The Space Telescope Science Institute (STScI) in Baltimore, Md. produced a composite image of Arp 147, a pair of interacting galaxies located about 430 million light years from Earth. In this picture, X-rays from the NASA’s Chandra X-ray Observatory are in pink, and optical data from the Hubble Space Telescope are red, green, and blue:

© X-ray: NASA/CXC/MIT/S.Rappaport et al, Optical: NASA/STScI

Arp 147 contains the remnant of a spiral galaxy (right) that collided with the elliptical galaxy on the left. This collision has produced an expanding wave of star formation that shows up as a blue ring containing a great number of massive young stars. These stars race through their evolution in a few million years or less and explode as supernovas, leaving behind neutron stars and black holes.

A fraction of the neutron stars and black holes will have companion stars, and may become bright X-ray sources as they pull in matter from their companions. The nine X-ray sources scattered around the ring in Arp 147 are so bright that they must be black holes, with masses that are likely ten to twenty times that of the Sun.

© Credit: X-ray: NASA/CXC/MIT/S.Rappaport et al, Optical: NASA/STScI

The X-ray picture reveals a few more things: another X-ray source is visible in the nucleus of the red galaxy on the left, as well as other objects unrelated to Arp 147. The X-ray source my be powered by a poorly-fed supermassive black hole; the other objects are a foreground star in the lower left of the image and a background quasar as the pink source above and to the left of the red galaxy.

The rate of star formation in the ring has been estimated thanks to Infrared observations with NASA’s Spitzer Space Telescope and ultraviolet observations with NASA’s Galaxy Evolution Explorer (GALEX): according to the authors, the most intense star formation may have ended some 15 million years ago.


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