Deep in the heart of the Milky Way galaxy
lurks an extraordinary black hole. Astronomers call it "supermassive."
It has been feeding on the core of our galaxy so long, the
hole has accumulated more than a million Suns of mass inside
its pinprick belly.
How
do we know it's there? You can't see a black hole.
It reveals itself whenever an errant star or cloud of gas
meanders too close. Matter falling into the hole is ripped
apart and superheated, emitting bursts of high-energy radiation
just before it disappears over the event horizon. Occasionally
a burp of X-rays emanates from the Milky Way's core, and astronomers
check off another meal.
Matter swirls into a growing supermassive black hole--an artist's
concept. [More]
Today
these burps are seldom, but among astronomers it is widely
thought that the Milky Way's "monster in the middle"
used to be more active--frighteningly so. Paul Martini of
Ohio State University (OSU) explains: "Billions of years
ago, when our galaxy was young, there was more 'food' in the
core—lots more gas and stars for the black hole to consume."
He believes there could have been "a real feeding frenzy"
lighting up the center of the Milky Way like a beacon visible
half-way across the Universe.
Could
this be true?
Finding
out requires traveling back in time--a trick, believe it or
not, astronomers are able to perform. "By looking at
galaxies billions of light years away, we can see them as
they were billions of years ago, " explains Martini. "This
can give us a clue to the state of the Milky Way when it was
young."
So,
in an effort led by OSU astronomy graduate student Jason Eastman,
Martini and colleagues used data from NASA's Chandra X-ray
Observatory to examine 12 clusters of galaxies ranging in
distance from 2.4 to 5.7 billion light years away. Their purpose:
to learn how galactic cores change over time.
What
they saw reminded Eastman of "piranhas in a very well-fed
aquarium." Younger galaxies tended to be very active;
supermassive black holes at their cores were furiously consuming
matter and producing copious X-rays in the process.
Older
galaxies, on the other hand, were relatively calm; the frenzy
was subsiding. "It's not that the black holes were no
longer hungry, " says Eastman, "they were just running
out of things to eat." The ratio of active X-ray cores
in the galaxies they analyzed, younger vs. older, was about
20 to 1.
|
A Chandra X-ray image of one of the clusters of galaxies used
in Eastman et al's study. [More]
|
"The
food, or fuel for a central black hole, is primarily thought
to be interstellar gas, " adds Martini. "It is likely
that an occasional star is also swallowed, but most researchers
agree that clouds of gas are the main fuel source."
Hence
the big picture: When galaxies are young, a black hole forms
at the core. Why? "Because that is the bottom of the
galaxy's gravitational potential well, " answers Martini.
"Gas, stars, even smaller black holes will settle to
the center of the galaxy over time." At first, gas is
abundant, and the black hole feeds greedily, announcing itself
to the cosmos via high-energy X-rays. As time passes, the
core is depleted of gas and feeding subsides. By the time
a galaxy is as old as the Milky Way (10+ billion years), the
central black hole has grown to millions of solar masses,
but only takes an occasional meal. The fish is hungry, but
the water is nearly empty.
A
note to the stars: Beware the Piranha.
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