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Starts With A Bang

Remarkable JWST trick lets us “see” dark matter

It's not only the gravity from galaxies in a cluster that reveal dark matter, but the ejected, intracluster stars actually trace it out.
This image reveals the intracluster light within the galaxy cluster SMACS-J0723, famed as the cluster from JWST's first deep-field image. After processing from the team of Mireia Montes and Ignacio Trujillo, the sources and distribution of this light have been revealed, with tremendous potential for applicability to other clusters and their internal dark matter distribution down the road.
(Credit: NASA, ESA, CSA, STScI)
Key Takeaways
  • Galaxy clusters are some of the most massive objects in all the Universe, bending space and revealing the presence of dark matter.
  • But it isn't just the effect of bent space and how it affects the light from background objects that reveals dark matter, but the extragalactic light within clusters does so, too.
  • When stars are ejected or form in the space between galaxies within a cluster, they go where the dark matter is, and measuring that intracluster light shows us dark matter as never before.

Dark matter remains one of nature’s greatest mysteries.

Our galaxy is embedded in an enormous, diffuse dark matter halo, indicating that there must be dark matter flowing through the solar system. While dark matter exists in a large, diffuse halo, the normal matter, because it experiences electromagnetic and collisional interactions, clumps and clusters together in the centers of these gravitational potential wells. The interplay between dark matter and normal matter is essential for understanding the mass distribution within individual galaxies.
(Credit: R. Caldwell and M. Kamionkowski, Nature, 2009)

Astrophysically, dark matter’s gravity accounts for multiple disparate observations.

A galaxy that was governed by normal matter alone (left) would display much lower rotational speeds in the outskirts than toward the center, similar to how planets in the Solar System move. However, observations indicate that rotational speeds are largely independent of radius (right) from the galactic center, leading to the inference that a large amount of invisible, or dark, matter must be present. These types of observations were revolutionary in helping astronomers understand the necessity for dark matter in the Universe.
Credit: Ingo Berg/Wikimedia Commons; Acknowledgement: E. Siegel

From individual, rotating galaxies

dark matter
According to models and simulations, all galaxies should be embedded in dark matter haloes, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The idea of dark matter has been with us for nearly a century, although not everyone remembers that fact.
Credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI)

to galaxies moving within clusters

The Coma Cluster of galaxies, as seen with a composite of modern space and ground-based telescopes. The infrared data comes from the Spitzer Space telescope, while ground-based data comes from the Sloan Digital Sky Survey. The Coma Cluster is dominated by two giant elliptical galaxies, with over 1000 other spirals and ellipticals inside. By measuring how quickly these galaxies move around inside the cluster, we can infer the cluster’s total mass. Very massive clusters, many thousands of times the Milky Way’s mass, are common in the modern universe, but very rare early on.
Credit: NASA / JPL-Caltech / L. Jenkins (GSFC)

to gravitational lensing

A distant, background galaxy is lensed so severely by the intervening, galaxy-filled cluster, that three independent images of the background galaxy, with significantly different light-travel times, can all be seen. In theory, a gravitational lens can reveal galaxies that are many times fainter than what could ever be seen without such a lens, but all gravitational lenses only take up a very narrow range of positions in the sky, being localized around individual mass sources.
Credit: NASA & ESA

to colliding galaxy clusters

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present.
(Credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland; University of Edinburgh, UK), R. Massey (Durham University, UK), T. Kitching (University College London, UK), and A. Taylor and E. Tittley (University of Edinburgh, UK))

to the large-scale cosmic web,

dark matter-free
The cosmic web that we see, the largest-scale structure in the entire Universe, is dominated by dark matter. On smaller scales, however, baryons can interact with one another and with photons, leading to stellar structure but also leading to the emission of energy that can be absorbed by other objects. Neither dark matter nor dark energy can accomplish that task; our Universe must possess a mix of dark matter, dark energy, and normal matter. Here, multiple galaxies are seen all bound together, within a surrounding halo of dark matter amidst the cosmic ocean.
Credit: Ralf Kaehler/SLAC National Accelerator Laboratory

the independent lines of evidence supporting dark matter are overwhelming.

A Hubble Space Telescope view of the galaxy cluster MACS 0416 is annotated in cyan and magenta to show how it acts as a ‘gravitational lens,’ magnifying more distant background sources of light. Cyan highlights the distribution of mass in the cluster, mostly in the form of dark matter. Magenta highlights the degree to which the background galaxies are magnified, which is related to how the mass is specifically distributed within the cluster. This technique can be used to hunt for features like distant supernovae, which can in turn be used to measure the Universe’s expansion rate.
Credit: STScI/NASA/CATS Team/R. Livermore (UT Austin)

We’ve even determined how it’s distributed within galaxy clusters.

A galaxy cluster can have its mass reconstructed from the gravitational lensing data available. Most of the mass is found not inside the individual galaxies, shown as peaks here, but from the intergalactic medium within the cluster, where dark matter appears to reside. More granular simulations and observations can reveal dark matter substructure as well, with the data strongly agreeing with cold dark matter’s predictions. Without the gravitational effects of dark matter, most galaxies would tear themselves apart during episodes of major star-formation.
Credit: A. E. Evrard, Nature, 1998

Now, a new method reveals dark matter’s presence more rigorously than ever before.

The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5 million light years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all. In addition to the sources of light from each individual galaxy within a cluster, a contribution exists from the light from stars existing between the galaxies: intracluster light. This can only be measured from space, but with the newfound power of JWST, may become our best tracer of dark matter ever.
(Credit: NASA/Digitized Sky Survey 2)

When galaxies interact within clusters, stars and tidal streams get stripped out.

Located within the Norma cluster of galaxies, ESO 137-001 speeds through the intracluster medium, where interactions between the matter in the space between galaxies and the rapidly-moving galaxy itself cause ram pressure-stripping, leading to a new population of tidal streams and intergalactic stars. Sustained interactions such as this can eventually remove all of the gas from within a galaxy, but also teach us that the galaxy, the cluster, and the gas within it is all made of matter, not antimatter.
Credit: NASA, ESA, CXC

This catapults stars into the intracluster medium: the space between galaxies.

The Tadpole Galaxy, shown here, has an enormous tail to it: evidence of tidal interactions. The gas that’s stripped out of one galaxy gets stretched into a long, thin strand, which contracts under its own gravity to form stars. The galactic element itself is comparable to the scale of the Milky Way, but the tidal stream alone is some ~280,000 light-years long: more than twice as large as our Milky Way’s estimated size. These features are common within galaxy clusters and will eventually lead to stars following the underlying dark matter distribution and creating the feature of intracluster light.
(Credit: NASA, H. Ford (JHU), G. Illingsworth (USCS/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS science team, and ESA)

Although individually unresolvable, those stars still shine, emitting faint intracluster light.

Here, galaxy cluster MACS J0416.1-2403 isn’t in the process of collision, but rather is a non-interacting, asymmetrical cluster. It also emits a soft glow of intracluster light, produced by stars that are not part of any individual galaxy, helping reveal normal matter’s locations and distribution. Gravitational lensing effects are co-located with the matter, showing that “non-local” options for modified gravity do not apply to objects like this. Clusters of galaxies contain all sorts of small-scale structures within them, from black holes to planets to star-forming gas and more.
Credit: NASA, ESA and M. Montes (University of New South Wales)

Because dark matter gravitationally attracts those stars, that intracluster light evolves as a dark matter tracer.

Along with lensing signals, this can map out dark matter substructure within galaxy clusters.

This Hubble image of massive galaxy cluster MACSJ 1206 shows the arcing and smeared-out features caused by the gravitational light-bending of the foreground galaxy cluster. Small-scale concentrations of dark matter, represented in blue, have been reconstructed based on the lensing data. Combining this lensing information with intracluster light information, which is another, independent tracer of dark matter, can reveal its presence and distribution as never before. We discover, through analyses like this, that all dark matter haloes consist of a rich set of dark matter substructures.
Credit: 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)

This technique has been successfully leveraged previously with Hubble, revealing suspicious and unexpected features.

This image shows the huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us. The huge mass of the cluster is bending the light from more distant objects. The light from these objects has been magnified and distorted due to gravitational lensing. The same effect is creating multiple images of the same distant objects. Meanwhile, the central location of the cluster clearly shows intracluster light: a remarkable tracer of dark matter.
(Credit: NASA, ESA, S. Rodney (John Hopkins University, USA) and the FrontierSN team; T. Treu (University of California Los Angeles, USA), P. Kelly (University of California Berkeley, USA) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI))

But now, the JWST offers even greater scientific potential.

This almost-perfectly-aligned image composite shows the first JWST deep field’s view of the core of cluster SMACS 0723 and contrasts it with the older Hubble view. The JWST image of galaxy cluster SMACS 0723 is the first full-color, multiwavelength science image taken by the JWST. It was, for a time, the deepest image ever taken of the ultra-distant Universe, with 87 ultra-distant galaxy candidates identified within it. They await spectroscopic follow-up and confirmation to determine how distant they truly are. but even from this first image, JWST observations suggested that the number and density of bright, early galaxies may pose a problem for astronomers.
Credit: NASA, ESA, CSA, and STScI; NASA/ESA/Hubble (STScI); composite by E. Siegel

Mireia Montes and Ignacio Trujillo analyzed the original JWST deep field for intracluster light.

This 3-panel animation shows the original JWST deep field, a color-inverted version, and then a contrast/brightness enhanced version designed to bring out the intracluster light. By properly calibrating, processing, and analyzing this data, Montes and Trujillo were able to reveal two contributions, one toward the center and one toward the outskirts, to this observed intracluster light.
(Credit: NASA, ESA, CSA, STScI; Processing: E. Siegel)

Additional processing and calibration revealed multiple contributors.

By properly calibrating the various contributions to reflected and external light effects and removing them, Montes and Trujillo were able to determine what fraction of the diffuse light is truly of intracluster origin, determining the stellar contributions and the profile of centrally distributed dark matter in the process.
(Credit: M. Montes & I. Trujillo, ApJ Letters, 2022)

Central mergers and outer accretion create this light.

Multiple features that contribute to the intracluster light, identified in the image here, can be teased out once the image is properly calibrated. The remaining light suggest that centrally, galactic mergers are the primary source of stars that contribute to the intracluster light, while on the outermore regions, galactic accretion plays a dominant role.
(Credit: M. Montes & I. Trujillo, ApJ Letters, 2022)

This “tracing” technique lets us see and map out dark matter as never before.

dark matter
This image showcases the massive, distant galaxy cluster Abell S1063. As part of the Hubble Frontier Fields program, this is one of six galaxy clusters to be imaged for a long time in many wavelengths at high resolution. The diffuse, bluish-white light shown here is actual intracluster starlight, which was captured for the first time only in 2018. It traces out the location and density of dark matter more precisely than any other visual observation, and with JWST data forthcoming, now holds unprecedented power for tracing out dark matter’s distribution within clusters as never before.
(Credit: NASA, ESA, and M. Montes (University of New South Wales))

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