Peering Far Back in Time to Uncover the Secrets of Galaxy Evolution
1 December 1992Looking far away and far back in time, NASA's Hubble Space Telescope (HST) has found some suspected ancestors of today's galaxies. The Hubble pictures reveal that star-forming galaxies were far more prevalent in the clusters of the younger universe than in modern clusters of galaxies near us today.
The results have important implications for theories of how galaxies have evolved since the beginning of the universe 15 billion years ago.
Serendipitously, the Hubble observations might have also discovered the most distant galaxy cluster yet seen. The cluster might be as far as ten billion light-years, at a "look- back" in time corresponding to the early epoch of galaxy formation.
The research was carried out by Drs. Alan Dressler of Carnegie Institution, Augustus Oemler of Yale University, James E. Gunn of Princeton University, and Harvey Butcher of the Netherlands Foundation for Research in Astronomy.
A principal goal for the Hubble Space Telescope is to trace galaxy evolution through direct observations. This is very difficult to achieve with ground-based telescopes because the tiny images of distant galaxies smear into faint blurs when viewed through Earth's atmosphere.
Hubble Space Telescope images of a pair of remote clusters of galaxies located four-billion light years away allow astronomers to distinguish, for the first time, the shapes of galaxies which existed long ago.
The pictures, taken with HST's Wide Field/Planetary Camera (in Wide Field Camera mode), are so detailed they show a full range of galaxy types inhabiting the universe of four billion years ago: elliptical, spiral, distorted and irregular forms.
The images also reveal galaxies in collision. Some are tearing material from each other, others are merging into single systems.
Dressler said the pictures are sharp enough to distinguish between various forms of spiral galaxies, whose distinctive swirl patterns are outlined by vigorous star formation. "This shows us that clusters billions of years ago contained not only the elliptical and SO galaxies (S-zero, lens- shaped featureless galaxies that may be the transition between spiral and elliptical) like those dominating their descendant clusters today, but also several times as many spiral galaxies."
The Case of the Disappearing Galaxies
"The new Hubble data are the first unambiguous sign of the influence of environment on the form of a galaxy," said Dressler, "Clearly, spirals were common in clusters in the distant past, but they have largely disappeared or changed form by now. What has been responsible for their demise?"
Based upon the HST pictures, and the results of earlier research with ground-based telescopes, the team thinks that the rapid decline in the spiral population can be explained by three mechanisms: merger, disruption, and fading.
HST reveals many examples of strong galaxy interactions or mergers in one of the clusters. This is evident by the presence of "tails" distorting the shapes of some galaxies. The tails are probably caused by tidal effects where the close gravitational pull between bypassing galaxies will stretch and disrupt their stellar distributions.
The result is that many ancient spirals might have merged to form giant elliptical galaxies (as proposed by MIT's Alar Toomre and Carnegie's Francois Schweizer), or simply been torn apart and dispersed by the violence of what Dressier calls the "Cuisinart environment."
Dressler, however, believes that violent collisions are not the whole story of the missing spiral galaxies. His earlier research with Gunn indicated that bursts of star formation were also much more common in the past. When the star formation subsided in many of these early spiral galaxies, they may have faded so that they are unnoticed in today's nearby clusters.
The Most Distant Galaxy Cluster Ever Seen?
The HST observations may have serendipitously discovered the farthest cluster of galaxies ever seen, located ten billion light-years away.
The HST picture resolves a cluster of about 30 very faint objects. "The smaller, more compact appearance of the objects suggests that they are in the background, much further away than the foreground galaxies," said Dressier.
Additional evidence comes from the presence of a quasar possibly among the faint objects. Ground-based spectral observations of the quasar's redshift (an indicator of cosmological distances) place the quasar at a distance of ten billion light-years.
Quasars are extraordinarily bright, active cores of primordial galaxies. Quasars were prevalent in the early universe and hence most are located out at ten billion light-years.
"Though the superposition of the cluster objects and the quasar could be a coincidence, both are so unusual that there is good reason to believe that all are members of the same cluster, at the same distance," said Dressler.
The bright spots which might accompany the quasar do not resemble the elliptical and spiral galaxies of today, according to Dressier. "Conceivably, the objects might not be separate galaxies but rather 'hot spots' in galaxies whose full, extended forms are too faint to be seen in the Hubble observations."
Dressler suggested that the small spots seen in the more distant cluster could be sites of vigorous star formation. This would explain their blue colors, because young, massive stars are brightest in blue and ultraviolet light.
"If the cluster is as far as the quasar, then it will offer an unprecedented opportunity to learn how galaxies formed. With so many objects, it seems likely that among them are the ancestors of common galaxies like our own Milky Way."
The team believes that the further study of this cluster and similar ones that could be a major breakthrough in seeing galaxies in the very act of formation.
When HST's full optical capabilities are restored during a Space Shuttle servicing mission in late 1993, HST will be able to resolve the morphology of these very young galaxies. Hubble will be capable of showing the evolution of galaxy form over a wide range of environments and in even earlier epochs. This will greatly aid astronomers in their efforts to understand this key piece of the cosmological puzzle.