ST. LOUIS — The universe's hefty black holes
are known to devour everything within their reach. Now astrophysicists have
found that some of the most massive of these sinkholes use high-energy jets to
stomp out nearby star formation.
The
finding, presented today here at a meeting of the American Astronomical Society
(AAS), solves a long-standing problem in galaxy-formation models.
"The
problem is that when you run these models and compare them to observations,
what you find is that the models over-predict stellar mass in low-mass galaxies
and in high-mass galaxies,"
said researcher Sugata Kaviraj, an astrophysicist at Oxford University in England. "There are too many stars."
To make the
models fit with observations, astronomers previously have relied on two means
for quenching
star formation.
In less
massive galaxies, exploding stars were added as the party crashers, imparting
enough energy to disperse close-knit bundles of gas that fuel star formation.
When a
galaxy beefs up to the equivalent of 10 billion suns, however, supernovae
don't have enough power to throw out star-forming gas. As the mass of a galaxy
increases, the gravitational pull holding onto that gas also soars, making it
tougher to give that gas the boot. And so theorists figured that in the beefier
galaxies, supermassive black holes, thought to reside at the centers of most
galaxies, could take over and stomp out star formation.
As gas
falls into the gravitational clutches of a supermassive black hole, the energy
gets spit out as a pair of laser-like jets from either end of the black hole.
These active black holes, called active galactic nuclei (AGN), have enough
energy to power 10 billion stars like the sun. They also can kick out gas from
hefty galaxies, theory had suggested.
"The
problem was that people used these supernovae and AGN prescriptions to match
the models to the observations," Kaviraj told SPACE.com. "However, there was no
evidence that this is actually how nature works."
To nail
down that evidence, Kaviraj and his colleagues looked at the level of
star-snuffing taking place in so-called post-starburst galaxies, which show
evidence of recent (within 1 billion years) star formation that was abruptly
halted. They used ultraviolet data collected by the orbiting space telescope,
NASA's Galaxy Evolution Explorer, and optical images from the Sloan Digital Sky
Survey.
The team studied
the relationship between galaxy mass and rate of star quenching, which in
simple terms means the number of stars snuffed out in a certain period of time.
If supernovae were the only means of kicking out gas, Kaviraj would expect the
rate of star quenching to decrease as the galaxy mass goes up.
And that's
what the researchers found for galaxies below 10 billion solar masses. Above
this weight, they found the opposite: As mass increased, star quenching also
boosted dramatically.
Kaviraj
said these observations support the AGN/jets explanation in the most massive
galaxies. Once galaxies grow to about 10 billion solar masses, when supernovae
can't kick out gas, active supermassive black holes take over. The black
hole's powerful jets must be kicking out the gas, at least in the
post-starburst galaxies studied, he said.
Astronomers
also have found that galaxies weighing more than 10 billion suns are more
likely to have AGNs than galaxies lighter than 10 billion suns.
While the
current results are based on nearby galaxies, the researchers hope to widen the
scope of the work to include more distant galaxies that date back to the peak
epoch of star formation some 10 billion years ago, when the universe was only
25 percent of its current age.
This
research was funded by a Leverhulme Early-Career Fellowship, a BIPAC fellowship
at the University of Oxford and a Research Fellowship at Worcester College, Oxford.
advertisement
