With Halloween coming up tomorrow, the Hubble Space Telescope team is celebrating by releasing a new Hubble image showing the dark cobwebs of galaxy cluster Abell 611. Located an incredible 3.2 billion light-years away, this view shows hundreds of galaxies that are bound together by gravity into one enormous structure.
Taken using Hubble’s Advanced Camera for Surveys and Wide Field Camera 3 instruments, the image combines both visible light and infrared observations.
This particular cluster is a favorite target for astronomers in their search to understand a spooky substance: dark matter. A big question in cosmology is why large structures like Abell 611 don’t fly apart, as they don’t seem to contain enough mass to hold themselves together with gravity. The leading explanation is that they do in fact contain much more mass than we can see, and this hypothetical unseen mass is referred to as dark matter.
However, searching for dark matter has proven extremely difficult. Dark matter does not interact with light, making it invisible, and it has never been directly detected. Cosmologists aren’t even sure what form dark matter might take, though many believe it is likely a particle of some kind. Even though we can’t see it directly, its effects on the universe are clear. As well as in places like Abell 611, we see similar effects on a large cosmic scale, in which more mass is required to explain the behavior of galaxies than we can observe.
Abell 611 is particularly useful for studying dark matter, because it has so many massive galaxies appearing close together that it demonstrates many examples of a phenomenon called gravitational lensing. This is where one massive object sits in front of another from our point of view, and the gravity of that intermediate object is so great that it bends spacetime, changing the appearance of the light coming from the background object. This is useful, as the intermediate object can act like a magnifying glass, bending the light from the further away galaxy and letting us see extremely distant objects.
You can see an example of this gravitational lensing in the middle of the image, where light from the galaxy to the left of the center has been smeared out into an arc shape by the mass of the galaxy cluster. As well as helping us see distant objects, the degree of lensing can also be used to work out the mass of the objects involved. When calculated this way, the mass of the cluster is far higher than the observed mass would suggest, supporting the idea that there must be some other unseen material, such as dark matter, filling up the cluster.