Credit: NASA, ESA, E. Meyer, W. Sparks, J. Biretta, J. Anderson, S.T. Sohn, and R. van der Marel (STScI), C. Norman (Johns Hopkins University), and M. Nakamura (Academia Sinica)
Magnetic Funnel Around a Supermassive Black Hole
More than thirteen years of observations from NASA's Hubble Space
Telescope have allowed astronomers to assemble time-lapse movies of a
5,000-light-year-long jet of superheated gas being ejected from a
supermassive black hole in the center of the giant elliptical galaxy
M87.
The movies promise to give astronomers a better understanding of how
active black holes shape galaxy evolution. While matter drawn
completely into a black hole cannot escape its enormous gravitational
pull, most infalling material drawn toward it first joins an orbiting
region known as an accretion disk encircling the black hole. Magnetic
fields surrounding the black hole are thought to entrain some of this
ionized gas, ejecting it as very high-velocity jets.
"Central supermassive black holes are a key component in all big
galaxies," said Eileen T. Meyer of the Space Telescope Science
Institute (STScI) in Baltimore, Md., the Hubble study's lead author.
"Most of these black holes are believed to have gone through an active
phase, and black-hole-powered jets from this active phase play a key
role in the evolution of galaxies. By studying the details of this
process in the nearest galaxy with an optical jet, we can hope to learn
more about galaxy formation and black hole physics in general."
The Hubble movies reveal for the first time that the jet's river of
plasma travels in a spiral motion. This motion is considered strong
evidence that the plasma may be traveling along a magnetic field, which
the team thinks is coiled like a helix. The magnetic field is believed
to arise from a spinning accretion disk of material around a black
hole. Although the magnetic field cannot be seen, its presence is
inferred by the confinement of the jet along a narrow cone emanating
from the black hole.
"We analyzed several years' worth of Hubble data of a relatively
nearby jet, which allowed us to see lots of details," Meyer said. "The
only reason you see the distant jet in motion at all over just a few
years is because it is traveling very fast."
Meyer found evidence for the magnetic field's suspected helical
structure in several locations along the jet. In the outer part of the
M87 jet, for example, one bright gas clump, called knot B, appears to
zigzag, as if it were moving along a spiral path. Several other gas
clumps along the jet also appear to loop around an invisible structure.
"Past observations of black hole jets couldn't distinguish between
radial motion and side-to-side motion, so they didn't provide us with
detailed information of the jet's behavior," Meyer explained.
M87 resides at the center of the neighboring Virgo cluster of roughly
2,000 galaxies, located 50 million light-years away. The galaxy's
monster black hole is several billion times more massive than our Sun.
In addition, the Hubble data provided information on why the jet is
composed of a long string of gas blobs, which appear to brighten and
dim over time.
"The jet structure is very clumpy. Is this a ballistic effect, like
cannonballs fired sequentially from a cannon?" Meyer asked. "Or, is
there some particularly interesting physics going on, such as a shock
that is magnetically driven?"
Meyer's team found evidence for both scenarios. "We found things that
move quickly," Meyer said. "We found things that move slowly. And, we
found things that are stationary. This study shows us that the clumps
are very dynamic sources."
The research team spent eight months analyzing 400 observations from
Hubble's Wide Field Planetary Camera 2 and Advanced Camera for Surveys.
The observations were taken from 1995 to 2008. Several team members,
however, have been observing M87 for 20 years. Only Hubble's sharp
vision allowed the research team to measure the jet's slight motion in
the sky over 13 years. Meyer's team also measured features in the hot
plasma as small as 20 light-years wide.
It's too soon to tell whether all black-hole-powered jets behave like
the one in M87. That's why Meyer plans to use Hubble to study three
more jets. "It's always dangerous to have exactly one example because
it could be a strange outlier," Meyer said. "The M87 black hole is
justification for looking at more jets."
The team's results will appear Aug. 22 in the online issue of The Astrophysical Journal Letters.
In addition to Eileen Meyer, other members of the science team are
William Sparks, John Biretta, Jay Anderson, Sangmo Tony Sohn, and
Roeland van der Marel of STScI; Colin Norman of Johns Hopkins
University, Baltimore, Md.; and Masanori Nakamura of Academia Sinica,
Taipei, Taiwan.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Eileen Meyer
Space Telescope Science Institute, Baltimore, Md.
410-516-5008
meyer@stsci.edu
