Image credit: ESO/NASA/JPL-Caltech/STScI
Monster black holes sometimes lurk behind gas and dust, hiding from
the gaze of most telescopes. But they give themselves away when material
they feed on emits high-energy X-rays that NASA's NuSTAR (Nuclear
Spectroscopic Telescope Array) mission can detect. That's how NuSTAR
recently identified two gas-enshrouded supermassive black holes, located
at the centers of nearby galaxies.
"These black holes are relatively close to the Milky Way, but they have remained hidden from us until now," said Ady Annuar, a graduate student at Durham University in the United Kingdom, who presented the results at the American Astronomical Society meeting in Grapevine, Texas. "They're like monsters hiding under your bed."
Both of these black holes are the central engines of what astronomers
call "active galactic nuclei," a class of extremely bright objects that
includes quasars and blazars. Depending on how these galactic nuclei
are oriented and what sort of material surrounds them, they appear very
different when examined with telescopes.
Active galactic nuclei are so bright because particles in the regions
around the black hole get very hot and emit radiation across the full
electromagnetic spectrum -- from low-energy radio waves to high-energy
X-rays. However, most active nuclei are believed to be surrounded by a
doughnut-shaped region of thick gas and dust that obscures the central
regions from certain lines of sight. Both of the active galactic nuclei
that NuSTAR recently studied appear to be oriented such that astronomers
view them edge-on. That means that instead of seeing the bright central
regions, our telescopes primarily see the reflected X-rays from the
doughnut-shaped obscuring material.
"Just as we can't see the sun on a cloudy day, we can't directly see
how bright these active galactic nuclei really are because of all of the
gas and dust surrounding the central engine," said Peter Boorman, a
graduate student at the University of Southampton in the United Kingdom.
Boorman led the study of an active galaxy called IC 3639, which is
170 million light years away. Researchers analyzed NuSTAR data from this
object and compared them with previous observations from NASA's Chandra
X-Ray Observatory and the Japan-led Suzaku satellite. The findings from
NuSTAR, which is more sensitive to higher energy X-rays than these
observatories, confirm the nature of IC 3639 as an active galactic
nucleus. NuSTAR also provided the first precise measurement of how much
material is obscuring the central engine of IC 3639, allowing
researchers to determine how luminous this hidden monster really is.
More surprising is the spiral galaxy that Annuar focused on: NGC
1448. The black hole in its center was only discovered in 2009, even
though it is at the center of one of the nearest large galaxies to our
Milky Way. By "near," astronomers mean NGC 1448 is only 38 million light
years away (one light year is about 6 trillion miles).
Annuar's study discovered that this galaxy also has a thick column of
gas hiding the central black hole, which could be part of a
doughnut-shaped region. X-ray emission from NGC 1448, as seen by NuSTAR
and Chandra, suggests for the first time that, as with IC 3639, there
must be a thick layer of gas and dust hiding the active black hole in
this galaxy from our line of sight.
Researchers also found that NGC 1448 has a large population of young
(just 5 million year old) stars, suggesting that the galaxy produces new
stars at the same time that its black hole feeds on gas and dust.
Researchers used the European Southern Observatory New Technology
Telescope to image NGC 1448 at optical wavelengths, and identified where
exactly in the galaxy the black hole should be. A black hole's location
can be hard to pinpoint because the centers of galaxies are crowded
with stars. Large optical and radio telescopes can help detect light
from around black holes so that astronomers can find their location and
piece together the story of their growth.
"It is exciting to use the power of NuSTAR to get important, unique
information on these beasts, even in our cosmic backyard where they can
be studied in detail," said Daniel Stern, NuSTAR project scientist at
NASA's Jet Propulsion Laboratory, Pasadena, California.
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.
For more information on NuSTAR, visit: http://www.nasa.gov/nustar - http://www.nustar.caltech.edu/
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
Elizabeth.landau@jpl.nasa.gov
Source: JPL-Caltech
