ST. LOUIS — As if in a cosmic hug, the spiral
arms of some galaxies wrap around themselves more tightly than others. The key
to the bear hug: Galaxies holding heftier black holes at their centers also
have more tightly wound spiral arms, an astronomer announced today.
The finding
gives astronomers a way to weigh so-called supermassive black
holes, which can have masses of millions to billions that of the sun, and
are thought to reside at the centers of galaxies.
"This
is a really easy way to determine the masses of these super-massive black holes
at the centers of galaxies that are very far away," said researcher Marc
Seigar, an astrophysicist at the University of Arkansas at Little Rock.
"This gives us a way to measure the size of these black holes out to
larger distances than ever before, up to 8 billion light-years away."
He
announced the results here at a meeting of the American Astronomical Society
(AAS).
Dark
behemoths
A black
hole is the evolutionary endpoint of a star packing at least 10 to 15 times the
mass of the sun. Once the fat star has burned out, there are no outward forces
to oppose gravity, causing the star to collapse in on itself. The stellar
remnant eventually collapses to a point of zero volume and infinite density
(called a singularity), and nothing, not even light, can escape its clutches.
Astronomers
can't plop hefty black holes onto a cosmic scale; they can't even see them.
Instead, they detect them indirectly and measure a black hole's mass by
observing the orbital speed of nearby stars affected by its tremendous
gravity. But this method is limited by the power of current telescopes.
"Even
with the Hubble Space Telescope, to be able to resolve the stars at the center
of the galaxy, you can only really look at the nearest 40 or so galaxies,"
Seigar said. "That's just maybe 20 million light-years away, something
like that."
Tight
hugs
The results
come from a study of 27 spiral galaxies, including the Milky Way and our
nearest neighbor the Andromeda Galaxy. Galaxies with the smallest black holes
at their centers also showed the loosest "hugs," with angles of up to
43 degrees between the arms and the central bulge.
The
tightest embraces came from galaxies boarding with the biggest black holes,
such as the Andromeda Galaxy (its central black hole weighs about 180 million
solar masses), which had just 7 degrees between the spiral arms and the central
bulges.
Seigar and
his colleagues are not sure why spiral arms wrap more tightly around heftier
supermassive black holes and more loosely for the lightweight supermassive
black holes.
But they
think dark
matter, mysterious matter thought to have played a critical role in the formation
of the first galaxies after the Big Bang, could be the driving force.
"We
think, although this is very speculative, the more concentrated the dark
matter, the larger a black hole you get," Seigar told SPACE.com.
"And also the more concentrated the dark matter the tighter the spiral
arms. So we think it's the dark matter that's driving everything."
(Of the
total matter in the universe, dark matter makes up 85 to 90 percent, and
visible matter (normal matter, baryonic matter) makes up the rest — about 10 to
15 percent. Of the total mass in the universe, about 74 percent is dark energy,
22 percent is dark matter and 4 percent or so is normal matter.)
The
dark-matter connection makes sense, he said: "The more dark matter you
have, the more matter you have, so the more stuff that's there to create first
of all a bulge, and the more mass there is to create a bigger black hole."
Seigar's
future research will aim to show whether dark matter is the driving force.
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