What happens when a galaxy falls in with the wrong crowd? The irregular galaxy NGC 1427A is a spectacular example of the resulting stellar rumble. Under the gravitational grasp of a large gang of galaxies, called the Fornax cluster, the small bluish galaxy is plunging headlong into the group at 600 kilometers per second or nearly 400 miles per second.
NGC 1427A, which is located some 62 million light-years away from Earth in the direction of the constellation Fornax, shows numerous hot, blue stars in this newly released image obtained by the Hubble Space Telescope. These blue stars have been formed very recently, showing that star formation is occurring extensively throughout the galaxy.
Galaxy clusters, like the Fornax cluster, contain hundreds or even thousands of individual galaxies. Within the Fornax cluster, there is a considerable amount of gas lying between the galaxies. When the gas within NGC 1427A collides with the Fornax gas, it is compressed to the point that it starts to collapse under its own gravity. This leads to formation of the myriad of new stars seen across NGC 1427A, which give the galaxy an overall arrowhead shape that appears to point in the direction of the galaxy's high-velocity motion. The tidal forces of nearby galaxies in the cluster may also play a role in triggering star formation on such a massive scale.
NGC 1427A will not survive long as an identifiable galaxy passing through the cluster. Within the next billion years, it will be completely disrupted, spilling its stars and remaining gas into intergalactic space within the Fornax cluster.
To the upper left of NGC 1427A is a background galaxy that happens to lie near Hubble's line of sight but is some 25 times further away. In contrast to the irregularly shaped NGC 1427A, the background galaxy is a magnificent spiral, somewhat similar to our own Milky Way. Stars are forming in its symmetric pinwheel-shaped spiral arms, which can be traced into the galaxy's bright nucleus. This galaxy is, however, less dominated by very young stars than NGC 1427A, giving it an overall yellower color. At even greater distances background galaxies of various shapes and colors are scattered across the Hubble image.
The Hubble Space Telescope's Advanced Camera for Surveys was used to obtain images of NGC 1427A in visible (green), red, and infrared filters in January 2003. These images were then combined by the Hubble Heritage team to create the color rendition shown here. Astronomers are using the data to investigate the star-formation patterns throughout the object, to verify a prediction that there should be a relation between the ages of stars and their positions within the galaxy. This will help them 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.
The disruption of objects like NGC 1427A, and even larger galaxies like our own Milky Way, is an integral part of the formation and evolution of galaxy clusters. Such events are believed to have been very common during the early evolution of the universe, but the rate of galaxy destruction is tapering off at the present time. Thus the impending destruction of NGC 1427A provides a glimpse of an early and much more chaotic time in our universe.
Acknowledgment: M. Gregg (Univ. Calif.-Davis and Inst. for Geophysics and Planetary Physics, Lawrence Livermore Natl. Lab.)