Tag Archives: galaxy clusters

This NASA/ESA Hubble Space Telescope image shows the massive galaxy cluster MACSJ 1206. Embedded within the cluster are the distorted images of distant background galaxies, seen as arcs and smeared features. These distortions are caused by the dark matter in the cluster, whose gravity bends and magnifies the light from faraway galaxies, an effect called gravitational lensing. This phenomenon allows astronomers to study remote galaxies that would otherwise be too faint to see. (NASA, ESA, G. Caminha (University of Groningen), M. Meneghetti (Observatory of Astrophysics and Space Science of Bologna), P. Natarajan (Yale University), the CLASH team, and M. Kornmesser (ESA/Hubble))

Astronomers Investigate Dark Matter’s Missing Ingredient

Our understanding of dark matter and its behavior could be missing a key ingredient. More gravitational lensing, the curving of spacetime and light by massive objects, could lead to the perfect recipe to solve this cosmic mystery. 

Despite comprising anywhere between 70–90% of the Universe’s total mass and the fact that its gravitational influence literally prevents galaxies like the Milky Way from flying apart, science is still in the dark about dark matter

As researchers around the globe investigate the nature and composition of this elusive substance, a study published in the journal Science suggests that theories of dark matter could be missing a crucial ingredient, the lack of which has hampered our understanding of the matter that literally holds the galaxies together. 

The presence of something missing from our theories of dark matter and its behavior emerged from comparisons of observations of the dark matter concentrations in a sample of massive galaxy clusters and theoretical computer simulations of how dark matter should be distributed in such clusters. 

Astronomers measured the amount of gravitational lensing caused by this cluster to produce a detailed map of the distribution of dark matter in it. Dark matter is the invisible glue that keeps stars bound together inside a galaxy and makes up the bulk of the matter in the Universe. (NASA, ESA, G. Caminha (University of Groningen), M. Meneghetti  (Observatory of Astrophysics and Space Science of Bologna), P. Natarajan (Yale University), and the CLASH team.)


Using observations made by the Hubble Space Telescope and the Very Large Telescope (VLT) array in the Atacama Desert of northern Chile, a team of astronomers led by Massimo Meneghetti of the INAF-Observatory of Astrophysics and Space Science of Bologna in Italy have found that small-scale clusters of dark matter seem to cause lensing effects that are 10 times greater than previously believed.

“Galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the Universe that are currently available reproduce well what we can infer from gravitational lensing,” says Meneghetti. “We have done a lot of testing of the data in this study, and we are sure that this mismatch indicates that some physical ingredient is missing either from the simulations or from our understanding of the nature of dark matter.”

Just Add Gravitational Lensing

The lensing that the team believes accounts for dark matter discrepancies is a factor of Einstein’s theory of general relativity which suggests that gravity is actually an effect that mass has on spacetime. The most common analogy given for this effect is the distortion created on a stretched rubber sheet when a bowling ball is placed on it.

This effect in space that results from a star or even a galaxy curving space and thus bending the path of light as it passes the object. Otherwise known as gravitational lensing it is commonly seen when a background object–which could be as small as a star or as large as a galaxy– moves in front of a foreground object and curves light from it giving it an apparent location in the sky. 

The gravitational lensing of a distant quasar by an intermediate body forms a double image seen by astronomers on Earth. (Lambourne. R, Relativity, Gravitation and Cosmology, Cambridge Press, 2010)

In extreme cases, where this lensing causes the paths of light to change in such a way that its arrival time at an observer is different, it can cause a background object to appear in the night sky at various different points. A beautiful example of this is an Einstein ring, where a single object appears multiple times forming a ring-like arrangement.

Because dark matter only interacts via gravity, ignoring even electromagnetic interactions — hence why it can’t be seen — gravitational lensing is currently the best way to infer its presence and map the location of dark matter clusters in galaxies.

 Returning to the ‘rubber sheet’ analogy from above, as you can imagine, a cannonball will make a more extreme ‘dent’ in the sheet than a bowling ball, which in turn makes a bigger dent than a golf ball. Likewise, the larger the cluster of dark matter — the greater the mass — the more extreme the curvature of space and therefore, light.

The gravitational microlensing effect results from the bending of space-time near an object of given mass that is predicted by Einstein’s general theory of relativity. An object, such as a star, crossing our line of sight to a more distant source star will affect the light from that star just like a lens, producing two close images whose total brightness is enhanced. If the lensing star is accompanied by a planet, one can (potentially) observe not only the principal effect from the star, but also a secondary, smaller effect resulting from perturbation by the planet. ( Beaulieu et al)

But now imagine what would happen if the bowling ball on the rubber sheet was surrounded by marbles. Though their individual distortions may be small, their cumulative effect could be considerable. The team believes this may be what is happening with smaller clusters of dark matter. These small scale clumps of dark matter enhance the overall distortion. In a way, this can be seen as a large lens with smaller lenses embedded within it.

Cooking Up A High-Fidelity Dark Matter Map

The team of astronomers was able to produce a high-fidelity dark matter map by using images taken by Hubble’s Wide Field Camera 3 and Advanced Camera Survey combined spectra data collected by The European Southern Observatory’s (ESO) VLT. Using this map, and focusing on three key clusters — MACS J1206.2–0847, MACS J0416.1–2403, and Abell S1063 — the researchers tracked the lensing distortions and from there traced out the amount of dark matter and how it is distributed.

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416.1–2403. A team of researchers used almost 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. (ESA/Hubble, NASA, HST Frontier Fields)

“The data from Hubble and the VLT provided excellent synergy,” says team member Piero Rosati, Università Degli Studi di Ferrara in Italy. “We were able to associate the galaxies with each cluster and estimate their distances.”

This led the team to the revelation that in addition to the dramatic arcs and elongated features of distant galaxies produced by each cluster’s gravitational lensing, the Hubble images also show something altogether unexpected–a number of smaller-scale arcs and distorted images nested near each cluster’s core, where the most massive galaxies reside.

The team thinks that these nested lenses are created by dense concentrations of matter at the center of individual cluster galaxies. They used follow-up spectroscopic observations to measure the velocity of the stars within these clusters and through a calculation method known as viral theorem, confirmed the masses of these clusters, and in turn, the amount of dark matter they contain. 

Abell S1063, a galaxy cluster, was observed by the NASA/ESA Hubble Space Telescope as part of the Frontier Fields programme. The huge mass of the cluster acts as a cosmic magnifying glass and enlarges even more distant galaxies, so they become bright enough for Hubble to see. (NASA, ESA, and J. Lotz (STScI))

This fusion of observations from these different sources allowed the team to identify dozens of background lensed galaxies that were imaged multiple times. The researchers then took this high-fidelity dark matter map and compared it to samples of simulated galaxy clusters with similar masses, located at roughly the same distances.

These simulated galaxy clusters did not show the same dark matter cluster concentrations — at least not on a small scale that is associated with individual cluster galaxies. 

The discovery of this disparity should help astronomers design better computer simulation models and thus develop a better understanding of how dark matter clusters. This improved understanding may ultimately lead to the discovery of what this abundant and dominant form of matter actually is. 


Original research: Meheghetti. M., Davoli. G., Bergamini. P., et al, ‘An excess of small-scale gravitational lenses observed in galaxy clusters,’ Science, [2020], 

Galaxy clusters Abell 399 (lower centre) and Abell 401 (top left). The galaxy pair is located about a billion light-years from Earth, and the gas bridge extends approximately 10 million light-years between them. (c) ESA

Superhot filaments of gas connect galaxy clusters

Galaxy clusters Abell 399 (lower centre) and Abell 401 (top left). The galaxy pair is located about a billion light-years from Earth, and the gas bridge extends approximately 10 million light-years between them. (c) ESA

Galaxy clusters Abell 399 (lower centre) and Abell 401 (top left). The galaxy pair is located about a billion light-years from Earth, and the gas bridge extends approximately 10 million light-years between them. (c) ESA

Astronomers have for the  first time confirmed a bridge of hot gas with a temperature of about 80 million degrees Kelvin connecting a pair of galaxy clusters 10 million light-years apart. The discovery is of particular importance since it might help shed light on the missing baryonic matter that has been puzzling scientists for decades.

The two galaxy clusters, Abell 399 and Abell 401, each contain hundreds of galaxies and are several billion light years away from Earth. In the early universe, filaments of gaseous matter pervaded the cosmos in a giant web, with clusters eventually forming in the densest nodes, according to the leading theory on the matter – this is called the warm-hot intergalactic medium (WHIM).

Despite the fact that we’ve yet to see any actual evidence or manage to pinpoint what exactly these are, the Universe is dominated by what’s ambiguously called dark matter and dark energy. What we can actually measure and see – stars, galaxies, cosmic clouds of dust and gas, and so on – only make up a tiny fraction of the Universe, less than 5%. This ‘white’ matter is commonly referred to among astronomers as baryonic matter.

Now, this baryonic matter can be generally detected by measuring the electromagnetic radiation it releases. When observing distant cosmic objects like far away galaxies and stars, however, the baryonic matter readings do not match those from nearby – there’s a mismatch between matter in the ancient Universe and the close Universe. About half of the  baryonic matter expected to be present in the local Universe is missing. So where is it?

Well, many scientists believe it lies in this warm-hot intergalactic medium or WHIM that I mentioned earlier. Cosmic simulations have revealed that both dark and baryonic matter are embedded in  a filamentary network, and that the WHIM might account for most of the baryonic matter in the local Universe. This network of tenuous gas ranges in temperature from 100,000 to several tens of millions of K and due to its extremely low density has proved very hard to detect.

Hot gas bridging galaxy clusters

This latest findings based on microwave and sub-millimetre wavelength observation using  ESA’s Planck satellite has brought new light into these theories.

“Although the WHIM is mainly organised in long and diffuse filaments, we expect to find it also in the proximity to galaxy clusters, which are the largest gravitationally-bound structures in the Universe,” explains José M. Diego, a Planck Collaboration scientist from the Instituto de Fisica de Cantabria (UC-CSIC) in Santander, Spain.

“Planck can detect galaxy clusters across the sky because the hot gas that fills them imprints a characteristic signature on the Cosmic Microwave Background known as the Sunyaev-Zel’dovich effect,” Diego adds. “Based on the same principle, Planck could be sensitive to gas from the WHIM, too”.

In other words, this Sunyaev-Zel’dovich or S-Z effect describes a phenomenon in which cosmic microwave background light interacts with the hot gas permeating these huge cosmic structures, which leads to energy distribution being modified in a characteristic manner.

“Detecting the WHIM via the Sunyaev-Zel’dovich effect is extremely challenging due to its low density,” comments Juan Macias-Perez, a Planck Collaboration scientist from the Laboratoire de Physique Subatomique et de Cosmologie in Grenoble, France. “The best chance to detect it is to look at the regions between pairs of nearby galaxy clusters that are interacting with one another: as they approach each other, gas in the inter-cluster region becomes denser and hotter, hence easier for us to spot,” he adds.

So the scientists looked at data collected by the Planck surveys, and looked for clusters that satisfy a somewhat delicate condition – close enough for intervening filaments to be detected, but also separate enough for Planck to be able to resolve as individual sources. Picky, picky, but they hit the jackpot eventually.

“A careful analysis revealed a ‘bridge’ of hot gas connecting two of the clusters in the sample: Abell 399 and Abell 401,” comments Diego.

By combining Planck data with archival X-ray observations from the German satellite Rosat, the astronomers found that the temperature of the gad bridge between the two galaxy clusters was roughly 80 million degrees Kelvin.

Early analysis suggests that it could be a mixture of the elusive filaments of the cosmic web mixed with gas originating from the clusters, but more data is needed for a through conclusion to be made. Next, the scientists are keen on studying another promising galaxy cluster pair – the composite system Abell 3391-Abell 3395, which is highly substructured and may in fact consist of three or four clusters.

“This discovery highlights the ability of Planck to probe galaxy clusters out to their outskirts and even beyond, allowing us to investigate the connection between intra-cluster gas and gas that is part of the cosmic web,” concludes Jan Tauber, Planck project scientist at ESA.

Findings were published in the journal Astronomy & Astrophysics.

source: ESA

Einstein’s theory passes tough test

Two studies put Einstein’s theory, the General Theory of Relativity to a test unlike any other before. The two teams used extensive observations from NASA’s Chandra X-ray Observatory to analyze galaxy clusters, the biggest objects in the Universe that are bound together by gravity (at least, that we know of). The first team produced results that dramaticaly “weaken” a competitor theory, while ther shows that Einstein’s theory works over a vast range of times and distances. Two thumbs up.

“If General Relativity were the heavyweight boxing champion, this other theory [“f(R) gravity”] was hoping to be the upstart contender,” said Fabian Schmidt of the California Institute of Technology in Pasadena, who led the study. “Our work shows that the chances of its upsetting the champ are very slim”

albert-einstein

Well if General Relativity were a heavyweight boxing champion, it would definitely be Cassius Clay. The point of the rival theory was to explaion why the Universe expands faster and faster. In the f(R) gravity theory, the cosmic expansion acceleration comes not from a form of energy, but rather from a modification of the gravitational force. The modification of the force also affects the rate at which small cosmic objects can grow over huge periods of time, thus opening the possibility of testing the theory with galaxy clusters observations.

What they found was that gravity is not different for distances of even 130 million light years.

“This is the strongest ever constraint set on an alternative to General Relativity on such large distance scales,” said Schmidt. “Our results show that we can probe gravity stringently on cosmological scales by using observations of galaxy clusters.”

The second study also tested the theory across cosmological periods and distances. The results fit General Relativity exactly.

“Einstein’s theory succeeds again, this time in calculating how many massive clusters have formed under gravity’s pull over the last five billion years,” said David Rapetti of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University and SLAC National Accelerator Laboratory, who led the new study. “Excitingly and reassuringly, our results are the most robust consistency test of General Relativity yet carried out on cosmological scales.”

However, this doesn’t solve the problem of the Universe expanding at an accelerated speed. It did eliminate an inaccurate theory, though. The matter, still, remains a mystery.

“Cosmic acceleration represents a great challenge to our modern understanding of physics,” said Rapetti’s co-author Adam Mantz of NASA’s Goddard Space Flight Center in Maryland. “Measurements of acceleration have highlighted how little we know about gravity at cosmic scales, but we’re now starting to push back our ignorance.”