Powerful new
computer simulations are allowing astronomers to understand how spiral
arms in galaxies form and survive. These simulations suggest that the
arms arise as a result of the influence of giant molecular clouds - star
forming regions or nurseries common in galaxies. Introduced into the
simulation, the clouds act as "perturbers" and are enough to not only
initiate the formation of spiral arms but to sustain them indefinitely.
In this frame from one such simulation, more than 100 million "stellar
particles" form the familiar shape of a spiral galaxy. The full
animation has been posted online.
Credit: Thiago Ize & Chris Johnson (Scientific Computing and Imaging Institute). Low Resolution Image (jpg)
This Hubble Space
Telescope photo of Messier 74 reminds us that spiral galaxies are some
of the most beautiful and photogenic residents of the universe. Nearly
70 percent of the galaxies closest to the Milky Way are spirals. New
research finds that spiral arms are self-perpetuating, persistent, and
surprisingly long lived.
Credit: NASA/ESA/Hubble Heritage Team. Low Resolution Image (jpg)
Cambridge, MA - Spiral
galaxies are some of the most beautiful and photogenic residents of the
universe. Our own Milky Way is a spiral. Our solar system and Earth
reside somewhere near one of its filamentous arms. And nearly 70 percent
of the galaxies closest to the Milky Way are spirals.
But despite their common shape, how galaxies like ours get and maintain
their characteristic arms has proved to be an enduring puzzle in
astrophysics. How do the arms of spiral galaxies arise? Do they change
or come and go over time?
The answers to these and other questions are now coming into focus as
researchers capitalize on powerful new computer simulations to follow
the motions of as many as 100 million "stellar particles" as gravity and
other astrophysical forces sculpt them into familiar galactic shapes. A
team of researchers from the University of Wisconsin-Madison and the
Harvard-Smithsonian Center for Astrophysics reports simulations that
seem to resolve long-standing questions about the origin and life
history of spiral arms in disk galaxies.
"We show for the first time that stellar spiral arms are not transient
features, as claimed for several decades," says UW-Madison
astrophysicist Elena D'Onghia, who led the new research along with
Harvard colleagues Mark Vogelsberger and Lars Hernquist.
"The spiral arms are self-perpetuating, persistent, and surprisingly long lived," adds Vogelsberger.
The origin and fate of the emblematic spiral arms in disk galaxies have
been debated by astrophysicists for decades, with two theories
predominating. One holds that the arms come and go over time. A second
and widely held theory is that the material that makes up the arms -
stars, gas and dust - is affected by differences in gravity and jams up,
like cars at rush hour, sustaining the arms for long periods.
The new results fall somewhere in between the two theories and suggest
that the arms arise in the first place as a result of the influence of
giant molecular clouds - star forming regions or nurseries common in
galaxies. Introduced into the simulation, the clouds act as "perturbers"
and are enough to not only initiate the formation of spiral arms but to
sustain them indefinitely.
"We find they are forming spiral arms," explains D'Onghia. "Past theory
held the arms would go away with the perturbations removed, but we see
that (once formed) the arms self-perpetuate, even when the perturbations
are removed. It proves that once the arms are generated through these
clouds, they can exist on their own through (the influence of) gravity,
even in the extreme when the perturbations are no longer there."
The new study modeled stand-alone disk galaxies, those not influenced by
another nearby galaxy or object. Some recent studies have explored the
likelihood that spiral galaxies with a close neighbor (a nearby dwarf
galaxy, for example) get their arms as gravity from the satellite galaxy
pulls on the disk of its neighbor.
According to Vogelsberger and Hernquist, the new simulations can be used
to reinterpret observational data, looking at both the high-density
molecular clouds as well as gravitationally induced "holes" in space as
the mechanisms that drive the formation of the characteristic arms of
spiral galaxies.
The team's research was published in the March 20 issue of The Astrophysical Journal and is available online.
This release is being issued jointly with UW-Madison.
Headquartered in Cambridge, Mass., the
Harvard-Smithsonian Center for Astrophysics (CfA) is a joint
collaboration between the Smithsonian Astrophysical Observatory and the
Harvard College Observatory. CfA scientists, organized into six research
divisions, study the origin, evolution and ultimate fate of the
universe.
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
Terry Devitt
Director of Research Communications
University of Wisconsin-Madison
608-262-8282
trdevitt@wisc.edu
