There is no middle ground when it comes to black holes, which tend
instead to be either petite or gargantuan, a new study suggests.
Black holes are known to exist in two classes: The stellar
variety result from exploded massive stars and typically pack the mass of a
few stars. The super-massive class can weigh millions or billions of stellar
masses and reside at the centers of galaxies. Astronomers have debated for
years whether a middleweight category exists, with evidence several times
suggesting they do but then being refuted.
Now, astronomers have scoured one of the few suspected hiding
spots for medium-sized black holes in a globular cluster, and conclude they are
rare or nonexistent.
"Some theories say that small black holes in globular
clusters should sink down to the center and form a medium-sized one, but our
discovery suggests this isn't true," said Daniel Stern, an astrophysicist
at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and co-author of the
study detailed in the Aug. 20 issue of Astrophysical Journal.
Scientists had thought that medium black holes might
lie hidden among millions of stars in globular clusters, which sit within
galaxies containing hundreds of billions of stars. Such black holes ranging in
size from 1,000 to 10,000 times the mass of the sun should sit inside globular
clusters like scaled-down versions of galactic black holes — at least in
theory.
Previous studies have hinted
at the existence of medium black holes, fingering star clusters with
suspiciously large masses.
To see if there was anything to this, Stern worked with
researchers led by Stephen Zepf, an astronomer at Michigan State University in
East Lansing, to probe a globular cluster located 50 million light-years away
in a neighboring galaxy. (A light-year is the distance light will travel in a year, or
about 6 trillion miles or 10 trillion kilometers.)
They eventually found the X-ray signature of an active black hole
in the globular cluster named RZ2109, using the European Space Agency's
XMM-Newton telescope.
Next, the researchers determined the size of the black hole by
using the W.M. Keck Observatory on Mauna Kea in Hawaii to get the chemical
fingerprint of the globular cluster. Computer simulations of the chemical
analysis revealed high-speed "winds" coming out of the black hole, indicating
that it was baby consuming too much material and spitting some of it out.
"If an intermediate-sized black hole were accreting this
material, it wouldn't be too big of a deal for it," Zepf said. "But
if a small black hole were accreting this material, it would be a lot for it to
take and therefore some material would be ejected in the form of high
winds."
The astronomers estimated that the black hole was relatively tiny
at just 10 times the mass of our sun.
"If a medium black hole existed in a cluster, it would either
swallow little black holes or kick them out of the cluster," Stern
explained.
Zepf suggested that medium-sized black holes might still lie hidden in dwarf
galaxies on the outskirts of larger galaxies such as our Milky Way, but it would
be difficult to track down.
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