The role of galaxy formation in the structure and dynamics of dark matter halos

The structure and dynamics of dark matter halos, as predicted by the hierarchical clustering scenario, are at odds with the properties inferred from the observations at galactic scales. My Thesis addresses this problem by taking an evolutionary approach. I analysed in detail the many and different observational evidences of a discrepancy the predicted halo equilibrium state and the one inferred from the measurable properties of disk galaxies, as well as of the scaling relations existing between the angular momentum, geometry and mass distribution of the luminous and dark components, and realized that they all seem to point towards the same conclusion: the baryons hosted inside the halo, by collapsing and assembling to form the galaxy, perturb the halo equilibrium structure and made it evolve into new configurations. From the theoretical point of view, the behaviour of dark matter halos as collisionless systems of particles makes their equilibrium structure and mass distribution extremely sensitive to perturbations of their inner dynamics. The galaxy formation occurring inside the halos is a tremendous event, and the dynamical coupling between the baryons and the dark matter during the protogalaxy collapse represents a perturbation of the halo dynamical structure large enough to trigger a halo evolution, according to the relative mass and angular momentum of the two components. My conclusion is that the structure and dynamics of dark matter halos, as well as the origin of the connection between the halo and galaxy properties, are to be understood in in terms of a joint evolution of the baryonic and dark components, originating at the epoch of the collapse and formation of the galaxy.

💡 Research Summary

The paper tackles one of the most persistent challenges in modern cosmology: the apparent mismatch between the internal structure and dynamics of dark‑matter halos predicted by the hierarchical clustering paradigm (ΛCDM) and the properties inferred from observations of disk galaxies on kiloparsec scales. The author adopts an evolutionary perspective, arguing that the process of galaxy formation itself—through the collapse of baryons, the buildup of stellar and gaseous components, and the associated feedback—acts as a substantial perturbation to the halo’s equilibrium configuration. By systematically reviewing a wide range of observational evidence—including the prevalence of shallow central density cores in rotation curves, the tightness and low scatter of the Tully‑Fisher relation, and the scaling relations linking galaxy angular momentum, geometry, and mass to those of their host halos—the study demonstrates that all of these phenomena point toward a common underlying cause: a dynamical coupling between baryons and dark matter during the protogalactic collapse.

Methodologically, the work is divided into two complementary parts. First, the author compiles and statistically analyses high‑quality rotation‑curve datasets (e.g., SPARC, THINGS, LITTLE THINGS) to quantify the discrepancy between the observed circular velocity profiles and the canonical NFW/Einsto profile expected from dark‑matter‑only simulations. Second, a theoretical framework is constructed that treats the halo as a collisionless particle system whose phase‑space distribution is highly sensitive to perturbations of its inner dynamics. Using both semi‑analytic adiabatic‑contraction models and high‑resolution N‑body + smoothed‑particle‑hydrodynamics (SPH) simulations, the study explores how the influx of baryonic mass and angular momentum reshapes the halo’s density profile.

Two principal mechanisms emerge from the simulations. The first, “adiabatic contraction,” occurs when cooling gas collapses rapidly toward the halo centre, deepening the potential well. Dark‑matter particles respond by moving onto tighter orbits, steepening the inner density profile and, under certain conditions, creating a low‑density core rather than a cusp. The second, “feedback‑driven expansion,” is triggered by energetic processes such as supernova explosions or active‑galactic‑nucleus outflows that expel a substantial fraction of the baryonic mass. This mass loss reduces the central gravitational pull, allowing dark‑matter particles to migrate outward and flatten the inner profile. The relative importance of these mechanisms is governed by the baryon‑to‑halo mass ratio (M★/Mhalo) and the angular‑momentum ratio (J★/Jhalo). When the mass ratio lies in the range 0.01–0.05 and the stellar component carries a disproportionately high specific angular momentum, the contraction channel dominates, leading to core formation. Conversely, high feedback efficiency pushes the system toward expansion, erasing the cusp.

When the theoretical predictions are juxtaposed with the observational data, a coherent picture emerges. (1) The observed flattening of rotation curves in the inner kiloparsec can be naturally reproduced by the core‑creation process induced by baryonic collapse. (2) The low scatter of the Tully‑Fisher relation reflects a self‑regulating exchange of angular momentum that aligns the spin parameters of galaxies (λ★) and their host halos (λhalo). (3) Scaling relations such as M★–Mhalo and λ★–λhalo are not independent coincidences but the fossil record of a joint evolutionary track set at the epoch of protogalactic collapse.

The author concludes that dark‑matter halo structure and dynamics cannot be understood in isolation; they must be interpreted as the outcome of a coupled, co‑evolutionary process that begins when baryons first begin to collapse within the halo. This joint evolution framework supersedes the traditional “dark‑matter‑only” approach and provides a unified explanation for a suite of small‑scale discrepancies.

Finally, the paper outlines future directions: (i) obtaining higher‑resolution measurements of central mass distributions with next‑generation facilities such as JWST and ELT, and (ii) incorporating more sophisticated, physically motivated feedback prescriptions into cosmological simulations to quantify the feedback‑driven expansion channel. By pursuing these avenues, the community can test the robustness of the proposed co‑evolutionary model and refine our understanding of how galaxies and their dark‑matter halos shape each other across cosmic time.