ESA’s Planck satellite finds the missing baryons

Where is the half of the ordinary matter in the universe not observed yet? Computer simulations of cosmological galaxy formation predict the existence of large intercluster filaments of hot and low-density gas, the so-called cosmic web. For the first time, ESA’s Planck satellite has observed one of such filaments in the merging cluster pair A399-A401; the discovery is based on the thermal Sunyaev-Zel’dovich effect. The observed temperature and baryon density are consistent with previous theoretical estimates, pointing towards a solution to the missing baryon conundrum. This new result is Part VIII of a series of 11 papers to be published by the Planck Collaboration in the scientific journal Astronomy & Astrophysics (the paper has 210 authors, 19 of which are affiliated with a Spanish institution) 1.

Based on the current experimental data, the contents of the universe, interpreted using the consensus cosmological model (or ΛCDM), are dark energy (72.8%), dark matter (22.7%) and ordinary (baryonic) matter (4.5%) 2. The baryon content can be classified in four classes based on the results of cosmological hydrodynamic galaxy formation simulations 3: (1) about the 12% is the cold gas condensed in galaxies, including stars and interstellar gas; (2) about the 26% is the warm gas, with temperature T < 105 K, in the clouds of neutral hydrogen in the intergalactic medium, between distant quasars and the Earth, that absorb ultraviolet light at the wavelength of the Lyman alpha line of hydrogen at a wavelength of 122 nm (the so-called Lyman-alpha forest); (3) about the 52% is the warm-hot gas, with 105 K < T < 107 K, mainly in unvirialized intercluster regions, i.e., filaments and oblate non-spherical regions where the equilibrium between the total kinetic and potential energies has not been reached, hence the so-called virial theorem does not apply; and (4) the last 12% is the hot X-ray emitting gas, with T > 107 K, in collapsed and virialized clusters of galaxies (regions having a nearly spherical shape as expected from the virial theorem). Before Planck satellite mission, there was no experimental evidence of the warm/hot gas, i.e., about half of the baryons of the universe had not been observed.

Planck satellite has observed the warm-hot intergalactic medium (WHIM) at low redshifts for the first time thanks to the thermal Sunyaev–Zel’dovich effect (tSZ), the distorsion of the cosmic microwave background (CMB) radiation due to the interaction of high-energy electrons of the WHIM with the low-energy CMB photons (a physical process referred to as inverse Compton scattering). Using the tSZ effect the Planck satellite can observe dense clusters of galaxies, although not without difficulties due to the small magnitude of the effect. However, the tSZ effect is independent of the redshift, hence both high and low redshift clusters can be observed.

A systematic search for the diffuse filamentary-like structures between pairs of merging clusters has resulted in its first identification by Planck satellite. The gas around clusters is expected to be hotter and denser than the WHIM in filaments, making its direct detection very difficult. However, the increase of the pressure of the gas between pairs of clusters in mutual interaction enhances the tSZ signal, making it easier to detect in such a case. Thanks to the full-sky coverage and wide range of frequencies, together with the extremely high sensitivity of the Planck satellite instruments have made possible the production of reliable enough maps of the tSZ emission of such filaments 4.

The Planck collaboration has studied 25 pairs of clusters of galaxies, but only found a clear signal compatible with an intercluster filament for two of these pairs: A399-A401 and A3391-A3395. The clearest one was observed in the case of the A399-A401 pair, being consistent with simulated data; hence, it may constitute the first detection of the tSZ effect between clusters. Figure 1 and 2 show the tSZ emission of the pair of clusters A399-A401 and a one-dimensional tSZ longitudinal profile, respectively [1].

Figure 1. The MILCA (Maximum Internal Linear Component Analysis) map of the Compton parameter for the A0399-A0401 (top) and A3391-A3395 (bottom) pair of clusters. | Credit: Planck Collaboration Ref. [1].

Figure 2. The 1D tSZ profile for the best-fit models of the A399 and A401 clusters; the red curve corresponds to a theoretical model. | Credit: Planck Collaboration Ref. [1].
In order to highlight the filament between the merging clusters, it is necessary to remove the contribution of each cluster by using a theoretical model. The new paper uses three different models for the clusters in order to obtain a model independent analysis of the WHIM filaments. Furthermore, to improve the quality of these models, the Planck data have been complemented with X-ray observations from the German X-ray telescope ROSAT (Röntgensatellit) of both merging clusters; it is important to note that ROSAT sees no signal in the intercluster region.

The amount of signal in the intercluster region was then used to constrain the parameters of the intercluster filament. The authors find that the temperature of the filament is (7.08 ± 0.85) keV, i.e., about 82 million degrees, with a central baryon density of (3.72 ± 0.17) 10−4 cm−3, about one Hydrogen molecule in the volume of a cube of side length 14 cm. These results qualitatively agree with hydrodynamical simulations in which the intercluster region between an interacting pair of clusters can show a significant tSZ signal. Although the intercluster region might be a mix of cluster and intergalactic material, a significant part of the signal seen by Planck corresponds to baryons outside and between the clusters.

The statistical significance of the new signal, under the interpretation of being the result of an intercluster filament, indicates that it is not caused by any known artifact in the data. But let us recall that the exact interpretation is open to speculation, as usual in Science for the first observation of any new phenomena. Only new observations of the tSZ signal of merging clusters and intercluster filaments would improve the modelling of the observed signal and reduce the uncertainties in the estimation of the excess associated to the intercluster region.

In summary, the new result of Planck mission points towards an expected solution of the missing baryon puzzle: the WHIM filaments of gas of the cosmic web connecting the galaxy clusters. But only new observations of gas bridges connecting other clusters will provide a conclusive answer.


  1. Planck Collaboration, “Planck intermediate results. VIII. Filaments between interacting clusters,” Astronomy & Astrophysics, Accepted for publication, November 20, 2012 [arXiv:1208.5911].
  2. N. Jarosik et al., “Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Sky maps, systematic errors, and basic results,” The Astrophysical Journal, Supplement Series, 192: 14, February 2011 [; arXiv:1001.4744].
  3. Renyue Cen, Jeremiah P. Ostriker, “Where Are the Baryons?,” The Astrophysical 514: 1-6, March 1999 [; arXiv:astro-ph/9806281].
  4. K. Dolag et al., “The imprints of local superclusters on the Sunyaev–Zel’dovich signals and their detectability with Planck,” Monthly Notices of the Royal Astronomical Society 363: 29-39, October 2005 [; arXiv:astro-ph/0505258].

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