The blue dots in this field of galaxies, known as the COSMOS field, show
galaxies that contain supermassive black holes emitting high-energy
X-rays.Credit: NASA/JPL-Caltech
Supermassive black holes in the universe are like a raucous choir
singing in the language of X-rays. When black holes pull in surrounding
matter, they let out powerful X-ray bursts. This song of X-rays, coming
from a chorus of millions of black holes, fills the entire sky -- a
phenomenon astronomers call the cosmic X-ray background.
NASA's Chandra mission has managed to pinpoint many of the so-called
active black holes contributing to this X-ray background, but the ones
that let out high-energy X-rays -- those with the highest-pitched
"voices" -- have remained elusive.
New data from NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR,
have, for the first time, begun to pinpoint large numbers of the black
holes belting out the high-energy X-rays. Or, in astronomer-speak,
NuSTAR has made significant progress in resolving the high-energy X-ray
background.
"We've gone from resolving just two percent of the high-energy X-ray
background to 35 percent," said Fiona Harrison, the principal
investigator of NuSTAR at Caltech in Pasadena and lead author of a new
study describing the findings in an upcoming issue of The Astrophysical
Journal. "We can see the most obscured black holes, hidden in thick gas
and dust."
The results will ultimately help astronomers understand how the feeding
patterns of supermassive black holes change over time. This is a key
factor in the growth of not only black holes, but also the galaxies that
host them. The supermassive black hole at the center of our Milky Way
galaxy is dormant now, but at some point in the past, it too would have
siphoned gas and bulked up in size.
As black holes grow, their intense gravity pulls matter toward them.
The matter heats up to scorching temperatures, and particles get boosted
to close to the speed of light. Together, these processes make the
black hole surroundings glow with X-rays. A supermassive black hole with
a copious supply of fuel, or gas, will give off more high-energy
X-rays.
NuSTAR is the first telescope capable of focusing these high-energy X-rays into sharp pictures.
"Before NuSTAR, the X-ray background in high energies was just one blur with no resolved sources," said Harrison. "To untangle what's going on, you have to pinpoint and count up the individual sources of the X-rays."
"We knew this cosmic choir had a strong high-pitched component, but we
still don't know if it comes from a lot of smaller, quiet singers, or a
few with loud voices," said co-author Daniel Stern, the project
scientist for NuSTAR at NASA's Jet Propulsion Laboratory in Pasadena,
California. "Now, thanks to NuSTAR, we're gaining a better understanding
of the black holes and starting to address these questions."
High-energy X-rays can reveal what lies around the most buried
supermassive black holes, which are otherwise hard to see. In the same
way that medical X-rays can travel through your skin to reveal pictures
of bones, NuSTAR can see through the gas and dust around black holes, to
get a deeper view of what's going on inside.
With NuSTAR's more complete picture of the supermassive black hole
populations, astronomers can begin to puzzle together how they evolve
and change over time. When did they start and stop feeding? What is the
distribution of the gas and dust that both feed and hide the black
holes?
The team expects to resolve more of the high-energy X-ray background over time with NuSTAR -- and better decipher the X-ray voices of our universe's rowdiest choir.
NuSTAR is a Small Explorer mission led by Caltech and managed by JPL
for NASA's Science Mission Directorate in Washington. NuSTAR was
developed in partnership with the Danish Technical University and the
Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences
Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC
Berkeley, and the official data archive is at NASA's High Energy
Astrophysics Science Archive Research Center. ASI provides the mission's
ground station and a mirror archive. JPL is managed by Caltech for
NASA.
