Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
How looking at a single, distant galaxy cluster can show us the invisible mass filling our space like nothing else.
“We find them smaller and fainter, in constantly increasing numbers, and we know that we are reaching into space, farther and farther, until, with the faintest nebulae that can be detected with the greatest telescopes, we arrive at the frontier of the known universe.” –Edwin Hubble
When you look out into the distant Universe, in most locations, you’ll find a field of faint, distant galaxies: beautiful, but nothing special.
The ‘parallel field’ of Abell 370 showcases a deep view of a region of space with no particularly massive or significant structure inside. This is what most of the Universe looks like, when imaged deeply enough. Image credit: NASA, ESA/Hubble, HST Frontier Fields.
However, in a few select locations, you’ll find massive clusters of galaxies, containing hundreds or even thousands of galaxies the size of the Milky Way and up.
Six billion light years away, Abell 370 is one of the most massive, dense ones discovered so far, but one galaxy, noticed early on, provided a hint of something more.
The distorted galaxy shown here is actually two images of a single galaxy located twice as far away as the rest of the galaxy; it is the effects of gravitational lensing that cause the odd appearance and multiple images. Image credit: NASA, ESA/Hubble, HST Frontier Fields.
The “stretched-out” galaxy you see here isn’t a distorted cluster member, but is instead two images of a single galaxy, twice as far away as the cluster itself.
An illustration of gravitational lensing showcases how background galaxies — or any light path — is distorted by the presence of an intervening mass, such as a foreground galaxy cluster. Image credit: NASA/ESA.
This phenomenon of gravitational lensing stretches galaxies into streaks and arcs, magnifying them, and creating multiple images.
The streaks of galaxies shown here are not representative of the actual shapes of the galaxies themselves, but rather the galaxies subject to the effects of the gravitational lens they pass through. Undistorted galaxies, like the one at the top left, are most likely in the foreground of the lens. Image credit: NASA, ESA/Hubble, HST Frontier Fields.
It also enables us to reconstruct the mass distribution of the cluster, revealing that it’s mostly due to dark matter.
The mass distribution of cluster Abell 370. reconstructed through gravitational lensing, shows two large, diffuse halos of mass, consistent with dark matter with two merging clusters to create what we see here. Image credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland), R. Massey (Durham University, UK), the Hubble SM4 ERO Team and ST-ECF.
There are two separate clumps present, showing that this is likely two clusters merging together.
Despite the presence of large, elliptical galaxies, the location where the mass density is greatest, indicated by the dotted circle, corresponds to no known massive galaxy or other structure based in normal matter. The only explanation for this is the presence of an invisible source of mass: dark matter. Image credit: NASA, ESA/Hubble, HST Frontier Fields / E. Siegel (annotation).
Most importantly, dark matter must be present — and present outside of the individual galaxies themselves — to explain these gravitational effects.
A 2009 image, based on only a fraction of the Hubble data available today, revealed some of the incredible structure in Abell 370. The current data, benefitting from 8 extra years, showcases even more information about the distant, massive Universe. Image credit: ESA/Hubble.
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