Put two massive galaxy clusters into collision and you have an astronomical laboratory for the study of dark matter, that much discussed and controversial form of matter that does not interact with light or a magnetic field. We learn about it through its gravitational effects on normal matter. In new work out of Caltech, two such clusters, each of them containing thousands of galaxies, are analyzed as they move through each other. Using data from observations going back decades, the analysis reveals dark and normal matter velocities decoupling as a result of the collision.
Collisions on galactic terms have profound effects on the vast stores of gas that lie between individual galaxies, causing the gas to become roiled by the ongoing passage. Counter-intuitively, though, the galaxies themselves are scarcely affected simply because of the distances between them, and for that matter between the individual stars that make up each.
We need to keep an eye on work like this because according to the paper in the Astrophysical Journal, so little of the matter in these largest structures in the universe is in the form we understand. That’s a telling comment on how much work we have ahead if we are to make sense of the structure of a cosmos we would like to explore. In fact, the authors make the case that only 15 percent of the mass in the clusters under study is normal matter, most of it in the form of hot gas but also locked up in stars and planets. That would make 85 percent of the cluster mass dark matter.
The clusters in question are tagged with the collective name MACS J0018.5+1626. All matter, including dark matter, interacts through gravity, while normal matter is also responsive to electromagnetism. That means that normal matter slows down in these clusters as the gas between the individual galaxies becomes turbulent and superheated, while the dark matter within the clusters moves ahead in the absence of electromagnetic effects. Lead author Emily Silich (a Caltech grad student working with principal investigator Jack Sayers) likens the effect to that of a collision between dump trucks carrying sand. “The dark matter is like the sand and flies ahead.”
Image: This artist’s concept shows what happened when two massive clusters of galaxies, collectively known as MACS J0018.5+1626, collided: The dark matter in the galaxy clusters (blue) sailed ahead of the associated clouds of hot gas, or normal matter (orange). Both dark matter and normal matter feel the pull of gravity, but only the normal matter experiences additional effects like shocks and turbulence that slow it down during collisions. Credit: W.M. Keck Observatory/Adam Makarenko.
Some years back we looked at the two colliding galaxy clusters known collectively as the Bullet Cluster (see A Gravitational Explanation for Dark Matter). There, the behavior of the component materials of the clusters has been analyzed in the study of dark matter, but the clusters are seen from Earth with a spatial separation. In the case of MACS J0018.5, the clusters are oriented such that one is moving toward us, the other away. These challenging observations made it possible to analyze the velocity differential between dark and normal matter for the first time in a cluster collision.
Caltech’s Sayers explains:
“With the Bullet Cluster, it’s like we are sitting in a grandstand watching a car race and are able to capture beautiful snapshots of the cars moving from left to right on the straightway. In our case, it’s more like we are on the straightway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed.”
I’m reminded of my previous post on Chris Lintott’s book, where the astrophysicist takes note of the role of surprise in astronomy. In this case, the scientists used the kinetic Sunyaev-Zel’dovich effect (SZE), a distortion of the cosmic microwave background spectrum caused by scattering of photons off high-energy electrons, to measure the speed of the normal matter in the clusters. With the two clusters moving in opposite directions as viewed from Earth, untangling the effects took Silich to data from NASA’s Chandra X-ray Observatory (another reminder of why Chandra’s abilities, in this case to measure extreme temperatures of interstellar gas, are invaluable).
Adds Sayers:
“We had this complete oddball with velocities in opposite directions, and at first we thought it could be a problem with our data. Even our colleagues who simulate galaxy clusters didn’t know what was going on. And then Emily got involved and untangled everything.”
Nice work! The analysis tapped many Earth- and space-based facilities. Data from the Caltech Submillimeter Observatory (CSO), now being relocated from Maunakea to Chile, go back fully twenty years. The European Space Agency’s Herschel and Planck observatories, along with the Atacama Submillimeter Telescope Experiment in Chile, were critical to the analysis, and data from the Hubble Space Telescope were used to map the dark matter through gravitational lensing. With the clusters moving through each other at 3000 kilometers per second – one percent of the speed of light – collisions like these are in Silich’s words “the most energetic phenomena since the Big Bang.”
Dark matter explains many phenomena including galaxy rotation curves, which imply more mass than we can see, and gravitational lensing has been used to show that visible mass is insufficient to explain the lensing effect. But we still don’t know what this stuff is, assuming it is real and not a demonstration of our need to refine General Relativity through theories like Modified Newtonian Dynamics (MOND). What we need is direct detection of dark matter particles, an ongoing effort whose resolution will shape our understanding of galactic structure and conceivably point to new physics.
The paper is Silich et al. 2024. “ICM-SHOX. I. Methodology Overview and Discovery of a Gas–Dark Matter Velocity Decoupling in the MACS J0018.5+1626 Merger,” Astrophysical Journal 968 (2): 74. Full text.
I have often thought that this is a process that can create low or free of dark matter galaxies. When galaxies in these clusters collide the dark matter from each galaxy simply moves through each other and onwards with the bulk of the stars but the gas in each galaxy collides creating new stars in a smaller galaxy in between that is essentially free of dark matter.
The more I hear about this “dark matter” the more its starting to sound like “the aether”. Remember the aether? It was everywhere, it was extremely important, but you just couldn’t see it or measure it. There was no way to tell it was there. It only gave away its presence because light waves needed something to wave, just like dark matter only gives away its presence by its gravitational attraction.
Mitchelson-Morely gave us the experimental evidence that there was no aether, and Einstein came up with the conceptualization that replaced it. Until something similar happens with dark matter, we’re going to be chasing our tails trying to find it, and rushing up blind alleys using it as explanations for phenomena.
So, can anyone tell me if dark matter is gravitationally affected by ordinary matter? I know the reverse is true, but is dark matter in a gravitational field accelerated? Does it have inertia? Momentum? Kinectic energy? Can it attract itself? Does it accumulate at the bottom of gravity wells? Is it measured in grams? And if there is no way to experimentally or observationally answer these questions, does it even make any sense to ask them?
Does dark matter obey relativistic mechanics and does it undergo Lorentz-Fitzgerald contraction when in motion? Can it be accelerated? Can it be accelerated to travel faster than light?
Ah, now there’s an interesting question…
The problem with comparisons with the “Mitchelson-Morely” (actually Michelson-Morley) experiment is that observations like this — and especially observations of the Bullet Cluster — are the rough equivalent of doing the M-M experiment and getting the observations predicted by luminiferous-ether theory.
As for the rest of your questions: the “cold dark matter” theory is a theory of relatively massive, slow-moving particles which don’t interact with electromagnetism (similar to how neutrinos behave) but otherwise obey all the laws of physics. So, yes, it’s gravitationally attracted by — and thus accelerated by — ordinary matter, has momentum and kinetic energy, can accumulate at the bottom of gravitational wells, obeys relativistic mechanics, cannot be accelerated to FLT velocities, etc., etc.
(Can we measure these directly? No, but we can test the effects of DM particle properties by doing simulations of large-scale-structure and galaxy formation — and cluster collisions! — and seeing whether we get results matching observations of galaxies, clusters, etc. And we generally do. For example, we get better matches to the data if the simulations include the gravitational pull of ordinary matter on dark matter.)
All of matter and energy in the universe interacts with the graviton, the theoretical force carrier of gravity. How it is done is not clear as their has not been a clear quantum theory of gravity.
@So, can anyone tell me if dark matter is gravitationally affected by ordinary matter?
By logical deduction I would say yes: if we speak of “matter” we can therefore think “electron” “quark” etc so of “particles” do not govern one or more of the 4 fundamental forces. If it is “dark antimatter” would have anihilation and release of energy, which obviously we do not observe (unless the movement of this dark matter is precisely due to an invible energy?) Remains the option of “exotic” particles or virtual ones that appear in extremely short time frames for our world? Mystery…I leave it to the specialists.
As usual there is one missing element in the puzzle that we will find one day. In the meantime, we must build our reasoning on a speculation that will become more refined over time.
Discussions of dark matter need some good illustrations, comparing otherwise similar spiral galaxies with distinctively different rotational rates coming and going.
The point here being that say a near twin to, say, Andromeda shows distinctly different rotational velocities due to invisible accumulations of matter. And that is essentially what Vera Ruben’s publications identified. One could suggest that the operation of gravity over galactic dimensions experiences something akin to turbulence, but suggesting that there is non-baryonic matter out there serves just as well for now. If a dark matter unit mass can be detected, that would help too.
But suggesting gravitational turbulence over spatial ocean lengths might result in us stuck with the same controversies with a more complicated hypothesis. With respect to spiral galaxies with altered rotational rates due to dark matter, I am not aware of significant alterations in distance candles such as Cepheid variable stars; so it does not seem as though physical laws for visible matter are affected much by the presence of dark matter concentrations. It is just that those Cepheids orbiting their galactic centers rotate about it with un-anticipated differences in velocity vs. the galactic overall luminosity. Perhaps features such as density waves of dust and gas that eventually form stars can be mapped against where concentrations of dark matter appear (sic) to accumulate.
Are we permitted to say “all matter matters”? Or maybe”matter does not matter”? Aethet did not pan out, but ether (the dimethyl variety) was an alternative to chloroform as an anesthetic agent “once upon a time”.
By the by, in two of the four schools of Buddhist philosophy and two of the six schools of Hindu philosophy — matter almost does not matter.
Though dark matter within galaxies seems to distinguish one galaxy’s rotational profile from another, seeing changes in galactic behavior is like trying to observe whether a refrigerator light turns off after you close the door.
Unless there’s a supernova, a nova or a Cepheid variation, a galaxy is going to look the same from one night to the next from the Chilean plateau – or observed from space.
Consequently, the thought occurred to me: A number of GR lens effects involve multiple images of the same galaxy. A possible reason is that a cluster of foreground galaxies (e.g. 4 or 5) perform the same lensing function but with different pathways. These pathways might actually provide different temporal snapshots of the distant galaxy. After all, the foreground galaxies have significant kilo-light year dimensions and slight different mega parsec distances from us. If the galaxy images are detailed enough, it might be possible to extract individual radial velocity profiles showing differences over time in the dark matter effect, possibly indicative of dark matter flow.
Maybe this has already been done?
I’m starting to wonder if dark matter and dark energy are today’s version of the luminiferous aether of the mid 19th century. Neither of these have been directly observed or created in a laboratory, which suggests to me that they do not actually exist.
It’s always amazing when one mind – like Emily Silich here in this instance – can step in and resolve something that has been an intractable problem for many very, very smart people up to that point.
Providing resolution both in the sense of finding an intellectual solution as well as in resolving – bringing into clear focus – our perception of a phenomenon. As if they were adjusting the focus on a telescope and brought a theretofore fuzzy amorphous image that had eluded us finally, and instantaneously, into sharp focus.
Kudos for having that inspiration that unlocked the order that lay behind the – perceived – chaos up to that point.
Sometimes that surprise in astronomy comes from within us, or, here from within Ms. Silich’s mind.
As for dark matter itself, I, too, remain skeptical – but, well, skeptical as a lay person, so big whoop – as it just sounds to me like an intellectual bandaid that speaks more to an issue with the current state of our own theoretical framework than to the underlying cosmos.
That is, what they were observing didn’t fit the then-current theoretical framework, so they slapped on – pardon me, postulated – dark matter as the solution.
And while it may an elegant solution, that just makes it all the more intellectually seductive to highly intelligent people.
Because it then allows them to “explain” other phenomena, despite the absence of underlying observations independently confirming the existence of dark matter in the first instance.
Or maybe they’ll conclusively confirm its existence via specific observations as we go forward.
But, until then, I’m just a bit skeptical (but, yes, again, big whoop).
A study from June 17 showed that “the rotation curves of galaxies remain flat for millions of light years with no end in sight.” The final statement in the study: “the obvious and inevitably controversial interpretation of this result is that dark matter is a chimera”.
Here is the link from phys.org:
https://phys.org/news/2024-06-mond-dark-rotation-galaxies-stay.html#:~:text=dark%20matter%3A%20Research%20suggests%20that%20rotation%20curves%20of%20galaxies%20stay%20flat%20indefinitely,-by%20Case%20Western&text=The%20primary%20technique%20Mistele%20used,Einstein's%20theory%20of%20general%20relativity.
This link at the bottom of the above article also states that “The data seems to support modified gravity over standard dark matter cosmology.”
It does go on to say that “The result is exciting, but it doesn’t conclusively overturn dark matter. The AQUAL model has its own issues, such as its disagreement with observed gravitational lensing by galaxies. But it is a win for the underdog theory, which has some astronomers cheering “Vive le MoND!””
A chimera in science is a combination of two distinct entities. A remarkable example was a woman who had genetic testing to donate bone marrow to her offspring, but was initially found to be not the mother: further testing found that she was a composite of two (sibling) zygotes, and the other zygote was the mother.
@Robin,
Somewhat pedandantic as “illusory” is also a definition:
Illusory and its synonyms also seem to be the first offerings when a thesaurus is used.
So what might be the chimera of baryonic matter and dark matter, if there is any intimate joining of the two rather than interaction? ;-)
Or a Chimera is “an imaginary monster compounded of incongruous parts”. I’m not saying DM is one or the other.
But we still don’t know what this stuff is, assuming it is real and not a demonstration of our need to refine General Relativity through theories like Modified Newtonian Dynamics (MOND).
Well, probably not things like MOND, because MOND fails to reproduce the observations of even isolated galaxy clusters, let alone colliding-cluster systems like this.
Good point, Peter. Thank you.
Does the original article explain how the velocity of dark matter, approaching from and receding from the cluster, was measured?
At first I thought this might have measured by its gravitational lensing effect on microwave background, but this is difficult to visualise: The motion described as being directly toward – or away from – the the observer. If that is the case, then it seems difficult to distinguish the lensing effect of the different ‘lobes’ of the cluster on the cosmic background. Essentially we have one lens in front of another… () () () (). How can the effect of any single aligned lens (normal or dark matter) be separated out from the others to give evidence for the article’s explanation?
“These challenging observations made it possible to analyze the velocity differential between dark and normal matter for the first time in a cluster collision.”
Does the original article explain how the velocity of dark matter, approaching from and receding from the cluster, was measured?
The velocity of the dark matter is assumed to be the same as the velocity of the galaxies, which can be measured from their spectra. E.g, in the Abstract: “We discover a velocity space decoupling of the DM and gas distributions in MACS J0018.5+1626, traced by cluster-member galaxy velocities and the kinematic Sunyaev–Zel’dovich effect, respectively.”
Looking at the original article, the paper describes statistical modelling and computer simulation for a number of different cluster mergers at various observational angles. It will be interesting to read more peer review of their analysis.
“In no case does a merger oriented along the line of sight pass the matching criteria.”
5.3. Viewing Angle
In both the first and second applications of the matching criteria to the simulations, there is a strong selection of simulation snapshots observed at viewing angles marginally offset from the merger axis. We plot the weighted distribution of selected viewing angles for simulation snapshots that pass the matching criteria for all the simulation variations in Figure 8. In particular, the matching algorithm prefers values of
that peak at ≈0.88 (i.e., inclined ≈28° from the merger axis). Quantitatively, the median of the selected viewing angles is
(≈32°) with an IQR of 0.12 (≈13°). The MACS J0018.5+1626 viewing angle is, therefore, likely between
(inclined ≈27°–40°). In no case does a merger oriented along the line of sight (
) pass the matching criteria.
Perhaps the right question is not so much what this dark matter is, but to know what it will bring us in terms of knowledge of the universe. How can it help us to move forward?
My thoughts about dark matter is that is our universe moving through a cloudy dark matter universe or are we comoving from the formation of the universe. That would have implications on the distribution of light and dark matter which we may be able to detect.
Looking back on the history of dark matter inquiry, so much of it rests on observations of spiral galaxies; i.e., galaxies with rotational rates or stars and dust rotating about the galactic center in violation of both Newton’s predictions or that expected for a 33 RPM plastic record.
Now what about elliptical galaxies?
It’s really hard to pin down elliptical motions. And reading over the literature they are quite often characterized as ellipsoidal, having axes in 3 dimensions. A couple of papers I dug up, though not close at hand, indicate that stellar motions appear much more random, yet defining a 3 dimensional overall form. Brownian perhaps, searching for a quick summary. Thus, one would surmise that that ellipticals have not told us very much about dark matter, if such a state exists within them at all. In fact, search as I might, I am yet to find overall angular rates associated with ellipticals. If ellipticals had histories of collisions with spirals, then whatever angular rate was inherent in the spiral, it gave up the ghost in the elliptical. Or else I missed something.
But why would an elliptical not have angular rates about its axes? After all, when galaxies collectively are examined, early examples often are irregular and as a result of collisions become the same. As a non specialist, it is hard for me to distinguish arguments about which galaxies originated as elliptical and then became spirals – and which spirals became irregular and then elliptical after mergers. And in between how is the aggregate of dark matter doing?
Before the dark matter mystery sprung on the astronomy community, I remember some musings about where had all the neutrinos of the universe gone, long time passing. So many processes and so many little packets of energy – and maybe mass – produced over 13 billion years of time. Were we all standing ankle deep in neutrinos? They had energy, but mass or subluminal velocity was uncertain. And then there was the discovery that neutrinos from the sun of a certain energy level – transitioned into three different forms in their passage past the Earth.
Perhaps it’s facetious to suggest that dark matter and neutrinos have some connection?
overhead, I had summarized dark matter origins referring to Vera Ruben’s survey work on spiral galaxies. She observed galaxies local enough that variations of radial velocity could be discerned in their spiral arms. Someone else noted a survey, on the other hand, which in summary noted that rotation rates of their galaxies were “flat”. Well, that would have problems for the visible distribution of mass too. A flat rotation rate would suggest rigid rather than Newtonian motion.
Going through the stacks of old Science magazines, a potential avalanche in the office closet that needed filing or to be thrown out, I came across some old articles that seem to have a different perspective now. Numerous ones, really. But the one iin front of me now is from Science (26 September 1986 – vol 233, pp. 1357-1460) summarizing a July conference at U California, Santa Cruz of that year.
Earlier, with the supervision of astronomer John Huchra, a “slice of the universe” sky survey had been undertaken to map the cosmos out in a 115 degree wide slice 6 degrees thick to a red shift distance representative of hundreds of millions of parsecs distance. Beside position estimates for the galaxies, there velocities associated with them that were not simply Hubble expansion. Some velocity elements were indicative of gravitational ties between galaxies and into clusters.
Detailed analysis indicated that galaxies were tied by gravitational forces for gigas of years.
But there was a discrepancy between mass associated with visible light and the dynamics observed. By about an order of magnitude pointing toward “dark matter”.
In addition to the cluster orbital motions such as globular clusters about our own galaxies or larger galaxies ( ours and Andromeda plus others) about a cluster center, there were flow patterns along the strands .
It was observed that the Milky Way galaxy as a body had a relative velocity of 600 km/sec with respect to the 2.7 K background cosmic radiation ( And I am still just getting used to thinking of the centerless GR cosmos – but evidently it has a spherical edge…) and local galaxy velocities it was noted that their relative velocities with the Milky Way are relatively small by comparison. So, much of our cosmic velocity is related to streaming – in a flow with a dark matter constituent that shapes it the streams.
To quote the article:
” Whatever one thinks of the streaming motions, however… there still remains the frothy large-scale structure itself. It is far from random. It cries out for an explanation. Thus, astronomers are inexorably led to the other half of the equation: dark matter.
” … In essence, the argument is that the spiral galaxies rotate too fast and the galaxis in clusters move too fast; the visible stars simply do not contain enough mass to hold these systems together by gravity. So something else must be making up the deficit, some kind of cosmic ectoplasm that permeates the galaxies, that has enormous mass, and that is utterly invisible.
“… Ordinary ‘baryonic ‘matter in the visible galaxies is little more than flotsam, drifting along wherever the dark matter carries it.”
Likely, much of this was written before dark energy was introduced into the picture. But dark energy as “work” would act on both conventional and unconventional matter.
Even though it is invisible, dark matter is hard to sweep away.
Maybe I’m barking up the wrong tree, but from https://arxiv.org/pdf/2403.04850 it sounds like they sicced ALMA on three elliptical galaxies and came up with something interesting — but short of a model of the dark matter distribution. It’s a pity I don’t actually understand the paper…
Hello, M.S.
Followed the link to the arxiv paper and came to a conclusion similar to yours:
Looked at 3 galaxies that were “lensed” and provided detailed post mortems.
Trouble was, I don’t think they were aware of our concern with the effects of dark matter. Rather, I suspect, was that given multiple images of the same galaxy or galaxies, they wanted to merge the images into one without “distortion”. Perhaps there are “local” time differences in images of the same galaxies though, and perhaps position or radial velocity data could be extracted at some later date as well. Should circumstances allow, that would be my own feedback to such research. You might try writing a shopping list too, just in case.
The paper in question is trying to infer the mass distribution (from the combination of dark matter and stars) of the lensing galaxies — the massive (probably elliptical) galaxies sitting in between us and the distant lensed galaxies. They’re not really interested in the latter, except as probes of the lensing galaxies’ mass distributions.
They note that in two of the three cases, the inferred spatial distribution of the total mass doesn’t really match the spatial distribution of the stars, which is potentially interesting. (I’ll note that on some levels, this is true for spirals, which have highly flattened distributions of stellar (and gas) mass, but rather round distributions of dark matter.)
Now what about elliptical galaxies?
Crudely speaking, elliptical galaxies are dominated by random motions in all directions: instead of mostly all moving in one direction with approximately circular orbits, the stars are all moving in different directions, often on fairly elongated/elliptical (or “radial”) orbits. You can do statistical modeling of the observed (Doppler-shifted) velocity dispersion to estimate how much mass is needed to keep the stars from expanding away. (If there’s some “bulk” rotation of the stars, you include that in the modeling as well.)
If you do this for globular clusters, which are dominated by random motions in a manner similar to elliptical galaxies, the result is that the mass of the stars is enough: you don’t need any extra (“dark”) matter to explain the stellar motions. But if you do this for elliptical galaxies, the stellar mass isn’t enough: you need dark matter. In fact, the most extreme results tend to be for very low-mass “elliptical” galaxies (more commonly called “dwarf spheroidal” galaxies) — these need a lot of dark matter relative to the amount of normal matter. Even in their very central regions, they are “dark matter dominated”, unlike massive spiral or elliptical galaxies.
There’s an additional approach you can use. Many (massive) elliptical galaxies are embedded in halos of very hot, X-ray-emitting gas. If you can measure the temperature and density profile of this gas (which you often can from the X-ray emission), you can estimate its pressure profile, and you can use it to test for dark matter: i.e., is the gravity provided by the stars (and the hot gas itself) strong enough to keep the gas confined to its halo, or should the gas have blown away billions of years ago? This is equivalent to asking if the gravity of all the mass in the Sun is enough to prevent the gas from expanding to infinity. In the case of the Sun, the answer is (obviously) yes; in the case of elliptical galaxies, the answer is no — so you need some extra (dark) mass to provide enough gravity to keep the gas from expanding away.
The upshot of all this is that, yes, elliptical galaxies appear to have dark matter, just like spiral galaxies.
My gut feeling is that people need to just throw data at the problem. The Bullet Cluster and this are two great examples. Is there any fundable way to get data about thousands of objects like this, or other types of objects with dark matter in tow? The more information about how far the dark matter diffuses outward, how it interacts with regular matter and itself, the more precise the physical models of these systems, the closer the question comes to an answer.
Does Dark Matter Really Exist? John Michael Godier’s podcast