What is left of the rest of the universe? The rest is matter we can think with relief. However, the matter that we know, which forms galaxies, their stars, nebulae, planets and the entire list of magnificent celestial objects, represents barely 4% of the total energy density balance of the universe (remember that energy and matter are equivalent ). The rest of the recipe, that remaining 26%, constitutes another great mystery: "dark matter."

In a bizarre comparison to humans, "normal" matter would be "extroverted" matter. It manifests itself in countless ways: we can detect it because it emits light (for example, in stars) or reflects it (like planets), it exerts the force of gravity by assembling stars, stellar systems, or the components of galaxies, it interacts with other "normal" matter by magnetic or electrical forces. That is, it can be detected directly with appropriate telescopes and instruments. In addition, it is a matter that makes us up, so we can study it and know it in depth.

However, most of the matter in the universe, dark matter, is... "introverted." That stuff can be everywhere, even in front of our best detectors, but we can't see it. Timidly, it only makes itself known through the gravitational force. It does not shine or interact with other forces of nature except through gravitation. And since it vastly exceeds ordinary matter in quantity, it conditions many properties of galaxies, such as their shape, kinematics, connection with other galaxies, and the distribution of matter within them.

Framed in the arduous efforts to understand the nature of dark matter and its influence on galaxies, Dr. Carlos R. Arguelles, IALP (CONICET-UNLP) researcher and professor at the UNLP Faculty of Exact Sciences and ICRANet, together with a team of researchers from Italy and France, has published March 1, 2023, an article that sheds new light on the subject, in the prestigious scientific journal The Astrophysical Journal.

This group of astronomers has built a theoretical model for the distribution of dark matter in galaxies, which predicts the formation of large regions called "halos." They propose that dark matter is a subatomic particle of the "fermion" class. Fermions constitute an extensive group of particles that encompasses, due to their quantum properties, for example, electrons, protons, and neutrons. Fermions differ from "bosons" by spin, a quantum property that makes their behavior different.

Among the bosons, the best-known particle is the photon (the particle of light). The proposed model considers only that dark matter fermions must be light (less massive than an electron), neutral (no electrical charge), and experience no force except gravity. Therefore, the researchers build their model using the general properties of fermions and gravity, drawing directly from known physical principles. In this way, the researchers demonstrate that the fermionic dark matter halo model can simultaneously explain many of the properties observed in ordinary matter (which is not enough to account for them), such as its distribution and their inner kinematics in different classes of galaxies.

One of the most important results of the work refers to the rotation of those galaxies whose matter is distributed in the form of a flat disk (a rotating disk made up of billions of stars and nebulae of gas and dust, as seen in the figure). In those cases, the model can account for the way in which the stars belonging to these galactic disks rotate under the influence of the dark matter halo, detailing their speeds from their innermost regions to their outer edges.

Astronomers use a graph of the rate of rotation velocity versus distance from the center of the galaxy: a "galactic rotation curve" (see inset figure below). For example, if the stars and components in the central region of a galaxy rotate faster than the stars at the edges of the galaxy, the rotation curve will have a decreasing slope. Dark matter was discovered precisely (back in the '60s and '70s) because the rotation curves had a peculiar shape. They were roughly flat in their outer regions, which required the existence of much more matter than observed.

The paper's authors used the observational rotation curves of 120 galaxies from the SPARC (Spitzer Photometry and Accurate Rotation Curves) database obtained with the Spitzer Space Telescope. With this huge amount of data, they distinguished the contribution to galactic rotation attributable to ordinary matter from that corresponding to dark matter, allowing to test the model. Next, they compared the predictions of this with the results of other previously published models that use different hypotheses about the distribution of dark matter, showing the differences and advantages of the proposed model.

One of the most relevant predictions they obtain is the distribution of dark matter from the galactic center to the outer edge. The fermionic halo model predicts a spherical distribution of heterogeneous dark matter, which is highly concentrated towards the center, being more dilute towards the outer region occupied by the disk. This result agrees with the observations of galactic rotation curves (for which data are only accessible in the outer parts) but include a novel prediction regarding its distribution in the most central region. This overall distribution of matter cannot be reproduced by other dark matter models based on numerical N-body simulations of current use (see inset figure above).

The fermionic dark matter halo model represents a potential advance in the interpretation of many observed properties of galaxies. Until now, these properties were explained by phenomenological models based on numerical simulations. Instead, this is a unifying model, based on the basic physical properties of fermions and gravity, on their ability to contribute, as dark matter, to the internal structure and dynamics of galaxies. In this way, the work of Arguelles and his collaborators brings us one step closer to the long-awaited goal of unveiling the unfathomable mystery of the nature of dark matter.

Research Report:"Galaxy rotation curves and universal scaling relations: comparison between phenomenological and fermionic dark matter profiles"

Related Links
UNLP Faculty of Exact Science
Stellar Chemistry, The Universe And All Within It