ESA XMM-Newton: Cosmic Collision in the Bullett Group

Composite image taken by ESA's XMM-Newton of the Bullet Group showing galaxies, hot gas (shown in pink) and dark matter (indicated in blue). 

Credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS

Galaxies are not as isolated as they at first glance may seem; on a cosmic scale they congregate in clumps along with dark matter and hot gas.

The colourful blob in this new composite image, based on data from several telescopes including ESA's XMM-Newton, is the group of galaxies known as the Bullet Group.

Its components appear to be clearly separated, with the hot gas partitioned from the rest of the mass within the group.

This is the smallest object ever found to show such an effect, which was caused by a merger in the group's past.