Isolated galaxy or corporate merger?

February 7, 2008
The elliptical galaxy NGC 1132 reveals the final result of what may have been a group of galaxies that merged together in the recent past. Another possibility is that the galaxy formed in isolation as a lone wolf in a universe ablaze with galaxy groups and clusters.

NGC 1132 is dubbed a "fossil group" because it contains enormous concentrations of dark matter, comparable to the dark matter found in an entire group of galaxies. NGC 1132 also has a strong X-ray glow from an abundant amount of hot gas that is normally only found in galaxy groups.

In visible light, however, it appears as a single, isolated, large elliptical galaxy. The origin of fossil-group systems remains a puzzle. They may be the end-products of complete merging of galaxies within once-normal groups. Or, they may be very rare objects that formed in a region or period of time where the growth of moderate-sized galaxies was somehow suppressed, and only one large galaxy formed.

Elliptical galaxies are smooth and featureless. Containing hundreds of millions to trillions of stars, they range from nearly spherical to very elongated shapes. Their overall yellowish color comes from the aging stars. Because ellipticals do not contain much cool gas, they no longer can make new stars.

This image of NGC 1132 was taken with Hubble's Advanced Camera for Surveys. Data obtained in 2005 and 2006 through green and near-infrared filters were used in the composite. In this Hubble image, NGC 1132 is seen among a number of smaller dwarf galaxies of similar color. In the background, there is a stunning tapestry of numerous galaxies that are much larger but much farther away.

NGC 1132 is located approximately 318 million light-years away in the constellation Eridanus, the River.