What Happens When Galaxies Collide?

          Astronomers are finding evidence that galaxies may be colliding much more often than they had previously anticipated.  In a recent study, of ultra-luminous infrared galaxies (extremely rare; 1 in a million) astronomer Kirk Borne has found strong visual evidence that about 30 percent of the 123 galaxies he studied within 3 billion light-years of Earth are involved in multiple galactic collisions.  Borne studied ultra-luminous infrared galaxies because it is believed that it takes several colliding galaxies to make them glow 100 to 1,000 times brighter than our milky way.  The glow is caused by a firestorm of star formation.  The high percentage of colliding galaxies was higher than expected and is making astronomers believe that collisions could be more common. (http://www.cnn.com/TECH/space/9911/22/hubble.galaxies/index.html)  Ohio State has also played a role in this discovery.  Astronomers at Ohio State undertook a study of 200 galaxies for over 6 hours each.  The “Ohio State InfraRed Imager and Spectrometer (OSIRIS) has revealed that many more galaxies appear to have had galactic interations than previously observed. The new finding coincide well with computer models that predict how much galactic interaction should have occurred.” (http://www.sciencedaily.com/releases/1999/04/990429055431.htm)

          When two galaxies collide, their stars pass each other, but their interstellar media collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation. The reason stars rarely collide is because the distance between stars is much greater than their size, making the odds of a collision much smaller.The following is what happens when two spiral galaxies collide. At first the encounter begins to disrupt the two galaxies and sends them into orbit around each other. As the collision continues a lot of the gas in the disk is pulled towards the center. Then gravitational forces tear out long streamers of stars called tidal tails. And finally the two centers of the galaxies approach each other and merge. The single galaxy left over from the collision become elliptical. This whole process usually takes 1.5 billion years. Due to the slowness of process there is little hope of observing any of the dynamics directly. Thus, to understand them we must look at the images of the hundreds of suspected collisional galaxies to observe in some detail what we are seeing and to predict how each collision will turn out.  

         A different outcome occurs when a small galaxy passes through a large one. This head-on collision causes the interstellar gas and dust to become condensed, which causes a wave of star formation to move out from the impact point like a ripple across the surface of a pond. Eventually it be comes a Ring galaxy as depicted on the bottom right.

                   Two spiral galaxies merging into an elliptical                                         A ring galaxy

        A study of the elliptical galaxy M86 and spiral galaxy NGC 4438 in the Virgo galaxy cluster by Professor and Chair of astronomy at Yale, Jeffrey Kenney, is showing why some of the biggest galaxies in the universe might stop forming stars.  Kenney believes that gravitational interactions between colliding galaxies might do the trick.  Kenney’s paper said “Low-velocity collisions between small- or medium-sized galaxies often produce an increase in the local star formation rate, but in high-velocity collisions that happen naturally between large galaxies, the energy of the collision can cause the gas to heat up so much that it cannot easily cool and form stars.”  This study suggests a plausible alternative to black holes as an explanation of what turns off star formation in the biggest galaxies.  (http://machineslikeus.com/news/galactic-collisions-may-prevent-stars-forming) Pieter van Dokkum of Yale University has done another study.  It seems that “collisions like these surprisingly spark very little, if any, new star formation. One possible reason is that the galaxy has already used up all its gas forming the stars already there.  Or, at a previous point the central black hole created so much energy that it pushed the gas out of these galaxies," van Dokkum said. "That is what's next on the agenda to figure out." (http://www.space.com/scienceastronomy/051212_mystery_monday.html)

          A new image from the Submillimeter Array captures the moment when galaxy 4C60.70 ripped a stream of material from its neighboring galaxy.  Now, the galaxies have merged and formed a single, massive black hole.  What this is showing is that black holes could have been common in much earlier stages of cosmic evolution than was previously considered possible.  (http://www.sciencedaily.com/releases/2008/10/081016124331.htm)