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A new high-energy X-ray image from NASA's Nuclear Spectroscopic
Telescope Array, or NuSTAR, has pinpointed the true monster of a
galactic mashup. The image shows two colliding galaxies, collectively
called Arp 299, located 134 million light-years away. Each of the
galaxies has a supermassive black hole at its heart.
NuSTAR has revealed that the black hole located at the right of the
pair is actively gorging on gas, while its partner is either dormant or
hidden under gas and dust.
The findings are helping researchers understand how the merging of
galaxies can trigger black holes to start feeding, an important step in
the evolution of galaxies.
"When galaxies collide, gas is sloshed around and driven into their
respective nuclei, fueling the growth of black holes and the formation
of stars," said Andrew Ptak of NASA's Goddard Space Flight Center in
Greenbelt, Maryland, lead author of a new study accepted for publication
in the Astrophysical Journal. "We want to understand the mechanisms
that trigger the black holes to turn on and start consuming the gas."
NuSTAR is the first telescope capable of pinpointing where
high-energy X-rays are coming from in the tangled galaxies of Arp 299.
Previous observations from other telescopes, including NASA's Chandra
X-ray Observatory and the European Space Agency's XMM-Newton, which
detect lower-energy X-rays, had indicated the presence of active
supermassive black holes in Arp 299. However, it was not clear from
those data alone if one or both of the black holes was feeding, or
"accreting," a process in which a black hole bulks up in mass as its
gravity drags gas onto it.
The new X-ray data from NuSTAR -- overlaid on a visible-light image
from NASA's Hubble Space Telescope -- show that the black hole on the
right is, in fact, the hungry one. As it feeds on gas, energetic
processes close to the black hole heat electrons and protons to about
hundreds of millions of degrees, creating a superhot plasma, or corona,
that boosts the visible light up to high-energy X-rays. Meanwhile, the
black hole on the left either is "snoozing away," in what is referred to
as a quiescent, or dormant state, or is buried in so much gas and dust
that the high-energy X-rays can't escape.
"Odds are low that both black holes are on at the same time in a
merging pair of galaxies," said Ann Hornschemeier, a co-author of the
study who presented the results Thursday at the annual American
Astronomical Society meeting in Seattle. "When the cores of the galaxies
get closer, however, tidal forces slosh the gas and stars around
vigorously, and, at that point, both black holes may turn on."
NuSTAR is ideally suited to study heavily obscured black holes such
as those in Arp 299. High-energy X-rays can penetrate the thick gas,
whereas lower-energy X-rays and light get blocked.
Ptak said, "Before now, we couldn't pinpoint the real monster in the merger."
NuSTAR is a Small Explorer mission led by the California Institute of
Technology in Pasadena and managed by NASA's Jet Propulsion Laboratory,
also in Pasadena, for NASA's Science Mission Directorate in Washington.
The spacecraft was built by Orbital Sciences Corporation, Dulles,
Virginia. Its instrument was built by a consortium including Caltech;
JPL; the University of California, Berkeley; Columbia University, New
York; NASA's Goddard Space Flight Center, Greenbelt, Maryland; the
Danish Technical University in Denmark; Lawrence Livermore National
Laboratory, Livermore, California; ATK Aerospace Systems, Goleta,
California, and with support from the Italian Space Agency (ASI) Science
NuSTAR's mission operations center is at UC Berkeley, with the ASI
providing its equatorial ground station located at Malindi, Kenya. The
mission's outreach program is based at Sonoma State University, Rohnert
Park, California. NASA's Explorer Program is managed by Goddard. JPL is
managed by Caltech for NASA.
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