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When Galaxies Collide

Under the guidance of Dr. Roger Lynds of the National Optical Astronomy Observatory, the Hubble Space Telescope has taken a picture of what appears to be the aftermath of a rare direct physical collision between two galaxies.

As a result of the tremendous pressures created in this awesome collision, a burst of new star formation has occurred. These stars appear to be outrunning local interstellar gas clouds, and eventually they should form their own galaxy.

NGC6745, The aftermath of two galaxies that have collided.

NGC6745 in the constellation of Lyra is a striking example of a galaxy-galaxy collision.

This means that, in 100 million years or so, future astronomers will classify NGC 6745 as a triple system, three galaxies in a nearly straight line! This group of galaxies is located in the constellation Lyra, about 206 million light-years from Earth.

When galaxies collide, the stars that comprise the major portion of the luminous mass of the colliding galaxies will almost never strike each other directly, since their physical size is so tiny compared with the vast spaces between them. Typically, the stars will pass freely between each other, with little obvious evidence in the aftermath that anything has happened.

However, it is fairly common to see tidal distortions in one or both of the colliding galaxies, caused by remote interaction between the large gravitational forces that each produces.

Gravitational shock waves ripple through the clouds of interstellar gas and dust in one or both galaxies. The resulting pressures and high densities at the shock wave fronts can trigger the birth of new stars, which then form new large-scale structures within the galaxies.

In the rare case of NGC 6745, the gas and dust drifting between the stars in the two galaxies actually collided. Wherever the interstellar clouds of two galaxies collide, they do not freely penetrate each other, but rather, suffer something called inelastic collision. If the relative velocity in such collisions is sufficiently high, the pressure at the collision interface will produce densities of material extreme enough to trigger star formation through gravitational collapse, in much the same way that star formation is triggered by gravitationally induced shock waves.

Because stars in the first few million years of their life are so extraordinarily luminous and blue, their distribution is often the most immediate and obvious indication that there has been a gravitational interaction - and also, in rare instances, an actual physical collision. The distribution of luminous blue star clusters in NGC 6745 clearly indicates such a case.

The dominant (or "target") galaxy in NGC 6745 is above center in the image, with its nucleus and two attendant spiral arms about half way between the center and the top of the image. [Some people see a pecking bird in this image, with the galactic nucleus about where the bird's heart would be.] Using established terminology in describing the sequence of events, it will be called Galaxy A.

The intruding galaxy, or Galaxy C, came from a region to the left of the upper-left corner of the image, on a curved trajectory, and then passed on the near side of the nucleus of Galaxy A. It is now located in the lower right corner of the image. Galaxy C [which resembles food that the bird is pecking at] induced shock waves in Galaxy A during its passage, including those responsible for the present spiral structure. It then collided with the interstellar medium of Galaxy A shortly after passing the nucleus.

In this scenario, the arc of bright blue clusters of stars along the upper-right edge of Galaxy A corresponds to the locus of star formation induced by the propagating gravitational bow shock from the passage of Galaxy C. The dense association of bright blue star clusters [the bird's "head"] protruding down and to the right of the lower endpoint of that arc, toward Galaxy C, are the stars produced by direct collision pressure in the physical encounter between the interstellar material of the two galaxies.

One of the indications that there actually was such a collision between galaxies A and C is that, at high contrast, the blue 'tendrils' of the star cluster distribution are seen to extend all the way into Galaxy C.

Today, Galaxy C seems to show little evidence of current star formation, suggesting it may have been largely stripped of its interstellar medium by the magnitude of the collision.

It is also interesting to note the abundance of ruddy dust cloud-like structures near the junction between the lower end of the arc in Galaxy A and the downward distribution of bright blue star clusters, although they do not extend very far. It appears that the interstellar clouds at the nexus of the collision did not receive enough momentum from Galaxy A to keep up with the newborn stars, which are simply outrunning the clouds and heading toward their destiny as a new galaxy of the future.

This image was taken in March 1996 by Hubble's Wide Field/Planetary Camera-2 and released from the archives as part of the Hubble Heritage Project. Earl J. O'Neil, Jr., of the University of Arizona's Steward Observatory is a co-investigator with Roger Lynds in an on-going study of NGC 6745 using Hubble and ground-based radio telescopes.

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