Huge galaxy group reveals dark matter

Image of a cluster of galaxies

This wide-field, deep image reveals thousands of galaxies crowding an area on the sky roughly as large as the full Moon. Some of the galaxies are relatively close. The more distant look just like faint blobs — their light has travelled through the Universe for eight billion years or more before reaching Earth.

A new wide-field image released by the European Southern Observatory shows many thousands of distant galaxies, especially a large group belonging to the massive galaxy cluster known as Abell 315.

As crowded as it may appear, this assembly of galaxies is only the proverbial “tip of the iceberg”, as Abell 315 — like most galaxy clusters — is dominated by dark matter.

And the huge mass of this cluster bends the light coming from more-distant background galaxies, distorting their observed shapes slightly.

When looking at the sky with the unaided eye, we mostly only see stars within our Milky Way galaxy and some of its closest neighbours. More distant galaxies are just too faint to be perceived by the human eye, but if we could see them, they would literally cover the sky.

The new image is both a wide-field and long-exposure one, and reveals thousands of galaxies crowding an area on the sky roughly as large as the full Moon. The full-size image shows the full extent of the galaxy cluster (click here).

These galaxies span a vast range of distances from us. Some are relatively close, as it is possible to distinguish their spiral arms or elliptical halos, especially in the upper part of the image. The more distant appear just like the faintest of blobs — their light has travelled through the Universe for eight billion years or more before reaching Earth.

This diagram shows how gravitational lensing works.

This diagram shows how gravitational lensing works. A distant galaxy (right) is actually directly behind the cluster's core. The orange arrow show how the galaxy's real position and shape get distorted by the cluster's gravity. The white arrow shows the true path of the galaxy's light if the cluster didn't get in the way.

Dominated by dark matter

Galaxy clusters are some of the largest structures in the Universe held together by gravity.

But there is more in these structures than the many galaxies we can see.

Galaxies in these giants contribute to only ten percent of the mass, with hot gas in between galaxies accounting for another ten percent. The remaining 80 percent is made of an invisible and unknown ingredient called dark matter that lies in between the galaxies.

The presence of dark matter is revealed through its gravitational effect: the enormous mass of a galaxy cluster acts on the light from galaxies behind the cluster like a cosmic magnifying glass, bending the trajectory of the light and thus making the galaxies appear slightly distorted.

By observing and analysing the twisted shapes of these background galaxies, astronomers can infer the total mass of the cluster responsible for the distortion, even when this mass is mostly invisible.

However, this effect is usually tiny, and it is necessary to measure it over a huge number of galaxies to obtain significant results: in the case of Abell 315, the shapes of almost 10,000 faint galaxies in this image were studied in order to estimate the total mass of the cluster, which amounts to over a 100,000 billion times the mass of our Sun.

Adapted from information issued by ESO / J. Dietrich / NASA.

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