Long-exposure images of the giant elliptical galaxy M87 by NASA’s
Chandra X-ray Observatory, together with radio observations, have
provided spectacular evidence of repetitive outbursts from the vicinity
of the galaxy’s supermassive black hole. Magnetized rings, bubbles,
plumes and jets ranging in size from a few thousand to a few hundred
thousand light years point to ongoing violent activity for hundreds of
millions of years.
“The hot X-ray emitting gas extending for hundreds of thousands of light
years around M87 reveals a record of episodes of black hole activity,”
said Paul Nulsen of the Harvard-Smithsonian Center for Astrophysics
(CfA) in Cambridge, Mass. and an author of an Astrophysical Journal
paper describing the latest Chandra results. “With these detailed
observations, we are beginning to understand how the central
supermassive black hole transfers enormous amounts of energy over vast
reaches of space.”
M87, located in the middle of the Virgo galaxy cluster, is surrounded by
an extensive atmosphere of multi-million degree Celsius gas. Chandra’s
long-exposure image has allowed astronomers to see in more detail
structures discovered by previous observations with Chandra and other
X-ray telescopes, to discover new features, and to make specific
comparisons with radio images, which trace the presence of high-energy
electrons in a magnetic field.
The picture that emerges is one in which the infall of material toward a
central supermassive black hole produces a magnetized jet of high-energy
particles that blasts away from the vicinity of the black hole at near
the speed of light. As a jet plows into the surrounding gas, a buoyant,
magnetized bubble of high-energy particles is created, and an intense
sound wave rushes ahead of the expanding bubble.
In Chandra’s image of M87, X-rays from the jet dominate the central
region of the galaxy. The jet is thought to be pointed at a small angle
toward the line of sight, out of the plane of the image. Bright arcs
around dark cavities of faint X-ray emission appear to be gas that has
been swept up on rising, buoyant bubbles that were created a few million
years ago (in M87 time — M87 is 50 million light years from Earth).
These bubbles, which rise like hot air from a fire or explosion in the
atmosphere, show up as bright regions in radio images. An alternative
interpretation, presented in the June 1, 2004 issue of Astrophysical
Journal Letters by Hua Feng of Tsinghau University in China and
colleagues, is that the rings are shock waves that surround the jet and
are seen in projection.
An image processed to bring out faint features reveals two circular
rings with radii of 45 thousand and 55 thousand light years,
respectively. These features are likely sound waves produced by earlier
explosions about 10 million and 14 million years ago, respectively. A
very faint arc at an even larger distance has a probable age of 100
million years.
Spectacular, curved X-ray plumes extending from the upper left to the
lower right illustrate in dramatic fashion how the central black hole
can affect the galaxy and its environment over huge distances. The arm
on the upper left extends more than 75 thousand light years, and the one
on the lower right more than 100 thousand light years from the center of
the galaxy. These features are thought to be gas carried out from the
center of the galaxy on buoyant bubbles created by outbursts tens of
millions of years ago.
A growing body of evidence from other galaxy clusters suggests that
episodic outbursts from supermassive black holes in giant, centrally
located galaxies are a common feature. These outbursts, which produce
magnetized jets and bubbles of high energy particles, along with mammoth
sound waves, could be due to the self-regulated inflow of gas into the
black hole — gas around the black hole cools and flows inward to feed
the black hole, producing an outburst which shuts down the inflow for a
few million years, at which point the cycle begins again. Or, the cause
could be a much more dramatic event, like the cannibalization of a
smaller galaxy, with the subsequent merger of two supermassive black
holes in the center.
The results from Nulsen’s team, which included William Forman and other
colleagues from the CfA, were based on approximately 40 hours of Chandra
observations with its Advanced CCD Imaging Spectrometer. Andrew Young of
the University of Maryland in College Park, and colleagues, have
published a paper identifying many of the X-ray features in M87 in the
November 10, 2002 issue of The Astrophysical Journal based on a shorter
Chandra observation.
NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for NASA’s Office of Space Science, Washington. Northrop
Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime
development contractor for the observatory. The Smithsonian
Astrophysical Observatory controls science and flight operations from
the Chandra X-ray Center in Cambridge, Mass.
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