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Starts With A Bang

How to make galaxies with the wrong amount of dark matter

Almost every large structure in the Universe displays a 5:1 dark matter-to-normal matter ratio. Here's how some galaxies defy that rule.
An image of a spiral galaxy taken by the JWST in space.
This composite JWST (above left) and Hubble (lower right) image of nearby spiral galaxy NGC 628, located 32 million light-years away, showcases the gas and dust network that traces out the spiral arms and the current star-forming regions within them. For large, high-mass spiral galaxies, it's very common to find a dark matter to normal matter ratio consistent with the cosmic average: 5-to-1. However, in extreme environments and for low mass galaxies, the ratios can be extreme, both in terms of producing galaxies overabundant and deficient in dark matter.
Credit: NASA, ESA, CSA, STScI, Janice Lee (STScI), Thomas Williams (Oxford), PHANGS Team
Key Takeaways
  • All throughout the Universe, if you measure large-scale cosmic structures, from large galaxies to galaxy clusters to the cosmic web and more, you find the same ratio of dark matter to normal matter: 5-to-1.
  • However, when we look at a few extreme cases, including at small galaxies close to large ones, at the aftermath of major galaxy collisions, and at very low-mass galaxies, we see two classes of exceptions.
  • Some galaxies appear to have lost large amounts of normal matter, leaving only the dark stuff, while other galaxies appear to be dark matter deficient or even dark matter-free. Here's how the Universe does it.

For nearly all large-scale objects, dark matter is a vital part of the story.

dark matter-free
The cosmic web that we see, the largest-scale structure in the entire Universe, is dominated by dark matter. Simulations of the large-scale structure of the Universe must include both dark matter and normal matter, including the effects of star-formation, feedback, and gas infall, as all of them are needed in order to predict the emergence of visible structures. Identifying which regions are dense and massive enough to correspond to star clusters, galaxies, galaxy clusters, and filaments, as well as determining when and under which conditions they form, is one of the great achievements of modern cosmology.
Credit: Ralf Kaehler/SLAC National Accelerator Laboratory

We see:

coma cluster zwicky dark matter
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. Gas-free, red-and-dead elliptical galaxies are very common, especially toward the cluster center, in large galaxy clusters such as this one. The speed of galaxies within the cluster can be used to help determine the cluster’s total mass.
Credit: NASA / JPL-Caltech / L. Jenkins (GSFC)
  • galaxies inside galaxy clusters rapidly zipping around,
galaxies velocity dispersion coma cluster
The velocities of the galaxies in the Coma Cluster, from which the total mass of the cluster can be inferred to keep it gravitationally bound. Note that this data, taken more than 50 years after Zwicky’s initial contentions, matches almost perfectly with what Zwicky himself contended way back in 1933: that far more mass must be present than stars, gas, plasma, dust, black holes, and all other forms of normal matter combined can account for.
Credit: G. Gavazzi, Astrophysical Journal, 1987
  • fast-orbiting stars and gas around spiral galaxies, even toward their outskirts,
dark matter rotation curve modified gravity
The extended rotation curve of M33, the Triangulum galaxy. These rotation curves of spiral galaxies ushered in the modern astrophysics concept of dark matter to the general field. The dashed curve would correspond to a galaxy without dark matter, which represents less than 1% of galaxies. Vera Rubin’s work throughout the 1970s was essential in demonstrating that galaxies practically universally require an explanation for this unexpected but robustly observed behavior.
Credit: Mario de Leo/Wikimedia Commons
  • large-magnitude gravitational lensing around galaxies and galaxy clusters,
evrard dark matter map nature 1998 galaxy cluster
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
  • mass separated from X-ray emitting gas during cluster collisions,
separation normal matter dark matter galaxy cluster
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. Without dark matter, these observations (along with many others) cannot be sufficiently explained.
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)
  • plus dark matter’s signature in the cosmic web,
baryon acoustic oscillations Ho'oleilana
The structure Ho’oleilana, a candidate for an individual baryon acoustic oscillation, can be identified visually by the human eye as a circular feature around 500 million light-years across. The red circle, shown in animation, makes the presence of this acoustic oscillation even clearer on scales of 155 Mpc or so: about 500 million light-years. This corresponds to the expected acoustic scale with an amplitude that matches the 5-to-1 dark matter-to-normal matter ratio expected from other lines of evidence.
Credit. R.B. Tully et al., ApJ, 2023
  • and cosmic microwave background.
planck cmb
Although we can measure the temperature variations all across the sky, on all angular scales, it’s the peaks and valleys in the temperature fluctuations that teach us about the ratio of normal matter to dark matter, as well as the length/size of the acoustic scale, where normal matter (but not dark matter) gets “bounced” outward from interactions with radiation. This radiation includes photons, which have a substantial cross-section with particles in the ionized plasma of the early Universe, and neutrinos, which do not.
Credit: NASA/ESA and the COBE, WMAP, and Planck teams; Planck Collaboration, A&A, 2020

They point toward one universal dark matter-to-normal matter ratio: 5-to-1.

how much dark matter
While the web of dark matter (purple, left) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red, at right) can severely impact the formation of structure on galactic and smaller scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Structure formation is hierarchical within the Universe, with small star clusters forming first, early protogalaxies and galaxies forming next, followed by galaxy groups and clusters, and lastly by the large-scale cosmic web.
Credit: Illustris Collaboraiton/Illustris Simulation

However, some galaxies are exceptional, defying that ratio.

dwarf galaxy WLM
This wide-field view shows the sky around the dwarf galaxy WLM in the constellation of Cetus (The Sea Monster). This picture was created from images forming part of the Digitized Sky Survey 2. The bluish clump in the center of the image is galaxy WLM; the bright, colored, spikey points, including the red and yellow ones, are simply foreground stars within our own Milky Way. Dwarf galaxies commonly come in a wide variety of morphologies, including the low-surface brightness variety. The lower in mass a galaxy such as this is, the greater its dark matter-to-normal matter ratio.
Credit: ESO/Digitized Sky Survey 2; Acknowledgement: Davide De Martin

The smallest, lowest mass galaxies display enhanced dark matter ratios.

An image of a dark black hole in space.
This “almost dark” galaxy, nicknamed Nube, is an incredibly diffuse galaxy found within a grouping of many other galaxies. It is thought that this ultra-diffuse galaxy, which has only a small smattering of stars inside a large mass of neutral hydrogen, owes its properties due to environmental factors. With so much hydrogen and so few stars, it represents a fascinating outlier among conventionally known galaxies.
Credit: M. Montes et al., A&A accepted, 2023

When stars form, energetic winds and radiation expel normal matter, escaping its gravitational pull.

cigar galaxy messier 82
This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds. The lower in mass the wind-driven starburst galaxy is, the more of its normal matter it loses during an event like this.
Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON)

The loss of normal matter creates elevated dark matter ratios.

dwarf galaxy segue 1 3
Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. As we discover smaller, fainter galaxies with fewer numbers of stars, we begin to recognize just how common these small galaxies are as well as how elevated their dark matter-to-normal matter ratios can be; there may be as many as 100 for every galaxy similar to the Milky Way, with dark matter outmassing normal matter by factors of many hundreds or even more.
Credit: Marla Geha/Keck Observatory

However, dark matter deficient galaxies also arise.

galaxies without dark matter
Across a wide range of masses, galaxies all fell along a relationship called the baryonic Tully-Fisher relation, where the observed/inferred rotational speed was determined empirically by the normal matter alone, irrespective of dark matter. The existence of a population of galaxies that does not follow this rule, as shown with the orange stars, provides strong evidence for a fundamentally different population: a set of galaxies without dark matter, following the grey line.
Credit: P.E. Mancera Piña et al., ApJL, 2019

Galaxies interact, both gravitationally and by speeding through a gas-rich medium.

This close-up look at the details from the tightly interacting pair of galaxies within Stephan’s Quintet showcases stellar streams and the interface of colliding gas, from which new stars arise. The new stars that form in these ripped-out streams may not remain gravitationally bound and undisturbed for long, but while they persist, will form collections of stars (or galaxies) that have no dark matter within them at all.
Credit: NASA, ESA, and the Hubble SM4 ERO Team

These interactions rip out normal matter, where star-formation ensues.

runaway galaxy pressure stripping
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, eliminating its ability to form new stars. Phenomena such as this allow us to conclude that the galaxy, the cluster, and the gas within it are all made of matter, not antimatter, while the tidal streams of new stars will contain practically no dark matter at all.
Credit: NASA, ESA, CXC

However, dark matter stays its original course, creating normal matter-only stellar clumps.

When galaxies like the spiral galaxy at right, D100, speed through a rich environment (like the Coma Cluster, which D100 is a member of), the friction with the environment can cause gas stripping, leading to the formation of stars and increasing the dark matter-to-normal matter ratio of the host galaxy. A few of these stripped star clusters that form, trailing the galaxy, could later re-form into a dark matter-free galaxy of their own.
Credit: NASA, ESA, M. Sun (University of Alabama), and W. Cramer and J. Kenney (Yale University)

Small galaxies can also experience tidal forces from larger ones.

penguin egg spitzer
This spectacular image was created with composite Spitzer and Hubble data, and shows a tidally distorted galaxy, rich in gas and actively forming new stars, merging with an old, gas-free elliptical galaxy made up of older stars. Poetically, this is called ‘the penguin and the egg,’ where the active star-forming regions of the Penguin may create a hostile environment for life, whereas the calm environment of the Egg may be among the best places for sustained life to emerge and thrive. Matter is being ripped off of the Penguin from the outside-in, with the innermost portions being the last to be disturbed but the outermore, dark matter-rich regions being disturbed earlier and by greater amounts.
Credit: NASA/JPL-Caltech

Matter gets torn apart from the outside-in, primarily removing dark matter first.

The galaxy NGC 1052-DF4, one of the two satellite galaxies of NGC 1052 determined to be devoid of dark matter internally, shows some evidence of being tidally disrupted; an effect more easily seen in the panel at right, once the surrounding light sources are accurately modeled and removed. Galaxies such as this are unlikely to live long in rich environments without dark matter to hold them together, but their formation mechanisms are still debated, and may arise from more than one mechanism.
Credit: M. Montes et al., ApJ, 2020

This temporarily creates dark matter-deficient, or even dark matter-free, galaxies.

galaxies without dark matter
Unlike other cosmological simulations. whose results are shown in the orange pentagon and blue hexagons, this current simulation by Moreno et al. actually reproduces dark matter deficient galaxies that are in agreement, for the first time, with the observed dark matter deficient galaxies NGC 1052-DF2 and NGC 1052-DF4.
Credit: J. Moreno et al., Nature Astronomy, 2022

Ironically, finding examples of galaxies without dark matter helps prove dark matter’s existence.

dark matter-free
The first galaxy detected that supports its existence without dark matter, NGC 1052-DF2, is shown here as imaged by Hubble. The follow-up imaging was done in order to determine whether it was gravitationally connected to NGC 1052, at ~64 million light-years away, or NGC 1042, much closer at 42 million light-years distant. The determination was 72 million light-years with an uncertainty of just a few percent, and hence, it is not connected to either. It cannot possess a typical amount of dark matter.
Credit: Z. Shen et al., ApJ, 2021

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