Radio Telescopes Provide Key Clue on Black Hole Growth (Forwarded)

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EMBARGOED For Release: 9:20 a.m., PST, Tuesday, January 9, 2007

Radio Telescopes Provide Key Clue on Black Hole Growth

Astronomers have discovered the strongest evidence yet found indicating
that matter is being ejected by a medium-sized black hole, providing
valuable insight on a process that may have been key to the development of
larger black holes in the early Universe. The scientists combined the
power of all the operational telescopes of the National Science
Foundation's National Radio Astronomy Observatory (NRAO) to peer deep into
the heart of the galaxy NGC 4395, 14 million light-years from Earth in the
direction of the constellation Canes Venatici.

"We are seeing in this relatively nearby galaxy a process that may have
been responsible for building intermediate-mass black holes into
supermassive ones in the early Universe," said Joan Wrobel, an NRAO
scientist in Socorro, NM. Wrobel and Luis Ho of the Observatories of the
Carnegie Institution of Washington in Pasadena, CA, presented their
findings to the American Astronomical Society's meeting in Seattle, WA.

Black holes are concentrations of matter so dense that not even light can
escape their powerful gravitational pull.

The black hole in NGC 4395 is about 400,000 times more massive than the
Sun. This puts it in a rarely-seen intermediate range between the
supermassive black holes at the cores of many galaxies, which have masses
millions to billions of times that of the Sun, and stellar-mass black
holes only a few times more massive than the Sun. Energetic outflows of
matter are common to both the supermassive and the stellar-mass black
holes, but the new radio observations of NGC 4395 provided the first
direct image of such a suspected outflow from an intermediate-mass black
hole.

The outflows presumably are generated by little-understood processes
involving a spinning disk of material being drawn toward the black hole at
the disk's center.

"An outflow from a black hole can regulate its growth by pushing back on
material being drawn toward it. This is an important aspect of black hole
development. Our observations offer new and unique information on how this
process works for intermediate-mass black holes," Ho said.

"Intermediate-mass black holes may have been the starting points for the
supermassive black holes that we now see throughout the Universe. By
studying this contemporary analog to those earlier objects, we hope to
learn how the less-massive ones grew into the more-massive ones," Wrobel
explained.

The black hole in NGC 4395 was added to a small number of known
intermediate-mass black holes in 2005, when a research team led by Brad
Peterson of the Ohio State University calculated its mass based on
ultraviolet observations. Other ultraviolet and X-ray observations gave
tantalizing hints that material might be flowing outward from the black
hole.

"Fortunately, this object also is detectable by radio telescopes, so we
could use very high precision radio observing techniques to make extremely
detailed images," Wrobel said. Wrobel and Ho used a technique called Very
Long Baseline Interferometry (VLBI), in which multiple radio-telescope
antennas are used together to simulate a much larger "virtual telescope,"
providing extremely great resolving power, or ability to see fine detail.

The astronomers used all of NRAO's telescopes in their coordinated VLBI
array, including the continent-wide Very Long Baseline Array (VLBA), the
27-antenna Very Large Array (VLA) in New Mexico, and the giant Robert C.
Byrd Green Bank Telescope (GBT) in West Virginia. The combination of
antennas spread far apart as well as the large amount of signal-collecting
area in this system allowed the scientists to make a detailed image of the
faint radio emission caused by fast-moving electrons in the suspected
outflow from the black hole interacting with magnetic fields.

The resulting image showed the suspected outflow stretching approximately
one light-year from the black hole. "This direct image bolsters the case
for an outflow that was suggested by the earlier indirect evidence from
the ultraviolet and X-ray observations," Wrobel said.

"By measuring the length of this suspected outflow, we offer a unique
constraint on theoretical models for how intermediate-mass black holes
operate," Ho said.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

IMAGE CAPTIONS:

[Image 1:
http://www.nrao.edu/pr/2007/imbh/ngc4395.hsa.new.jpg (90KB)]
VLBI image of extended radio emission from core of NGC 4395, indicating
suspected outflow powered by black hole. CREDIT: Wrobel & Ho, NRAO/AUI/NSF

[Image 2:
NGC_4395_UGC_7524_IRAS_12233+3348_irg.jpg (198KB)]
Optical (visible light) image of NGC 4395. CREDIT: David W. Hogg, Michael
R. Blanton, and the Sloan Digital Sky Survey Collaboration