Cosmic Cannibalism

One of the most important ways that galaxies evolve is through interaction with other galaxies. We see substantial evidence for collisions between smaller galaxies triggering the formation of massive luminous stars. In some cases, galaxies appear to be swallowing other galaxies whole.
Caught in the Act
The adjacent image taken using the 1 m European Southern Observatory telescope, illustrates an extreme example of this. This false color image is of a distant cluster of galaxies called Abel 3827, which lies at a redshift of 0.1, corresponding to a distance of about 1.5 billion light years. In the enhanced image we see that the dominant central galaxy of the cluster (the bright central region) has swallowed five other galaxies in the cluster (the five large yellow blobs inside the central region). In many other clusters of galaxies one finds a large galaxy near the center, suggesting that galactic cannibalism in clusters may be relatively common.

Cannibalism in the Early Universe?
The following figure shows a Very Large Telescope image of the distant galaxy 1138-262 and its vicinity. This galaxy, which is a strong source of radio waves, is in the southern constellation Hydra (the Water Snake), at a distance of about 10 billion light years from Earth. The light in this image left the galaxy when the Universe was only about 20 percent of its present age.

The false color image image is of Lyman-alpha hydrogen emission, and marks regions having a high concentration of ionized hydrogen. The cloud of ionized hydrogen around the galaxy is about 500,000 light years across (five times the size of the visible Milky Way).

A Massive Galaxy Assembles Itself?
Observations of this galaxy by ROSAT in X-rays and the VLA at radio frequencies suggest that it is surrounded by a hot gas, much like that observed more locally in clusters of galaxies. Furthermore, the Hubble Space Telescope has found that the galaxy appears to be comprised of many clumps. The preliminary interpretation is that this is a massive galaxy in the process of formation by absorbing smaller galaxies in a distant rich cluster. If this is a correct interpretation, it may shed important light on how galaxy and cluster structure grew in the early Universe. In this interpretation, the large cloud of ionized hydrogen gas is the material from which the smaller galaxies that are being absorbed into the giant galaxy are forming.