Researchers using the National Science Foundation's Very Large Array (VLA) radio telescope have imaged a "spectacular and complex structure" in giant elliptical galaxy M87, the central galaxy of the Virgo Cluster of Galaxies. Their work both resolves a decades-old observational mystery and revises current theories about the origin of X-ray emission coming from gas surrounding the galaxy.
This new VLA image shows the "central engine", or massive central object, of the galaxy M87, spewing out jets of subatomic particles at nearly the speed of light. The VLA image is the first to show detail of a larger structure that originally was detected by radio astronomers more than a half-century ago. Analysis of the new image indicates that astronomers will have to revise their ideas about the physics of what causes X-ray emission in the cores of many galaxy clusters.
These results were announced on January 7, 1999 by Frazer Owen of the National Radio Astronomy Observatory (NRAO) in Socorro, NM, Jean Eilek of the New Mexico Institute of Mining and Technology (NM Tech) in Socorro, NM, and Namir Kassim of the Naval Research Laboratory in Washington, DC, at the American Astronomical Society's meeting in Austin, TX.
The new observations show two large, bubble-like lobes, more than 200,000 light-years across, that emit radio waves. These lobes, which are intricately detailed, apparently are powered by gravitational energy released from the galaxy's central engine. They think that material is flowing outward from the galaxy's core into these large, bright, radio-emitting 'bubbles.'
The newly-discovered "bubbles" sit inside a region of the galaxy known to be emitting X-rays. Theorists have speculated that this X-ray emission arises when gas that originally was part of the Virgo Cluster of galaxies, cools and falls inwards onto M87 itself, at the center of the cluster. Such "cooling flows" are commonly thought to be responsible for strong X-ray emission in many galaxy clusters.
The new structures found in M87 show that the story is much more complicated. Our knowledge about radio jets suggests that the energy being pumped into this region from the galaxy's central engine exceeds the energy being lost in the X-ray emission. This system is more like a heating flow than a cooling flow. Astronomers have to revise their ideas about the physics of what's going on in regions like this.
M87, discovered by the French astronomer Charles Messier in 1781, is the strongest radio-emitting object in the constellation Virgo. Its jet was described by Lick Observatory astronomer Heber Curtis in 1918 as "a curious straight ray ... apparently connected with the nucleus by a thin line of matter." In 1954, Walter Baade reported that the jet's light is strongly polarized. M87's X-ray emission was discovered in 1966. M87 is the largest of the thousands of galaxies in the Virgo Cluster. The Local Group of galaxies, of which our own Milky Way is one, is part of the Virgo Cluster's outskirts.
The galaxy's radio emissions first were observed by Australian astronomers in 1947, but the radio telescopes of that time were unable to discern much detail. They could, however, show that there is a structure more than 100,000 light-years across.
Subsequent radio images, particularly those made using the sharp radio "vision" of the VLA, were primarily aimed at studying the inner 10,000 light-years or so, and showed great detail in the galaxy's jet. Astronomers even have followed the motions of concentrations of material within the jet over time. These observations, however, did not show much about the larger structure that was seen by earlier radio astronomers, leaving its details largely a mystery.
Giant elliptical galaxy M87 is situated at the dynamical center of the nearby Virgo Cluster of Galaxies. It is one of the strongest radio sources in the sky. Its radio structure has been studied on scales ranging from 50 kpc (about 50,000 light years), the largest extent of the source, down to 0.08 pc (1/4 light year). The latter dimension is still larger than the central massive object, about 100 times larger than the corresponding Schwarzschild diameter of a presumable central black hole - the reason why it is appropriate to be careful with conclusions from these observations on "black hole physics".
NRAO radio image of M87's jet, revealing a lot of detail. This is a radio image of the resulting jet as observed by the space VLBI mission. The VLBI image of the galaxy Virgo A was produced by Bill Junor (NRAO), John Biretta (STScI), Frazer Owen (NRAO), and Mitch Begelman (University of Colorado).
Last Modification: 2 Sep 2001, 13:45 MET