Black Holes Key to Spiral Arm Hugs
The black hole at the center of the Triangulum Galaxy has a mass that is no more than 1,500 times the mass of the sun. Its spiral arms are loosely wound at an angle of 43 degrees.
Credit: NASA/Swift Science Team/Stefan Immler

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 "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.