By using the best available measurements of the cosmic microwave background (CMB) from the Planck satellite, Dr. Carlos Hernández Monteagudo from the Centro de Estudios de Física del Cosmos de Aragón (CEFCA) and his collaborators have detected vast amounts of hidden baryonic matter around middle-size galaxies in the local universe. To arrive at this new result, the team has used the latest Planck data release and data from the Sloan Digital Sky Survey.

This new study solves the problem of the missing baryons around 200.000 local galaxies, and also contributes to the understanding of the distribution of baryonic matter in galaxies, groups and clusters of galaxies, an essential ingredient to know how these objects form and evolve.

Baryons are particles, such as protons and neutrons, that, together with electrons, make up the atoms found in the Universe. Nowadays, It is well known that around 90% of all baryons created in the first moments of the Universe after the Big Bang reside in the form of a diffuse, hot and ionized gas, esentially composed of protons and free electrons. However, only a small part of this gas, the hottest fraction, could be detected up to now.

By observing the fluctuations of the CMB light along the direction of those galaxies, the team has been able to detect clouds of ionized gas extending to distances significantly larger than the size of the galaxies. This corresponds to the so-called "missing" or "hidden" baryons surrounding those galaxies, whose spatial distribution is very close to the distribution expected for dark matter around those objects. The dark matter is the dominant form of matter in the Universe, which has not been directly detected or characterized yet.

“We have discovered a kick experienced by the CMB photons when they encounter moving electrons in ionized clouds. This leads to a tiny fluctuation in the temperature of the CMB around each of those galaxies, which we have nevertheless managed to detect statistically after stacking the CMB measurements in the direction of all of our 200,000 galaxies” states Dr. Hernández-Monteagudo. The high quality of Planck data allowed the team to reveal the presence of ionized gas out to a distance of 20 to 30 times the typical distance of luminous matter to the center of the galaxy. "Within those distances we have found as much gas as theory predicts we should find", he concludes.

This work, led by CEFCA, has been published in Physical Review Letters, in collaboration with researchers working in other institutions in Spain (IFCA in Cantabria, IAC in Spain) and other countries (UK, Germany and France).

C. Hernández-Monteagudo, Yin-Zhe Ma, F. S. Kitaura, W. Wang, R. Génova-Santos, J. Macías-Pérez, and D. Herranz. Evidence of the Missing Baryons from the Kinematic Sunyaev-Zeldovich Effect in Planck Data. Physical Review Letters. DOI: http://dx.doi.org/10.1103/PhysRevLett.115.191301