Astronomers on Verge of Seeing Black Hole!

Astronomers Say They Are on Verge of Seeing a Milky Way Black Hole!
By Dennis Overbye, N Y Times Staff Writer, Thursday, November 3, 2005


Astronomers said they had moved one notch closer to seeing the unseeable
today.

Using a worldwide array of radio telescopes to obtain the most detailed
look yet at the center of the Milky Way galaxy, they said that they had
narrowed the size of a mysterious fountain of energy there to less than
half that of the Earth's orbit about the Sun.

The result strengthens the case that the energy is generated by a
massive black hole gobbling stars and gas, they said. And it leaves
astronomers on the verge of seeing the black hole itself as a small dark
shadow ringed with light, in the blaze of radiation that marks the
galaxy's center.

Up until now the existence of black holes -- objects so dense that not
even light can escape them- has been surmised by indirect measurements
of, say stars or gas swirling in their grip. Seeing the black hole's
shadow would require the ability to see about twice as much detail as
can now be discerned. Such an observation could provide an important
test of Albert Einstein's theory of general relativity, his theory of
gravity, which predicts black holes can exist.

"We're getting tantalizingly close to being able to see an unmistakable
signature that would provide the first concrete proof of a supermassive
black hole at a galaxy's center," Zhi-Qiang Shen of the Shanghai
Astronomical Observatory, one of the leaders of an international team of
radio astronomers, said in a news release. Their report appears today in
the journal Nature.

Fred Lo, director of the National Radio Astronomy Observatory in
Charlottesville, Va., said that achieving the extra resolution could
take several years and would probably require new radio telescopes.
"We're not there yet, but in time no question, we will get there." He
added that seeing the shadow, would be "proof of the pudding" that
Einstein was right and that black holes exist.

In an accompanying commentary, Christopher Reynolds of the University of
Maryland wrote that such observations "will herald a new era in probing
the structure and properties of some of the most enigmatic objects in
the universe."

Other experts, however, said it might be difficult, even if the extra
resolution could be achieved, to untangle the detailed properties of the
black hole from its tangled blazing surroundings. In the last few
decades, astronomers have identified thousands of probable black holes.
These include objects billions of times as massive as the Sun at the
centers of galaxies, where, it is theorized, gas and dust swirling
toward doom are heated and erupt with jets of x-rays and radio energy.

But the putative holes are all too far away for astronomers to discern
the signature feature of a black hole, a point of no return, called the
event horizon, in effect, an edge of the observable universe, from which
nothing can return. Instead, the evidence for black holes rests mostly
on showing that to much invisible mass resides in too small a space to
be anything but a black hole.

The center of our own galaxy is about 26,000 light years away, in the
direction of Sagittarius. The new observations conclude that at the
center of the Milky Way, an amount of invisible matter equal to the mass
of four million Suns is crammed into a region no larger than half the
size of Earth's orbit around the Sun, about 90 million miles. That small
size eliminates the most likely alternative explanation of the galactic
center fireworks, a cluster of stars, the radio astronomers said. Such a
dense cluster would collapse in 100 years.

Even more conclusive proof would come from the observation of the black
hole's shadow, which would be about five times the size of the event
horizon and appear about as big as a tennis ball on the moon according
to calculations by Eric Agol of the University of Washington, Heino
Falcke of the Max Planck Institute for Radio Astronomy, in Germany, and
Fulvio Melia of the University of Arizona.

"For most people, seeing is believing," said Dr. Agol, who added that
observations of the shadow could in principle be used to test whether
general relativity is correct in such strange conditions and to measure
how fast the black hole is spinning.

Holland Ford, a Johns Hopkins astronomer who has used the Hubble Space
Telescope to investigate black holes at the centers of galaxies, said,
"It would be very exciting to begin to resolve the black hole's event
horizon."

Martin Rees of Cambridge University in England, who along with Donald
Lynden-Bell first proposed a black hole as the energy source at the
Milky Way's center in 1971, said he was encouraged by this progress. But
he cited studies suggesting that the shadow could be washed out by
radiation or particles in front of the black hole, making definitive
measurements hard.

Moreover, as all the astronomers pointed out, getting to the next level
of detail will require building new radio telescopes that operate at
shorter wavelengths (and high frequencies) than the Very Long Baseline
Array with which the present observations were carried out.

According to the laws of optics, seeing smaller details requires shorter
wavelengths than the present array can handle. Short wavelengths, Dr. Lo
explained, can also best see through the haze of electrons that surround
black holes and scatter radio waves, blurring images. The best hope, he
said, is the Atacama Large Millimeter Array, or ALMA, which is to be
built by an international consortium high in the mountains of Chile.

"It's something I've been working on for 30 years, its been a long
saga," Dr. Lo said, referring to the quest for the black hole. For a
long time he explained, astronomers were peering through a haze, but
"now we're seeing the thing in itself."
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