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Starts With A Bang

Are the “missing baryons” simply too hot to see?

There are two types of missing, or "dark" matter: baryonic (made of normal matter) and non-baryonic. Have we finally found the normal stuff?
X-ray view cartwheel galaxy
This composite image of the Cartwheel galaxy group shows optical data overlaid with X-ray data (purple) from NASA's Chandra X-ray observatory. While the luminous matter is clustered together into stars, much more of the normal matter may be present in a very hot, circumgalactic region surrounding the most massive galaxy in the cluster, as hinted at by X-ray light.
Credit: X-ray: NASA/CXC; Optical: NASA/STScI
Key Takeaways
  • All throughout the Universe, it isn't just luminous stars and galaxies that fill the cosmos, but unseen forms of gravitating matter, known collectively as dark matter.
  • Most of this dark matter cannot be made of any of the known particles making up the Standard Model, but some of what's missing is simply "normal stuff" that's gone perpetually unseen.
  • Thanks to a series of new studies at X-ray wavelengths, we might have exposed the greatest hint as to where they're hiding: as hot, ionized plasmas surrounding massive galaxies, including our own.

Dark matter remains a great unsolved cosmic mystery.

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

Unlike what we see, our Universe is mostly non-luminous.

This comparison image, showing the same region as imaged by Hubble’s eXtreme Deep Field (top) and JWST’s JADES survey (bottom) showcases a selection of many ultra-distant galaxies found in the young Universe. When we observe the Universe at great distances, we’re seeing it as it was in the distant past: smaller, denser, hotter, and less evolved. However, it’s only the luminous matter (i.e., stars) that is typically seen by our telescopes; most of the matter, including most of the normal matter and all of the dark matter, is non-luminous.
Credit: NASA, ESA, CSA, STScI (JWST); ESA/Hubble & NASA and the HUDF09 team (Hubble)

To form galaxies,

spiral galaxy comparison dark matter
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

galaxy clusters,

X-ray lensing Abell 2744 animation
This four-panel animation shows the individual galaxies present within Abell 2744, Pandora’s Cluster, alongside the X-ray data from Chandra (red) and the lensing map constructed from gravitational lensing data (blue). The mismatch between the X-rays and the lensing map, as shown across a wide variety of X-ray emitting galaxy clusters, is one of the strongest indicators favoring the presence of dark matter.
Credit: X-ray: NASA/CXC/ITA/INAF/J.Merten et al, Lensing: NASA/STScI; NAOJ/Subaru; ESO/VLT, Optical: NASA/STScI/R.Dupke; Animation by E. Siegel

and the large-scale cosmic web,

dark matter
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role.
Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn

both normal and dark matter are required.

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. Regions of space must grow to be overdense by a large enough amount before dark energy comes to dominate the Universe’s expansion if they ever want to form a bound structure. Once dark energy takes over, it’s already too late.
Credit: Illustris Collaboraiton/Illustris Simulation

Today, our Universe’s composition is:

  • 5% normal matter,
  • 27% dark matter,
  • and 68% dark energy.
dark matter dark energy density contents
The matter and energy content in the Universe at the present time (left) and at earlier times (right). Note how dark matter and dark energy dominate today, but that normal matter is still around. At early times, normal matter and dark matter were still important, but dark energy was negligible, while photons and neutrinos were also quite important. The expansion rate is determined by the actual, instantaneous value for density, not by the distribution of the pie chart.
Credit: NASA/WMAP science team, modified by E. Siegel

But much of the normal matter remains elusive.

milky way twin analogue
The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. Despite the detectable presence of gas, dust, stars, black holes, and more, the amount of normal matter seen here is insufficient to add up to the expected amount of normal matter across the Universe.
Credit: ESA/Hubble & NASA

Galaxies contain stars, planets, gas, dust, and black holes, but not enough.

microlensing black holes
By observing a region of sky toward the galactic center over time, Hubble captured a large number of Milky Way stars and was able to tell how their brightness varied over time. One of those stars, highlighted in the inset panels from August of 2011 to August of 2017, brightened significantly but briefly: consistent with being caused by a microlensing event from a passing, intervening black hole. The observed microlensing rate is consistent with astronomers’ estimates that there are approximately 100 million roving black holes within our galaxy: not enough to account for the “missing baryons” required by the Universe.
Credit: ASA, ESA, Kailash Sahu (STScI); Processing: Joseph DePasquale (STScI)

The intergalactic medium, rich in ionized plasma, helps, but insufficiently so.

This visualization of the Laniakea supercluster, which represents a collection of more than 100,000 estimated galaxies spanning a volume of over 100 million light-years, shows the distribution of dark matter (shadowy purple) and individual galaxies (bright orange/yellow) together. Despite the relatively recent identification of Laniakea as the supercluster which contains the Milky Way and much more, it’s not a gravitationally bound structure and will not hold together as the Universe continues to expand. A large amount of the normal matter in the Universe isn’t found between the galaxies: in the intergalactic medium.
Credit: Tsaghkyan/Wikimedia Commons

Where’s the rest of the Universe’s normal matter?

M84 galaxy activity
Although an optical view of most galaxies will show only their stellar extent, an X-ray map, particularly for energetic galaxies, can reveal a much larger, more diffuse halo of normal matter surrounding galaxies. As shown here for galaxy M84 within the Virgo cluster, X-ray emissions go on for tens of thousands of light-years beyond the stellar extent of the galaxy.
Credits: X-ray: NASA/CXC/Princeton Univ/C. Bambic et al.; Optical: SDSS; Radio: NSF/NRAO/VLA/ESO; Image processing: NASA/CXC/SAO/N.Wolk

X-ray astronomy suggests a solution to this “missing baryons” problem: the circumgalactic medium.

circumgalactic medium simulation
These simulations of the circumgalactic medium, from the mid-2010s, shows how the expected density, temperature, and heavy element abundances will change as a function of cosmic time for a typical massive galaxy. The “missing baryons” problem may be solved by the presence of a very hot, very sparse circumgalactic medium.
Credit: EAGLE, Schaye et al., 2015, Oppenheimer et al., 2016

This sparse, diffuse material surrounds every massive galaxy: ranging far beyond their stellar extents.

energy injection fermi bubble
In the main image, our galaxy’s antimatter jets are illustrated, blowing ‘Fermi bubbles’ in the halo of gas surrounding our galaxy. In the small, inset image, actual Fermi data shows the gamma-ray emissions resulting from this process. These “bubbles” arise from the energy produced by electron-positron annihilation: an example of matter and antimatter interacting and being converted into pure energy via E = mc^2. Here on Earth, more positrons than can be explained by conventional physics are seen via direct cosmic ray experiments, putting forth the exciting possibility that dark matter might be the cause of both that excess and the galactic center gamma rays.
Credit: David A. Aguilar (main); NASA/GSFC/Fermi (inset)

With such low densities, very high temperatures are achieved.

simulation of ionized magnesium around a massive galaxy
This map shows the expected emission from ionized magnesium around a star-forming galaxy. Note that the emissions go out far beyond the stellar and disk extents of the galaxy itself: out well into the circumgalactic medium. Energy injection can cause low-density regions to achieve tremendous temperatures.
Credit: Max Planck Institute for Astrophysics

Its constituent atoms, as a result, lose most of their electrons.

first ionization energy atoms
This graph shows the first ionization energy of each atom in the periodic table by atomic number. This is how much energy it takes to strip the most loosely-held electron off of the atom entirely and ionize it. Elements in the first group of the periodic table, particularly lithium, sodium, potassium, rubidium, and so on, lose their first electron much more easily than any other elements. At even higher temperatures and energies, multiple electrons can be stripped off of an atom, creating a state of greater ionization.
Credit: Glrx/Wikimedia Commons

Several recent X-ray studies have detected this hot, ionized, galaxy-surrounding material.

Perseus cluster Chandra
NASA’s Chandra X-ray telescope detected the diffuse X-rays surrounding the Perseus galaxy cluster: evidence of a diffuse, hot, highly ionized collection of normal matter surrounding it.
Credit: NASA/CXC/Stanford/I.Zhuravleva et al.

Light from distant sources must travel through the circumgalactic medium to reach us.

warm-hot intergalactic medium sculptor wall
Galaxies can be found along, nearby, and within cosmic filaments. There is often both neutral and ionized matter within the haloes of these galaxies as well as along their line-of-sights, so when that light arrives, those absorption features seen in their spectra can tell us what the density and temperature of matter was in their own circumgalactic mediums, as well as for intervening galaxies and our own Milky Way.
Credit: CXC/M. Weiss; NASA/CXC/Univ. of California Irvine/T. Fang

Ionized atoms can absorb light at very specific frequencies.

quasar absorption lines
Distant sources of light — from galaxies, quasars, and even the cosmic microwave background — must pass through clouds of normal matter. The absorption features we see enable us to measure many features about the intervening gas clouds, including the abundances of the light elements inside and the degree of ionization.
Credit: Ed Janssen/ESO

We see this for our Milky Way,

silicon sulfur ionized circumgalactic medium
Ions of silicon and sulfur, with all but one of their electrons stripped away, have been detected in the halo of the Milky Way galaxy due to their absorption features. The stacked absorption signal from many galaxies, combined, is shown here.
Credit: A. Lara-Dl et al., ApJ, 2023

including omnidirectionally,

milky way sources circumgalactic
This map shows the extragalactic sources whose X-ray light spectra were stacked together to extract the absorption features of highly ionized silicon and sulfur. Similar studies with oxygen indicate that a diffuse, massive, very hot circumgalactic medium of normal matter is found around nearly all Milky Way analogue galaxies.
Credit: A. Lara-Dl et al., ApJ, 2023

for many different extragalactic sources,

detection ionized oxygen absorption lines
Absorption lines indicating the presence of six-times-ionized oxygen around several different galaxies is shown, pointing to evidence that a normal matter-rich circumgalactic medium is present around most Milky Way-like galaxies.
Credit: F. Nicastro et al., ApJL submitted, 2023

and even around faraway galaxies.

X-ray absorption Oxygen VII
Light from a distant galaxy shows key absorption lines from six-times ionized oxygen from its own circumstellar medium, providing strong evidence that the “missing baryons” around galaxies may actually be found in their circumgalactic mediums.
Credit: S. Mathur et al., MNRAS, 2023

The hot, diffuse circumgalactic medium may finally solve our Universe’s “missing baryons” problem.

circumgalactic medium
This illustration of the circumgalactic medium, in context with the rest of the galaxy, shows how the diffuse gas in most directions is a mix of gas from all sources together, including gas that has been recycled many times.
Credit: J. Tumlinson et al., ARAA, 2017

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