An image of a distant galaxy that has been smeared into an arc due to immense gravitational forces is puzzling astronomers because it simply shouldn’t exist. The apparent arc shape of the galaxy is an artefact of a phenomenon known as gravitational lensing.

The gravitational arc in distant galaxy cluster IDCS J1426.5+3508 shouldn’t exist. Credit: NASA/ESA

In a similar manner that a lens in a refractor telescope will bend light in order to make a distant object easier to see, massive galaxy clusters can be used as a “lens” to see galaxies located behind them.

The immense mass of the galaxy cluster “lens” will bend the light rays of the distant source galaxy so that the source appears distorted, magnified, or is split into multiple images. Astronomers use these gravitational giants as nature’s telescope to look for otherwise undetectable background galaxies. This type of gravitational lensing is known as strong lensing, and there are many magnificent examples in the not too distant Universe.

Observing objects at large distances from Earth is also a way of looking back in time, as we see light that was emitted billions of years ago. The further that one peers back in time, the less massive the galaxies become. So finding a massive galaxy cluster 10 billion light years away came as quite a surprise to astronomers.

The galaxy cluster, known as IDCS J1426.5+3508, is the most massive in existence at that redshift, a measure of distance, and is at least five times larger than neighbouring clusters. The cluster was discovered using NASA’s Spitzer Space Telescope along with archived images from the National Optical Astronomy Observatory’s Deep Wide Field Survey. A second surprise was revealed in the form of the mysterious arc, in observations made using the Hubble Space Telescope.

The light rays from a distance galaxy are distorted by the mass of the intervening galaxy cluster, causing the image of the galaxy to be stretched into unusual shapes. Credit: Karen Teramura, University of Hawai’i Institute for Astronomy

At this distance, the arc just shouldn’t exist. Source galaxies should be too faint to cause visible arcs and the chance of even finding such an arc is less than one per cent. Statistical analyses predict that, at best, only arcs that are a fraction of the brightness as this stubborn arc should exist.

“What we actually did was ask the question of whether such an arc should exist behind any cluster at this redshift without also requiring that it be behind a cluster as massive as this one,” explains Anthony Gonzalez from the University of Florida, and lead author on one of the Astrophysical Journal papers about the arc. “If we were to instead ask the question of whether the arc should exist at this redshift behind a cluster this massive, then the odds of finding the arc would be even worse.”

The source galaxy was determined to be a star forming galaxy that is 10 to 13 billion light years away. Future observations should help to better pinpoint the exact distance, and upcoming searches might discover any other rogue arcs that may be lurking in the distant Universe.

“In one of these we are looking at a region that is ten times wider in area than the search in which this cluster was found and should enable us to search for similar clusters at these redshifts,” Gonzalez tells Skymania News. “This search, like the current program, is based upon a survey with Spitzer. We are also separately searching for the rarest, most massive galaxy clusters all-sky using data from the WISE mission.”

A galaxy cluster that is much closer to Earth displays many stretched images of background galaxies.

Astronomers have yet to find a satisfactory explanation as to why the arc exists. One suggestion is that distant clusters could have a greater population of galaxies at the core, but even this is still not enough to account for the mass needed to cause the arc. Another suggestion is that the subtle fluctuations in matter right after the Big Bang are not quite what is expected from standard predictions, and thus it is possible for massive clusters to form. The jury is still out on this one.

Here is a link to the original paper.