Andrew Yee[_1_]
June 13th 07, 06:09 PM
News Service
Stanford University
Stanford, Caifornia
Contact:
Dawn Levy, News Service
(650) 725-1944
Comment:
Stelios Kazantzidis, Kavli Institute for Particle Astrophysics and Cosmology
(650) 926-4786
June 6, 2007
Scientists reveal how supermassive black holes bind into pairs during galaxy
mergers
By Dawn Levy
Picture the Milky Way galaxy -- a disk of stars and gas, a stellar spheroid
and an enormous halo of dark matter. It spirals around a black hole that is
supermassive -- about three million solar masses. The Milky Way's total mass
is about 100 billion solar masses -- enormous to us but average among
galaxies.
Then imagine that galaxy encountering its identical twin. The first galaxy
merges with the second to produce a galaxy that's even grander and greater.
Cosmologists think that's how galaxies grow -- through a complex process of
continuous mergers.
Now, using supercomputers to simulate galaxy mergers, scientists at Stanford
and elsewhere have seen the formation of a new type of structure -- a
central disk of gas that can be from a hundred to a few thousand light years
wide and from a few hundred million to a billion solar masses. They report
the first simulated formation of a supermassive black hole (SMBH) pair in
the June 7 edition of Science Express, an online version of Science
magazine.
"The theory of General Relativity that Einstein developed about 90 years ago
which describes the behavior of gravity has been verified in many of its
predictions," says Stanford co-author Stelios Kazantzidis of the Kavli
Institute for Particle Astrophysics and Cosmology. "However, there is one
major consequence of this theory that has yet escaped verification, and this
is the existence of gravitational waves. Due to the fact that coalescing
SMBH binaries constitute the most powerful sources of gravitational-wave
emission in the universe, it is of primary importance to establish the
necessary conditions leading to the merger of two SMBHs."
Kazantzidis and Lucio Mayer from ETH Zurich (the Swiss Federal Institute of
Technology) and the University of Zurich, the main authors, conceived the
project and designed and conducted the numerical simulations. Monica Colpi
from the University Milano-Bicocca and Piero Madau from the University of
California-Santa Cruz helped interpret the results. Thomas Quinn from the
University of Washington and James Wadsley from McMaster University wrote
the code used to perform the simulations.
A SMBH binary consists of two SMBHs orbiting around their center of mass.
The black holes may or may not merge depending on the existence of a
mechanism that can extract angular momentum from their orbit and reduce
their relative separation, Kazantzidis said.
"A pair of SMBHs may interact with the stars around them or with the gas,"
he said. "Both the gas and the stars exert a frictional force onto the black
holes. This frictional force extracts energy from the orbital motion of the
SMBHs. As a result, the separation between them gradually shrinks." It is
unclear whether it is the friction of the stars or that of the gas that
dominates the process.
To model a galaxy merger, scientists first use a computer program that
builds galaxies based on observations and theoretical predictions. For
example, all galaxies are surrounded by extended, massive dark matter halos,
and spiral galaxies contain a disk of stars. In all self-consistent models
of disk galaxies these two components must be included.
"Once the galaxy models are constructed, we must parameterize the orbits of
involved galaxies and set them on a collision course," Kazantzidis said.
The scientists found that when the merging galaxies contain some gas, their
SMBHs will form a binary system in most cases. Once paired, the black holes
may keep reducing their separation until they are about as far apart as our
solar system's width, Kazantzidis said.
"At this point we predict that they should begin producing strong
gravitational waves," he said. "Since the emission of waves extract energy
from the binary black hole, the two holes will ultimately merge, less than a
billion years after they had initially formed a binary."
This paper reports the first time that a galactic merger is followed through
the formation of a gravitationally bound SMBH binary and down to scales of
only a few light years.
While many earlier authors have investigated the formation of SMBH binaries
using supercomputer simulations of galaxy collisions, Kazantzidis and his
colleagues followed the processes over a wide range of spatial and temporal
scales.
"With very few exceptions, earlier studies did not explore the role of gas
in driving the evolution of a SMBH pair," Kazantzidis added. "Theoretical
and observational evidence do suggest that the central regions of merger
remnants contain significant amounts of gas. Our simulations provide new
insights into how SMBHs form pairs and coalesce, and underscores the vital
role of the gaseous component of galaxies in determining the fate of SMBHs."
Average galaxies such as the Milky Way do not contain such impressive
gaseous nuclear disks, suggesting that mergers are important in forming
these structures, Kazantzidis said. "Our simulations were the first to
report the formation of such nuclear disks in galaxy mergers. It is natural
to assume that the large reservoir of gas present in these nuclear disks
represents the fuel that feeds the central supermassive black holes and
powers the active galactic nucleus."
The results are good news for LISA, or the Laser Interferometer Space
Antenna experiment, a space-based observatory designed to detect
gravitational waves using laser interferometry over astronomical distances.
The LISA detector is ideal for probing merging supermassive black holes such
as those simulated by Kazantzidis and colleagues. A joint venture of NASA
and the European Space Agency, LISA is expected to begin observations around
2015.
"Detecting the gravitational waves from the merging supermassive black holes
will not only be the strongest confirmation of one of the most fundamental
theories of physics (General Relativity) but also a confirmation of our
general scenario for galaxy formation and evolution," Kazantzidis says.
"This is one of the most beautiful examples of the deep connections between
fundamental physics and astrophysics."
Simulations were performed on supercomputers at the Pittsburgh
Supercomputing Center, the University of Zurich and ETH Zurich. "The numbers
of calculations needing to be computed is astronomical," Kazantzidis said.
"This is why we resort to powerful supercomputers capable of performing very
large numbers of calculations per second. Supercomputer simulations allow us
to effectively compress the vast cosmological timescales that apply to the
largest structures in our universe down to weeks of virtual time. This gives
us a completely new view of the universe from what is available to us by
observation alone."
What's more, the supercomputers provide the closest thing to a physics lab
that is available to the astrophysicist. "The numerical simulations we
conducted constitute the most extensive calculations ever performed on this
topic, consuming several months of supercomputing time each at various
supercomputer centers around the world," he said.
While the merger that the scientists simulated -- one where both galaxies
have equal mass -- is less probable than a merger of unequal galaxies, it's
not at all improbable, Kazantzidis said. In fact, it's the fate of our own
Milky Way.
"Our galaxy is in a collision course with its largest neighbor, Andromeda,
due to take place in approximately three billion years time," Kazantzidis
said. "The result of this collision will be the destruction of the disks and
the formation of an elliptical galaxy."
Support for the work came from the U.S. Department of Energy, NASA and the
Alexander von Humboldt Foundation.
-30-
Editor Note:
Photos of galaxies merging are available on the web at
http://newsphotos.stanford.edu/Hole.jpg [172KB]
and
http://newsphotos.stanford.edu/Hole_composite.jpg [169KB]
Relevant Web URLs:
* Kavli Institute for Particle Astrophysics and Cosmology
http://www-group.slac.stanford.edu/kipac/
Stanford University
Stanford, Caifornia
Contact:
Dawn Levy, News Service
(650) 725-1944
Comment:
Stelios Kazantzidis, Kavli Institute for Particle Astrophysics and Cosmology
(650) 926-4786
June 6, 2007
Scientists reveal how supermassive black holes bind into pairs during galaxy
mergers
By Dawn Levy
Picture the Milky Way galaxy -- a disk of stars and gas, a stellar spheroid
and an enormous halo of dark matter. It spirals around a black hole that is
supermassive -- about three million solar masses. The Milky Way's total mass
is about 100 billion solar masses -- enormous to us but average among
galaxies.
Then imagine that galaxy encountering its identical twin. The first galaxy
merges with the second to produce a galaxy that's even grander and greater.
Cosmologists think that's how galaxies grow -- through a complex process of
continuous mergers.
Now, using supercomputers to simulate galaxy mergers, scientists at Stanford
and elsewhere have seen the formation of a new type of structure -- a
central disk of gas that can be from a hundred to a few thousand light years
wide and from a few hundred million to a billion solar masses. They report
the first simulated formation of a supermassive black hole (SMBH) pair in
the June 7 edition of Science Express, an online version of Science
magazine.
"The theory of General Relativity that Einstein developed about 90 years ago
which describes the behavior of gravity has been verified in many of its
predictions," says Stanford co-author Stelios Kazantzidis of the Kavli
Institute for Particle Astrophysics and Cosmology. "However, there is one
major consequence of this theory that has yet escaped verification, and this
is the existence of gravitational waves. Due to the fact that coalescing
SMBH binaries constitute the most powerful sources of gravitational-wave
emission in the universe, it is of primary importance to establish the
necessary conditions leading to the merger of two SMBHs."
Kazantzidis and Lucio Mayer from ETH Zurich (the Swiss Federal Institute of
Technology) and the University of Zurich, the main authors, conceived the
project and designed and conducted the numerical simulations. Monica Colpi
from the University Milano-Bicocca and Piero Madau from the University of
California-Santa Cruz helped interpret the results. Thomas Quinn from the
University of Washington and James Wadsley from McMaster University wrote
the code used to perform the simulations.
A SMBH binary consists of two SMBHs orbiting around their center of mass.
The black holes may or may not merge depending on the existence of a
mechanism that can extract angular momentum from their orbit and reduce
their relative separation, Kazantzidis said.
"A pair of SMBHs may interact with the stars around them or with the gas,"
he said. "Both the gas and the stars exert a frictional force onto the black
holes. This frictional force extracts energy from the orbital motion of the
SMBHs. As a result, the separation between them gradually shrinks." It is
unclear whether it is the friction of the stars or that of the gas that
dominates the process.
To model a galaxy merger, scientists first use a computer program that
builds galaxies based on observations and theoretical predictions. For
example, all galaxies are surrounded by extended, massive dark matter halos,
and spiral galaxies contain a disk of stars. In all self-consistent models
of disk galaxies these two components must be included.
"Once the galaxy models are constructed, we must parameterize the orbits of
involved galaxies and set them on a collision course," Kazantzidis said.
The scientists found that when the merging galaxies contain some gas, their
SMBHs will form a binary system in most cases. Once paired, the black holes
may keep reducing their separation until they are about as far apart as our
solar system's width, Kazantzidis said.
"At this point we predict that they should begin producing strong
gravitational waves," he said. "Since the emission of waves extract energy
from the binary black hole, the two holes will ultimately merge, less than a
billion years after they had initially formed a binary."
This paper reports the first time that a galactic merger is followed through
the formation of a gravitationally bound SMBH binary and down to scales of
only a few light years.
While many earlier authors have investigated the formation of SMBH binaries
using supercomputer simulations of galaxy collisions, Kazantzidis and his
colleagues followed the processes over a wide range of spatial and temporal
scales.
"With very few exceptions, earlier studies did not explore the role of gas
in driving the evolution of a SMBH pair," Kazantzidis added. "Theoretical
and observational evidence do suggest that the central regions of merger
remnants contain significant amounts of gas. Our simulations provide new
insights into how SMBHs form pairs and coalesce, and underscores the vital
role of the gaseous component of galaxies in determining the fate of SMBHs."
Average galaxies such as the Milky Way do not contain such impressive
gaseous nuclear disks, suggesting that mergers are important in forming
these structures, Kazantzidis said. "Our simulations were the first to
report the formation of such nuclear disks in galaxy mergers. It is natural
to assume that the large reservoir of gas present in these nuclear disks
represents the fuel that feeds the central supermassive black holes and
powers the active galactic nucleus."
The results are good news for LISA, or the Laser Interferometer Space
Antenna experiment, a space-based observatory designed to detect
gravitational waves using laser interferometry over astronomical distances.
The LISA detector is ideal for probing merging supermassive black holes such
as those simulated by Kazantzidis and colleagues. A joint venture of NASA
and the European Space Agency, LISA is expected to begin observations around
2015.
"Detecting the gravitational waves from the merging supermassive black holes
will not only be the strongest confirmation of one of the most fundamental
theories of physics (General Relativity) but also a confirmation of our
general scenario for galaxy formation and evolution," Kazantzidis says.
"This is one of the most beautiful examples of the deep connections between
fundamental physics and astrophysics."
Simulations were performed on supercomputers at the Pittsburgh
Supercomputing Center, the University of Zurich and ETH Zurich. "The numbers
of calculations needing to be computed is astronomical," Kazantzidis said.
"This is why we resort to powerful supercomputers capable of performing very
large numbers of calculations per second. Supercomputer simulations allow us
to effectively compress the vast cosmological timescales that apply to the
largest structures in our universe down to weeks of virtual time. This gives
us a completely new view of the universe from what is available to us by
observation alone."
What's more, the supercomputers provide the closest thing to a physics lab
that is available to the astrophysicist. "The numerical simulations we
conducted constitute the most extensive calculations ever performed on this
topic, consuming several months of supercomputing time each at various
supercomputer centers around the world," he said.
While the merger that the scientists simulated -- one where both galaxies
have equal mass -- is less probable than a merger of unequal galaxies, it's
not at all improbable, Kazantzidis said. In fact, it's the fate of our own
Milky Way.
"Our galaxy is in a collision course with its largest neighbor, Andromeda,
due to take place in approximately three billion years time," Kazantzidis
said. "The result of this collision will be the destruction of the disks and
the formation of an elliptical galaxy."
Support for the work came from the U.S. Department of Energy, NASA and the
Alexander von Humboldt Foundation.
-30-
Editor Note:
Photos of galaxies merging are available on the web at
http://newsphotos.stanford.edu/Hole.jpg [172KB]
and
http://newsphotos.stanford.edu/Hole_composite.jpg [169KB]
Relevant Web URLs:
* Kavli Institute for Particle Astrophysics and Cosmology
http://www-group.slac.stanford.edu/kipac/