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Newton, Fermi and Dirac meet in Galaxies

In an article published in August 2014 in the journal Monthly Notices of the Royal Astronomical Society, scientists from the Observatory of Paris, Pierre and Marie Curie University and SISSA of Trieste (Italy), describe the main observed properties of galaxies with a new theory which involves dark matter and gravity. The Universe is composed of 81% dark matter, different from the ordinary visible matter formed by atoms. Invisible, dark matter mainly manifests through the action of gravity; it is one of the essential components of galaxies.

The proportion of dark matter in the total mass of the galaxies varies according to their sizes: 95% for large diluted galaxies (spiral, elliptical and others) up to 99.99% for compact dwarf galaxies. In such proportions, dark matter and gravity play a decisive role in the formation and structure of galaxies.

Using a theory involving these two components, a European team of scientists, including a scientist from SISSA managed to reproduce the observed properties of galaxies, namely their circular velocities, their density distributions and the relationship between their masses, sizes and velocities.

Warm Dark Matter

This new theoretical approach is based on the supposed existence of the dark matter called "warm", as opposed to the "cold" dark matter. The "warm" dark matter is composed of particles with mass in the scale of thousand electron volts (i.e. 1 keV, in this units an electron has a mass of 511 keV). In the "cold" dark matter, the particles are much heavier and slower. The work is based on the microscopic properties of particles of warm dark matter, that is to say, their quantum properties (such as the spin and the statistics associated with the particles): a repulsive pressure is generated, acting against the attractive force of gravity.

For this work, ten different independent data sets of galaxies have been used, each set with galaxy masses ranging from 5 × 10^9 to 5 × 10^11 solar masses.
The rotation curves of galaxies and density distributions obtained from the above WDM model results consistent with these latter observed quantities. And a remarkable feature emerges: the theoretical rotation curves and density profiles are universal: they are well defined functions according to the galaxy of mass. The compact dwarf galaxies are thus close to the high dense gas (Fermi gas or gas of degenerate fermions), while larger and intermediate galaxies (spiral, elliptical and others) are close to the classical diluted gas (known as Boltzmann ). One, of the many candidates for this particle is the "sterile neutrino". These studies also confirm the small importance of the baryonic matter (the "ordinary" matter), being a small fraction (up to fe per cent) of the galaxy mass. 

This novel theoretical approach to the structure of galaxies puts together gravitation and quantum properties of dark matter. It has its counterpart in the statistical approach used for atoms (known under the name of "Thomas-Fermi"), with gravitation playing the role of the electric potential; in other words, "Newton, Fermi and Dirac meets together in galaxies through dark matter," points out Paolo Salucci, one of the authors of the study.