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NGC 6745: The Astronomer's Story


by Roger Lynds and Earl O'Neil Jr.

The picture portrays the aftermath of a direct mechanical collision between two galaxies, as distinct from more distant gravitational interactions. In the latter, more common situation we customarily see tidal distortions in one or both galaxies, possibly with the addition of large scale structures created by pressure-induced star formation triggered by the passage of gravitational shock waves through the gaseous and particulate interstellar media in one or both galaxies. However, in the case of NGC 6745 pictured here, two galaxies actually collided.

Interacting Galaxy System NGC 6745

When galaxies collide, the stars that normally comprise the major portion of the luminous masses of the two galaxies will almost never collide with each other but will pass rather freely between each other with little prominent and immediate evidence in the aftermath that anything has happened. The reason is that the physical size of individual stars is so tiny compared with their typical separations that the probability of physical encounter is vanishingly small.

However, the situation is quite different for the interstellar media in the two galaxies - media consisting largely of clouds of atomic and molecular gases and of tiny particles of matter, dust, strongly coupled to the gas. Wherever the interstellar clouds of the two galaxies collide, they do not freely interpenetrate but, rather, suffer inelastic collision. If the relative velocity in such collisions is sufficiently high, the ram pressure at the collision interface will produce material densities sufficiently extreme as to trigger star formation through gravitational collapse, in much the same way that star formation is triggered by the high material densities in 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 stars and clusters of such stars in NGC 6745 indicates such a case.

Using established terminology in describing the sequence of events, the dominant ('target') galaxy, galaxy (a), in NGC 6745 is above center in the illustration, with its nucleus and two attendant spiral arms about half way between the center and the top of the illustration. The intruding galaxy, galaxy (c), came from a region to the left of the upper-left corner on a gravitationally curved trajectory, passed on the near side of the nucleus of galaxy (a) - inducing shock waves in its passage, including those responsible for the present spiral structure, collided with the interstellar medium of galaxy (a) shortly after passing the nucleus, and has now reached its present location at the lower-right corner of the illustration.

In this scenario the arc of bright blue clusters of stars along the upper-right boundary of galaxy (a) corresponds to the locus of star formation induced by the propagating gravitational shock from the passage of galaxy (c). The dense association of bright blue star clusters seen to be coextensive with and protruding down and to the right of the lower terminus of that arc toward galaxy (a) comprises the stars produced by ram pressure in the physical encounter between the interstellar media 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). Galaxy (c) seems to show little evidence of current star formation and may have been largely shorn of its interstellar medium.

It will also be noted that there is an abundance of ruddy structures indicating dust clouds near the junction between the lower terminus of the arc and the downward distribution of bright star clusters, but that these do not extend very far into this distribution. It appears then that these stars clusters are simply outrunning interstellar clouds that did not receive sufficient momentum exchange from galaxy (a) to keep up. A result will be that future astronomers 100 million years or so from now will classify NGC 6745 as a triple system, three galaxies in a nearly straight line.