The galaxy NGC 4395 is shown here in infrared light, captured by NASA's Spitzer Space Telescope. Image credit: NASA/JPL-Caltech. ›Full image and caption
How do you grow a supermassive black hole that is a million to a
billion times the mass of our sun? Astronomers do not know the answer,
but a new study using data from NASA's Wide-field Infrared Survey
Explorer, or WISE, has turned up what might be the cosmic seeds from
which a black hole will sprout. The results are helping scientists piece
together the evolution of supermassive black holes -- powerful objects
that dominate the hearts of all galaxies.
Growing a black hole is not as easy as planting a seed in soil and
adding water. The massive objects are dense collections of matter that
are literally bottomless pits; anything that falls in will never come
out. They come in a range of sizes. The smallest, only a few times
greater in mass than our sun, form from exploding stars. The biggest of
these dark beasts, billions of times the mass of our sun, grow together
with their host galaxies over time, deep in the interiors. But how this
process works is an ongoing mystery.
Researchers using WISE addressed this question by looking for black
holes in smaller, "dwarf" galaxies. These galaxies have not undergone
much change, so they are more pristine than their heavier counterparts.
In some ways, they resemble the types of galaxies that might have
existed when the universe was young, and thus they offer a glimpse into
the nurseries of supermassive black holes.
In this new study, using data of the entire sky taken by WISE in
infrared light, up to hundreds of dwarf galaxies have been discovered in
which buried black holes may be lurking. Infrared light, the kind that
WISE collects, can see through dust, unlike visible light, so it's
better able to find the dusty, hidden black holes. The researchers found
that the dwarf galaxies' black holes may be about 1,000 to 10,000 times
the mass of our sun -- larger than expected for these small galaxies.
"Our findings suggest the original seeds of supermassive black holes
are quite massive themselves," said Shobita Satyapal of George Mason
University, Fairfax, Va. Satyapal is lead author of a paper published in
the March issue of Astrophysical Journal.
Daniel Stern, an astronomer specializing in black holes at NASA's Jet
Propulsion Laboratory, Pasadena, Calif., who was not a part of the new
study, says the research demonstrates the power of an all-sky survey
like WISE to find the rarest black holes. "Though it will take more
research to confirm whether the dwarf galaxies are indeed dominated by
actively feeding black holes, this is exactly what WISE was designed to
do: find interesting objects that stand out from the pack."
The new observations argue against one popular theory of black hole
growth, which holds that the objects bulk up in size through galaxy
collisions. When our universe was young, galaxies were more likely to
crash into others and merge. It is possible the galaxies' black holes
merged too, accumulating more mass. In this scenario, supermassive black
holes grow in size through a series of galaxy mergers.
The discovery of dwarf galaxy black holes that are bigger than
expected suggests that galaxy mergers are not necessary to create big
black holes. Dwarf galaxies don't have a history of galactic smash-ups,
and yet their black holes are already relatively big.
Instead, supermassive black holes might form very early in the
history of the universe. Or, they might grow harmoniously with their
host galaxies, feeding off surrounding gas.
"We still don't know how the monstrous black holes that reside in
galaxy centers formed," said Satyapal. "But finding big black holes in
tiny galaxies shows us that big black holes must somehow have been
created in the early universe, before galaxies collided with other
galaxies."
Other authors of the study include: N.J. Secrest, W. McAlpine and
J.L. Rosenberg of George Mason University; S.L. Ellison of the
University of Victoria, Canada; and J. Fischer of the Naval Research
Laboratory, Washington.
WISE was put into hibernation upon completing its primary mission in
2011. In September 2013, it was reactivated, renamed NEOWISE and
assigned a new mission to assist NASA's efforts to identify the
population of potentially hazardous near-Earth objects. NEOWISE will
also characterize previously known asteroids and comets to better
understand their sizes and compositions.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages and
operates the NEOWISE mission for NASA's Science Mission Directorate. The
WISE mission was selected competitively under NASA's Explorers Program
managed by the agency's Goddard Space Flight Center in Greenbelt, Md.
The science instrument was built by the Space Dynamics Laboratory in
Logan, Utah. The spacecraft was built by Ball Aerospace &
Technologies Corp. in Boulder, Colo. Science operations and data
processing take place at the Infrared Processing and Analysis Center at
the California Institute of Technology in Pasadena. Caltech manages JPL
for NASA.
More information on WISE and NEOWISE can be found online at: http://www.nasa.gov/wise, http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov
