When worlds collide: Cornell astronomers investigate cosmic forcesthat produce new galaxies (Forwarded)

News Service
Cornell University
Ithaca, New York

Media Contact:
Lauren Gold, Cornell News Service
(607) 255-9736

Nov. 30, 2005

When worlds collide: Cornell astronomers investigate cosmic forces that
produce new galaxies

By Lauren Gold

When galaxies collide (as our galaxy, the Milky Way, eventually will
with the nearby Andromeda galaxy), what happens to matter that gets spun
off in the collision's wake?

With help from the Spitzer Space Telescope's infrared spectrograph
(IRS), Cornell astronomers are beginning to piece together an answer to
that question. Specifically, they are gaining new insight into how some
ubiquitous dwarf galaxies form, interact and arrange themselves into new
systems.

Dwarf galaxies, with stellar masses around 0.1 percent that of the Milky
Way, are far more common than their more massive spiral or starburst
counterparts. Some may be primordial remnants of the big bang; but
others -- called tidal dwarfs -- formed later as a result of
gravitational interactions after galactic collisions.

To understand which dwarf galaxies are tidal in origin and how those
galaxies differ from primordial dwarf galaxies, Cornell researcher Sarah
Higdon and her colleagues studied a system called NGC 5291, which is 200
million light years from Earth and stretches a distance roughly four
times the span of the Milky Way. At the system's center are two
colliding galaxies; behind them trail a string of much smaller dwarfs.

The researchers focused on the system because they knew from earlier
analyses that the trailing dwarfs were formed tidally as a result of the
central collision. Until recently, though, they hadn't been able to look
closely enough at the tidal dwarfs to catalog their properties for
comparison with those of similar galaxies.

Spitzer's sharp eye has changed that. Using it to look for compounds
that indicate star-forming activity, Higdon's team found that when it
comes to fostering new star formation, the colliding galaxies at the
system's center are fairly dull. The exciting place to be, they found,
is in the tidal dwarfs at the system's edges.

Specifically, the team found that the tidal dwarfs show strong emission
from organic compounds, found in crude petroleum, burnt toast and (more
relevantly) stellar nurseries, known as PAHs -- for polycyclic aromatic
hydrocarbons. And for the first time, the researchers detected warm
molecular hydrogen -- another indicator of star formation, and one that
has never before been directly measured in tidal dwarf galaxies.

"We know molecular hydrogen is out there. Now we have the sensitivity to
measure it," Higdon said.

Higdon and Cornell colleagues James Higdon and Jason Marshall describe
the features of the NGC 5291 system in a forthcoming issue of the
Astrophysical Journal.

"Nearly everything at some stage interacts," Higdon said. "This is a
part of the puzzle. But we've only just started looking. We don't know
how long lived [the tidal dwarf galaxies] will be, or how many formed
like this."

Next, the team plans to search for new tidal dwarf galaxies using the
Spitzer surveys and compare their properties to the newly cataloged
galaxies in NGC 5291.

The Spitzer telescope is the last of NASA's Great Observatories. The
IRS, one of three science instruments on the observatory, is managed by
NASA's Jet Propulsion Laboratory and was built by Ball Aerospace &
Technologies Corp. under the direction of Cornell professor of astronomy
Jim Houck.

Related Information:

* Spitzer Space Telescope
http://www.spitzer.caltech.edu/
* First detection of PAHs and warm molecular hydrogen in tidal dwarf
galaxies
http://xxx.lanl.gov/abs/astro-ph/0511422
* Cornell News Service's Spitzer page
http://www.news.cornell.edu/releases/SIRTF/

[NOTE: An image supporting this release is available at
http://www.spitzer.caltech.edu/Media...ig05-021.shtml ]