Black holes get a bad rap in popular culture for swallowing
everything in their environments. In reality, stars, gas and dust can
orbit black holes for long periods of time, until a major disruption
pushes the material in.
A merger of two galaxies is one such disruption. As the galaxies
combine and their central black holes approach each other, gas and dust
in the vicinity are pushed onto their respective black holes. An
enormous amount of high-energy radiation is released as material spirals
rapidly toward the hungry black hole, which becomes what astronomers
call an active galactic nucleus (AGN).
A study using
NASA's NuSTAR telescope shows that in the late stages of galaxy mergers,
so much gas and dust falls toward a black hole that the extremely
bright AGN is enshrouded. The combined effect of the gravity of the two
galaxies slows the rotational speeds of gas and dust that would
otherwise be orbiting freely. This loss of energy makes the material
fall onto the black hole.
"The further along the merger is, the more enshrouded the AGN will
be," said Claudio Ricci, lead author of the study published in the
Monthly Notices Royal Astronomical Society. "Galaxies that are far along
in the merging process are completely covered in a cocoon of gas and
dust."
Ricci and colleagues observed the penetrating high-energy X-ray
emission from 52 galaxies. About half of them were in the later stages
of merging. Because NuSTAR is very sensitive to detecting the
highest-energy X-rays, it was critical in establishing how much light
escapes the sphere of gas and dust covering an AGN.
The study was published in the Monthly Notices of the Royal Astronomical Society.
Researchers compared NuSTAR observations of the galaxies with data from
NASA's Swift and Chandra and ESA's XMM-Newton observatories, which look
at lower energy components of the X-ray spectrum. If high-energy X-rays
are detected from a galaxy, but low-energy X-rays are not, that is a
sign that an AGN is heavily obscured.
The study helps confirm the longstanding idea that an AGN's black
hole does most of its eating while enshrouded during the late stages of a
merger.
"A supermassive black hole grows rapidly during these mergers," Ricci
said. "The results further our understanding of the mysterious origins
of the relationship between a black hole and its host galaxy."
NuSTAR
is a Small Explorer mission led by Caltech and managed by NASA's Jet
Propulsion Laboratory 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.
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Source: JPL-Caltech/News
