Hearts of Galaxies Close in for Cosmic Train Wreck

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Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

NEWS RELEASE: 2009-050 March 16, 2009

Hearts of Galaxies Close in for Cosmic Train Wreck

PASADENA, Calif. -- A new image from NASA's Spitzer Space Telescope offers a rare view of
an imminent collision between the cores of two merging galaxies, each powered by a black hole
with millions of times the mass of the sun.

The galactic cores are in a single, tangled galaxy called NGC 6240, located 400-million light
years away in the constellation Ophiuchus. Millions of years ago, each core was the dense center
of its own galaxy before the two galaxies collided and ripped each other apart. Now, these cores
are approaching each other at tremendous speeds and preparing for the final cataclysmic collision.
They will crash into each other in a few million years, a relatively short period on a galactic
timescale.

The spectacular image is online at:
http://www.nasa.gov/mission_pages/spitzer/multimedia/spitzer-20090316.html . It combines
visible light from NASA's Hubble Space Telescope and infrared light from Spitzer. It catches the
two galaxies during a rare, short-lived phase of their evolution, when both cores of the interacting
galaxies are still visible but closing in on each other fast.

"One of the most exciting things about the image is that this object is unique," said Stephanie
Bush of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a
new paper describing the observation in an upcoming issue of the Astrophysical Journal.
"Merging is a quick process, especially when you get to the train wreck that is happening. There
just aren't many galactic mergers at this stage in the nearby universe."

NGC 6240 is already putting out huge amounts of infrared light, an indication that a burst of star
formation is underway. The extra infrared radiation is common in interacting galaxies; as the two
galaxies interact, dust and gas swept up by the collision form a burst of new stars that give off
infrared light. Such galaxies are called luminous infrared galaxies. Spitzer's infrared array
camera can image the extra heat from newly formed stars, even though their visible light is
obscured by thick dust clouds around them.

The blob-like shape of the galaxy is due to the sustained violence of the collision. Streams of
millions of stars are being ripped off the galaxy, forming wispy "tidal tails" that lead off NGC
6240 in several directions. But things are about to get even more violent as the main event
approaches and the two galactic cores meld into one.

In the center of NGC 6240, the two black holes in the cores will whip up a frenzy of radiation as
they careen towards one another head-on, likely transforming the galaxy into a monster known as
an ultra-luminous infrared galaxy, thousands of times as bright in infrared as our Milky Way.

Another fascinating aspect of this rare object is that no two galactic mergers are the same. "Not
only are there few objects at this stage, but each object is unique because it came from different
progenitor galaxies," said Bush. "These observations give us another layer of information about
this galaxy, and galactic mergers in general."

Infrared light taken by Spitzer's infrared array camera at 3.6 and 8.8 microns (red) shows cold
dust and radiation from star formation; visible light from Hubble (green and blue) shows hot gas
and stars.
Other authors of this paper include Zhong Wang, Margarita Karovska and Giovanni Fazio, all of
the Harvard-Smithsonian Center for Astrophysics.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope
mission for NASA's Science Mission Directorate, Washington. Science operations are conducted
at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech
manages JPL for NASA.

More information about Spitzer is at http://www.spitzer.caltech.edu/spitzer and
http://www.nasa.gov/spitzer .

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