S. Wilson, (301) 405 1519, wilson@astro.umd.edu
PHOTO CREDIT: Neil M. Nagar, Andrew S. Wilson (University of Maryland), Heino Falcke (Max Planck Institut fur Radioastronomie, Bonn, Germany) and James S. Ulvestad (National Radio Astronomy Observatory).
The Figure shows the radio images of four galaxies - Messier 84, Messier 89, NGC 4278 and NGC 6500 - made with the Very Long Baseline Array of the National Radio Astronomy Observatory. The bar at the lower left of each image is 3.3 light years (1 parsec) in length. All four galaxies show a bright nucleus plus extended radio emission, indicative of plasma jets being ejected from the nucleus.
This material was presented to the American Astronomical Society meeting in Atlanta, GA on January 13, 2000. These images are available at: