A sideswiping near-collision between two spiral galaxies in the constellation of the great dog has left the swirling participants visibly rumpled and bent out of shape, a team of astronomers announced Thursday.
The aftermath of the galactic near-miss is shown by an image from the Hubble Space Telescope. The picture shows two galaxies that appear to be colliding. In fact, the two are moving apart after grazing within 100,000 light-years of one another. Viewed from Earth, the closest approach would have occurred 40 million years ago.
The close encounter has left both galaxies distorted by their mutual gravitational pulls, according to Debra Elmegreen an astronomer at Vassar College in Poughkeepsie, New York. Elmegreen leads the team that used Hubble to zoom in on the two galaxies.
Scientists believe the two disk-shaped galaxies are locked in each other's orbits, slowly falling ever closer together. Elmegreen calculates that the galaxies swipe past one another every few hundred million years.
"They probably formed in a similar region. They've just been doing a little dance for a while and eventually we expect that they'll merge, but they have a few more orbits around each other before that happens," Elmegreen said.
The pair is found about 114 million light-years away from Earth in the direction of the constellation Canis Major. That constellation can be seen following Orion the hunter across the nighttime sky.
The larger of the two objects (seen at the right side of the Hubble image) is called NGC 2207. It is about 25 percent more massive than the Milky Way, and some 140,000 light years across. The smaller galaxy, known as IC 2163 (on the left side of the Hubble image), is about two-thirds as massive as the Milky Way and about 100,000 light years across, Elmegreen said.
At closest approach, the two galaxies were oriented perpendicular to each other, somewhat like two Frisbees forming a T-shape, except that the disks would have been separated by a distance roughly equal to the diameter of one of the Frisbees. The tidal forces on the smaller IC 2163 acted to stretch out the galaxy's spiral arms -- opening them up and smearing dust and gas from a tight spiral-arm into more of a fan shape.
This force is what made the galaxy look more like an eye, than a typical spiral, Elmegreen said.
With the larger NGC 2207, the gravitational tidal force was exerted perpendicular to the plane of the galaxy, Elmegreen said. "So rather than forming long tidal arms, it had distortions that caused the gas to move very rapidly above and below the disk," she said.
For the past decade, Elmegreen and her colleagues have been looking for galaxies that are, in her words, "gently interacting."
Scientists understand a good deal about how stars form in isolated galaxies, and in recent years have been investigating the process in regions where galaxies collide. Violently-colliding galaxies appear to be hotbeds of star formation, creating large clumps of young stars called globular clusters.
Wondering how star-formation is affected by galactic interactions on the tame side of full-scale collisions, Elmegreen's team has searched for pairs like NGC 2207 and IC 2163, and examined the age and distribution of stars.
What the team has determined is that star-formation in these interacting galaxies is about the same as it is in isolated galaxies.
"The star-forming regions -- we've looked at a couple hundred different clusters in each of these galaxies -- and they look like they form at about the same rate and about the same proportion of high and low-mass ones as in normal disks," Elmegreen said.
The conclusion:
"You can shake gas clouds in different ways, but they still end up doing the same things in terms of star formation," she said.
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