SANTA CRUZ DE TENERIFE, Feb. 15 (EUROPE PRESS) –
A team of researchers from the Cosmology group of the Instituto de Astrofísica de Canarias (IAC) has obtained one of the most precise measurements of the mass contained in clusters of galaxies and its relationship with the amount of hot gas in these structures. To do this, the dynamics of the galaxies in 570 clusters selected from the catalogs provided by the Planck satellite (ESA) have been studied.
This study has culminated a four-year observational effort, in which more than 10,000 spectra have been obtained at the Telescopio Nazionale Galileo (TNG) and the Gran Telescopio Canarias (GTC), located at the Roque de los Muchachos Observatory, as well as data from the public archive of the Sloan Digital Sky Survey (SDSS). The result allows us to infer the amount of dark matter that exists in the Universe.
Galaxy clusters are the largest gravitationally bound structures in the Universe. The characterization of its spatial and temporal distribution provides one of the most powerful tools in modern cosmology to determine the origin and energy content of the Universe, the rate of expansion during cosmic evolution, and the details of the formation process of all structures in the Universe. large scale that we observe in the Universe today.
These immense groups are not only made up of galaxies, which barely make up 2% of the total mass of the clusters. In addition, they contain extremely hot gas, which accounts for 15% of their mass, and a lot of dark matter, more than 80% of the total mass. This ‘multicomponent’ nature of galaxy clusters means that they can be studied using a wide variety of observational techniques. Thus, while the hot gas can be seen in X-rays and microwaves, the galactic component is mainly visible in the optical and infrared.
On the other hand, to study the distribution of dark matter, several indirect methods are used. Generally, the total mass (including the dark component) is plotted using the gravitational lensing effect produced by the large concentration of matter in clusters on images of objects along their line of sight.
These measurements typically provide high accuracy. However, this new study has used the measurement of the galaxies’ velocities to infer the total gravity present in these structures, achieving for the first time a precision comparable to measurements of gravitational lensing.
Through the microwave maps of the Planck satellite, it has also been possible to estimate how much gas mass the clusters contain and, indirectly, to infer the total mass. One would expect all of these techniques to agree, yielding a single value for the mass of the clusters. However, this does not happen, because each technique suffers from certain intrinsic differences. But by comparing them, these differences can be known precisely and, therefore, a better understanding of the real mass of galaxy clusters can be obtained.
HISTORICAL MILESTONE.
With this goal in mind, the team of researchers has used the two catalogs PSZ1 and PSZ2 of galaxy clusters detected by the Planck satellite, which offer an estimate of the mass of the clusters from their microwave signal. On the other hand, the dynamic mass of these clusters has been estimated by measuring the radial velocities of the galaxies with spectroscopy techniques.
These observations have been made at the Telescopio Nazionale Galileo (TNG) and the Gran Telescopio Canarias (GTC) at the Roque de los Muchachos Observatory. “We have made a great observational effort over the years, with which we have explored more than 400 galaxy clusters from the PSZ1 and PSZ2 catalogs,” says Rafael Barrena, astrophysicist at the IAC and member of the group of researchers.
“The results obtained for Planck’s PSZ1 catalog show that mass estimates based on the amount of gas provide values 20% lower than those based on the dynamics of their galaxies”, says Antonio Ferragamo, an IAC researcher who has focused his thesis doctorate in the present study. “These results are confirmed for the clusters in Planck’s PSZ2 catalog and are compatible with measurements that other authors have obtained using different techniques (X-rays, gravitational lensing, etc.),” he explains.
For his part, Alejandro Aguado, a researcher at the IAC who has also completed his doctoral thesis based on the results of this work, stresses that “the importance of the study lies, on the one hand, in the high precision obtained for the dynamic measurement of the , with an error of less than 10%, which is a milestone in the history of measurements of this magnitude, and on the other hand, its consistency with the value estimated by the Planck consortium (Planck Col. XX, 2014)”. And he adds: “Today, very few measurements offer such a low statistical error and this is due to the large number of cumulus clouds used in the sample.”
NEW QUESTIONS IN COSMOLOGY
From a cosmological perspective, the confirmation of this value raises new questions. “With data from the Planck satellite we find that there is an apparent tension between the amount of dark matter in the Universe predicted from galaxy cluster abundance measurements and that inferred from maps of the cosmic microwave background (CMB), fossil radiation from the origins of the Universe”, explains José Alberto Rubiño, astrophysicist at the IAC and member of the group of researchers.
“Dark matter values inferred from galaxy cluster abundances are 5% higher than those based on CMB measurements, and the latter in turn indicate that the number of galaxy clusters we have detected should have been slightly higher,” he says.
The results found with this work, which have been recently published in the specialized journal ‘Astronomy & Astrophysics’, show that the abundances of galaxy clusters and the CMB data produce slightly different cosmological models, a tension that was already anticipated by the collaboration Planck.
“If the difference between the mass inferred from the gas and the dynamic mass had turned out to be around 35% instead of the 20% we measured, this slight tension between the reconstructed cosmological models would have disappeared,” says Rubiño.
“Resolving this discrepancy may provide us with a tool to determine the mass of neutrinos in the future through their effects in Cosmology or even give us indications of the need to introduce modifications to the standard cosmological model or to General Relativity”, underlines the researcher.
“The explanation for this apparent discrepancy is one of the great questions in current cosmology, which remains after our work and which will have to be investigated further in years to come”, says Barrena. “Space missions such as EUCLID (ESA) or Litebird (JAXA/ESA), in which the IAC actively participates, will play an essential role in the very near future”, she concludes.
Other IAC researchers who have participated in this project are Alina Streblyanska, Denis Tramonte, Ricardo Génova Santos, Angela Hempel and Heidi Lietzen.
