More small-scale dark matter gravitational lenses than expected in galaxy clusters

James Marshall
September 13, 2020

A team of researchers used nearly 200 images of distant galaxies, whose light has been bent and magnified by this huge cluster, combined with the depth of Hubble data to measure the total mass and dark matter content of this cluster more precisely than ever before. The three inset images magnify three separate galaxies within the cluster, which appear to be producing gravitational lenses far stronger than they should be.

New data from the Hubble Space Telescope unveils much higher concentrations of dark matter than previously believed in some galaxies, by over an order of magnitude.

The gravity that's left indicates how much dark matter is in the Universe - and, from what we can tell, it's a lot.

It would be, let's say, very optimistic to claim that we have a good handle on the dark matter situation. The fact that these models get the big picture so right has been a strong argument in their favor.

But a new study shows that the same models get the details wrong - with a full-scale sequence. Gravitational lensing is pretty cool because it allows astronomers to see, for example, a galaxy that would otherwise be obscured by a closer one in front of it. Gravity distorts space itself and can do so by refracting light like a lens. A attractive example of this is an Einstein ring, where a single object appears multiple times forming a ring-like arrangement.

Dark matter's presence is known only through its gravitational pull on visible matter in space. This is one of the many sources that support the dark side.

There is an unusual mismatch between our theoretical models of how dark matter should be spread across galaxy clusters, and the way it actually seems to be behaving in those clusters, the researchers said. These models, as they move forward, provide an explanation of how the distribution of that dark object should have been at different points in the history of the Universe up to the present.

At the moment, there's no telling what the answer is to this new part of the dark matter mystery.

According to these models, the Universe was built step by step.

Because a higher concentration of dark matter in a cluster gives a more dramatic light-bending effect, the team surmised that the presence of the nested lenses were being produced by the gravity of dense pockets of dark matter inside the individual cluster galaxies that had magnified and warped the passing light. Its existence is only known by its attractiveness to the visible matter in space. By examining the output of these models, you can see the expected distribution of dark matter around the cluster. So they turned to galaxy clusters, which hide a huge amount of dark matter. By measuring the lensing distortions, astronomers could trace out the amount and distribution of dark matter.

Meanwhile, in the real universe...


This Hubble Space Telescope image shows the massive galaxy cluster MACS J1206. Follow-up imaging using the Very Large Telescope helped identify the distance of those objects based on how much their light was shifted to the red end of the spectrum by the expansion of the Universe-the larger the redshift, the more distant the object. This allowed researchers to determine which objects were behind the galaxy cluster, thus potential candidates for gravitational lensing.

The reference: "An excess of micro-gravitational lenses observed in galaxy clusters" by Massimo Mingetti, Guido Davoli, Pietro Bergamini, Piero Rosati, Priamvada Natarajan, Carlo Giocoli, and Gabriel B. Kamenya, R., Francesco Calura, Claudio Grillo, Amata Mercurio and Eros Vanzella, 11 September 2020, Science. They did so to identify possible lens locations and where they could create the greatest distortion.

The two did not match. There were significantly more areas that generated high distortion in the real-Universe galaxy than there were in the model. These distortions are caused by the amount of dark matter in the cluster, whose gravity bends and amplifies light from distant galaxies.

This isn't the first discrepancy of the sort we've seen. Dark matter models also predict that there should be more dwarf satellite galaxies around the Milky Way and that they should be more diffuse than they are. But if we adjust our models to further propagate these galaxies, we are less likely to see even smaller structures in galaxies. So, rather than finding two problems that could both be solved by making one adjustment, the two issues appear to need adjusting in opposite directions. "But we don't yet know whether this is telling us something about our computations and simulations, or whether it's telling us something fundamental about dark matter". However, since the big picture of the universe is largely correct for both, the problem would be subtle and consequently hard to identify if these results were confirmed independently. Gravitational lensing uses the light being emitted by a distant source in the universe as a lens, with the light becoming distorted as a result of the distribution of matter between the source and the object it is traveling towards.

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