A curious pair of galaxies

Two interacting galaxies create Arp 261's chaotic and unusual structure.Provided by ESO, Garching, Germany
By | Published: March 16, 2009 | Last updated on May 18, 2023
This color composite image of Arp 261 was created from images obtained using the FORS2 instrument on the ESO Very Large Telescope (VLT), at the Paranal Observatory in Chile. Located in the mountains of the Atacama Desert, the Paranal Observatory enjoys some of the clearest and darkest skies on the whole planet. This image was created from images through blue, green, red and infrared filters, and the total exposure time was 45 minutes.
March 16, 2009
The European Space Observatory’s (ESO) Very Large Telescope (VLT) has taken the best image ever of a strange and chaotic duo of interwoven galaxies. The images also contain some surprises – interlopers both far and near.

Sometimes objects in the sky that appear strange, or different from normal, have a story to tell and prove rewarding scientifically. This was the idea behind Halton Arp’s catalog of Peculiar Galaxies that appeared in the 1960s. One of the oddballs listed there is Arp 261, which has now been imaged in more detail than ever before using the FORS2 instrument on ESO’s VLT. The image contains several surprises.

Arp 261 lies about 70 million light-years distant in the constellation Libra. Its chaotic and unusual structure is created by the interaction of two galaxies that are engaged in a slow motion, but highly disruptive close encounter. Although individual stars are unlikely to collide in such an event, the huge clouds of gas and dust crash into each other at high speed, leading to the formation of bright new clusters of hot stars that are seen in the picture. The paths of the existing stars in the galaxies are also disrupted, creating the faint swirls extending to the upper left and lower right of the image. Both interacting galaxies were probably dwarfs not unlike the Magellanic Clouds orbiting our own galaxy.

The images used to create this picture were not taken to study the interacting galaxies, but to investigate the properties of the inconspicuous object just to the right of the brightest part of Arp 261 and close to the center of the image. This is an unusual exploding star, called SN 1995N, that is thought to be the result of the final collapse of a massive star at the end of its life, a so-called core collapse supernova. SN 1995N is unusual because it has faded very slowly – and still shows clearly on this image more than 7 years after the explosion took place. It also is one of the few supernovae to have been observed to emit X rays. It is thought that these unusual characteristics are a result of the exploding star being in a dense region of space and that the material blasted from the supernova plows into it and creates X rays.

Apart from the interacting galaxy and its supernova, the image also contains several other objects at wildly different distances from us. Starting close to home, two small asteroids in our solar system between the orbits of Mars and Jupiter happened to cross the images as they were being taken and show up as the red-green-blue trails at the left and top of the picture. The trails arise as the objects are moving during the exposures and also between the exposures through different colored filters. The asteroid at the top is 14670 and the one to the left is 9735. They are probably less than 3 miles (5 kilometers) across. The reflected sunlight from these small bodies takes about 15 minutes to get to Earth.

The next closest object is probably the bright star at the bottom. It may look bright, but it is still about 100 times too faint to be seen with the unaided eye. It most likely is a Sun-like star and about 500 light-years from us – 20 million times farther away than the asteroids. Arp 261 and the supernova are about 140,000 times farther away than this star, but still in what astronomers would regard as our cosmic neighborhood. More distant still, perhaps some 50-100 times further away than Arp 261, lies the cluster of galaxies visible on the right of the picture. There is no doubt, however, that a more-remote object lies unrecognized among the faint background objects seen in this image.

For great galaxy images from amateur astrophotographers, visit Astronomy.com’s Online Reader Gallery : Galaxies album.