Cornell Astronomers Ask: What Happens When Worlds Collide?

Our home in the Milky Way Galaxy -- a collection of gas, dust, and hundreds of billions of stars -- lies about two million light years away from the the Andromeda Galaxy, which is about the same size and shape as the Milky Way. Current measurements suggest that, in about five billion years, the Milky Way and Andromeda galaxies will collide.

When galaxies collide, what happens to matter that gets spun off in the collision's wake? The stars that comprise the major portion of the luminous mass of each of the two galaxies will almost never collide with each other, but will pass rather freely between each other with little damage. This occurs because the physical size of individual stars is tiny compared to their typical separations, making the chance of physical encounter relatively small. The space between our Sun and our nearest stellar neighbor, Proxima Centauri (part of the Alpha Centauri triple system), for example is a vast 4.3 light-years.

However, the situation is quite different for the interstellar matter -- material consisting largely of clouds of tiny particles of debris and dust that is strongly coupled to the interstellar gas. Wherever the interstellar clouds of the two galaxies collide, they do not freely move past each other, but rather suffer a damaging collision. High relative velocities cause ram pressures at the surface of contact between the interacting interstellar clouds. This pressure, in turn, produces material densities sufficiently extreme as to trigger star formation.

With help from the Spitzer Space Telescope's infrared spectrograph, Cornell astronomers are beginning to piece together an answer to the question “What happens when worlds collide?” 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.

"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."

For More Information:

http://www.news.cornell.edu/stories/Nov05/Higdon.Spitzer.lg.html