Stephan's Quintet of Galaxies

Great meaning and mystery dwell in a patch of sky no larger than a thumbtack held at arm's length. Nothing about this piece of sky has been easily understood ever since its discovery. Inhabited here is the great far-off malestrom of Stephan's Quintet. The five galaxies in the center of this image (there is another to the left) seem to dance with graceful promenades. This would imply that all five galaxies are located near to each other in space. However, long ago astronomers noted that the bluish spiral galaxy (NGC 7320, bottom galaxy) had a recessional velocity, due to the expansion of the universe, that is considerably smaller than the other four galaxies. Since these velocities imply distances, it would seem that NGC 7320 could not be party to this galactic gala.

For years, many argued that chains of stars between NGC 7320 and the other galaxies positively placed it at the same distance. Only recently was the issue finally put to rest, quite literally resolved, by the sharp vision of the Hubble Space Telescope. Individual stars, clusters, and nebulae are quite clearly seen in NGC 7320 and not in any of the other galaxies owing to its foreground front seating. And so from what once seemed an irreparable rend in the theory of cosmological redshift has now been satisfyingly sewn back together as a good description of our expanding universe.

Just as quickly as one nagging question was answered by the Hubble Space Telescope, did another equally mysterious question arise. Near to the nucleus of NGC 7319 (top left galaxy) a quasar shines brightly. As labeled in the inset below (and detected in the data shown here) this quasar once again seems to play the game of next galaxy on the dance card. Quasars are generally described as being super luminous galaxies formed during the early universe. But if this quasar is associated with NGC 7319, the understanding of quasars and the scale of the universe is once again in jeopardy. The reason that this quasar is so puzzling is that there is very little absorption of its light due to the effect of the gas and dust of NGC 7319. Perhaps, as some astronomers suggest, some quasars are actually the stripped cores of devoured galaxies that have been subsequently spit out by the surviving galaxy such as NGC 7319. Many other galaxies seem to have a high number of detected quasars near to them. This could be an observational bias or perhaps in this case the light of the quasar just happens to shine through a fortuitous window of NGC 7319. Only the future will tell the fate of these far-off mysteries.