Return to Heritage Home Page Current Image Gallery Archive Information Center Hubble Art Search
Return to Heritage Home Page Current Release Home Page Caption Fast Facts Biographies Supplemental Material Original Images

Dwarf Irregulars and Galaxy Clusters

Galaxies were once viewed as immutable objects, persisting in their present form for most of the history of the universe. Over the last decade, however, astronomers have come to understand that many galaxies experience cataclysmic events which can drastically alter their appearance. Galaxies interact with each other through gravity; close passages can remove material from one and add it to another. Outright collisions can merge two galaxies into one, perhaps even changing one type of galaxy into another. Smaller galaxies can be completely pulled apart and destroyed by the tidal forces exerted by larger galaxies. These various interactions were probably more common at early times in the universe when galaxies were first being assembled, but even at the present epoch, it is possible to find spectacular examples of galaxy interactions and even destruction.

Illustration Credit: NASA, ESA, Z. Levay and L. Frattare (STScI) Image Credit: A. Karick and M. Gregg (LLNL/UC,D) using the Michigan Curtis Schmidt Telescope at CTIO: digital color composite/mosaic in B, V and I bands. Image Courtesy: M. Drinkwater (University of Queensland).

Galaxy clusters, where hundreds or even thousands of galaxies inhabit dense quarters, provide a busy environment, an excellent place to look for intense galaxy interactions. Even the relatively innocent-looking nearby (60 million light years) Fornax cluster has an example of cataclysmic galaxy evolution in progress. Fornax appears to be a quiet place, even a bit dull, being dominated by old galaxies, ellipticals and S0's mostly. This appearance is deceiving, however; X-ray images reveal a more turbulent picture of a cluster in the late stages of a recent merger of a sizable subgroup with the main cluster. A recent closer look at the relative motions of the galaxies has revealed additional evidence for not one but two subunits of galaxies colliding with the main group.

In retrospect, Fornax contains a prominent visual clue that the cluster is not entirely serene: NGC1427A, the only gas-rich irregular galaxy in the cluster, has an unusual "swoosh'' morphology with prominent blue knots of ongoing star formation. It also has a high velocity relative to the main cluster (about 600 km/s) and is probably part of one of the subgroups making its first passage through the main cluster. NGC1427A is relatively isolated, though, so its unusual appearance and vigorous star formation cannot be blamed on interaction with any particular large companion, but instead must be caused by the cluster as a whole. The motions of the stars which make up NGC1427A are distorted, and part of the galaxy appears to be breaking away to the north. This is highly suggestive that NGC1427A will not long survive as an identifiable galaxy passing through the cluster, but will be disrupted, spilling its stars and gas into the intergalactic regions of the Fornax cluster.

Such events, now relatively rare in rich clusters, were probably quite common when hundreds of galaxies were first being assembled into clusters. The stars and gas ripped from countless galaxies in a cluster contribute to building up the giant galaxies found at the heart of most clusters, and also account for the recently discovered ghostly (but substantial) intergalactic stellar populations.

Astronomers are using the data from galaxies like NGC 1427A to investigate the star formation patterns throughout the object, to determine if there is a relation between the ages of stars and position within the galaxy. This will help to understand how the gravitational influence of the cluster has affected the internal workings of this galaxy and how this galaxy has responded to passing through the cluster environment. Because it has a relatively high velocity with respect to the main cluster, after NGC1427A is disrupted in about 1 billion years, its individual stars, star clusters, and remaining gas will be spread far and wide throughout Fornax, contributing to the thin intergalactic sea of stars and the million degree gas which glows brightly in X-rays.

The disruption of objects like NGC1427A, and even larger galaxies like our own Milky Way, are an integral part of the formation and continued evolution of galaxy clusters. The impending destruction of NGC1427A provides but a glimpse of early and much more chaotic times in the Universe.