Category Archives: Highlights on physics papers

Il ruolo nascosto delle anisotropie nel plasmare la formazione delle strutture nelle simulazioni cosmologiche N-body

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Il ruolo nascosto delle anisotropie nel plasmare la formazione delle strutture nelle simulazioni cosmologiche N-body, di Francesco Sylos Labini, Physical Review D

In questo lavoro mostriamo che la formazione dei filamenti nelle simulazioni cosmologiche N-body deriva dall’amplificazione di anisotropie già presenti nelle condizioni iniziali. Tali anisotropie non dovrebbero essere presenti se le condizioni iniziali riproducessero correttamente il campo di densità previsto dai modelli cosmologici standard, vale a dire un campo uniforme e statisticamente isotropo.


Concludiamo pertanto che le strutture filamentari osservate nelle simulazioni cosmologiche N-body sono in larga misura spurie, nel senso che riflettono artefatti delle condizioni iniziali piuttosto che esiti fisici genuini dei modelli cosmologici sottostanti. Per giungere a queste conclusioni, abbiamo sviluppato un nuovo strumento statistico — la distribuzione angolare delle distanze a coppie e la sua varianza dipendente dalla scala — in grado di identificare strutture filamentari e di mettere in relazione in modo inequivocabile le misure osservative con le previsioni teoriche.

The Hidden Role of Anisotropies in Shaping Structure Formation in Cosmological N-Body Simulations

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The Hidden Role of Anisotropies in Shaping Structure Formation in Cosmological N-Body Simulations by Francesco Sylos Labini, Physical Review D 2026

In this work we show that the formation of filaments in cosmological N-body simulations arises from the amplification of anisotropies already present in the initial conditions. Such anisotropies should not occur if the initial conditions correctly reproduce the density field predicted by standard cosmological models, namely a uniform and statistically isotropic field.

We therefore conclude that the filamentary structures observed in cosmological N-body simulations are largely spurious, in the sense that they reflect artefacts of the initial conditions rather than genuine outcomes of the underlying cosmological models. To reach these conclusions, we have developed a new statistical tool—the angular distribution of pairwise distances and its scale-dependent variance—which is capable of identifying filamentary structures and relating observational measurements unambiguously to theoretical predictions.

Large-Scale Galaxy Correlations from the DESI First Data Release

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We quantify galaxy correlations using two distinct three-dimensional samples from the first data release of the Dark Energy Spectroscopic Instrument (DESI): the Bright Galaxy Sample (BGS) and the Luminous Red Galaxy Sample (LRGS). Specifically, we measure the conditional average density, defined as the average density of galaxies observed around a typical galaxy in the sample. To minimize boundary effects, we adopt a conservative criterion: only galaxies for which a spherical volume of radius r, centered on them, is fully contained within the survey footprint are included in the computation. For the BGS, we construct four volume-limited subsamples in order to eliminate biases arising from luminosity-dependent selection effects. By contrast, the LRGS is approximately volume-limited by design. The resulting samples span different depths, providing an opportunity to test the stability of statistical measurements across survey volumes of increasing size. Our results show that the conditional average density follows a power-law decay, ⟨n(r)⟩∝r−0.8, without exhibiting any transition to homogeneity within the survey volume. The large statistics of the DESI samples also allow us to demonstrate that finite-size effects become significant as r approaches the boundaries of the sample volumes. Consistently, we find that the distribution of density fluctuations follows a Gumbel distribution – characteristic of extreme-value statistics – rather than a Gaussian distribution, which would be expected for a spatially homogeneous field. These findings confirm and extend the trends previously observed in smaller redshift surveys, supporting the conclusion that the galaxy distribution does not undergo a transition to spatial homogeneity within the probed scales, up to r∼400~\text{Mpc}/h.

Comments:14 pages 12 figures
Subjects:Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as:arXiv:2511.21585 [astro-ph.CO]
 (or arXiv:2511.21585v1 [astro-ph.CO] for this version)
 https://doi.org/10.48550/arXiv.2511.21585

The Hidden Role of Anisotropies in Shaping Structure Formation in Cosmological N-Body Simulations

Highlights on physics papers,

Francesco Sylos Labini

Initial conditions in cosmological N-body simulations are typically generated by displacing particles from a regular cubic lattice using a correlated field derived from the linear power spectrum, often via the Zel’dovich approximation. While this procedure reproduces the target two-point statistics (e.g., the power spectrum or correlation function), it introduces subtle anisotropies due to the underlying lattice structure. These anisotropies, invisible to angle-averaged diagnostics, become evident through directional measures such as the Angular Distribution of Pairwise Distances. Analyzing two Cold Dark Matter simulations with varying resolutions, initial redshifts, and box sizes, we show that these anisotropies are not erased but are amplified by gravitational evolution. They seed filamentary structures that persist into the linear regime, remaining visible even at redshift z=0. Our findings demonstrate that such features are numerical artifacts — emerging from the anisotropic coupling between the displacement field and the lattice — not genuine predictions of an isotropic cosmological model. These results underscore the importance of critically reassessing how initial conditions are constructed, particularly when probing the large-scale, quasi-linear regime of structure formation.

Comments:20 pages, 17 figures
Subjects:Cosmology and Nongalactic Astrophysics (astro-ph.CO); Statistical Mechanics (cond-mat.stat-mech)
Cite as:arXiv:2508.13765 [astro-ph.CO]
 (or arXiv:2508.13765v1 [astro-ph.CO] for this version)
 https://doi.org/10.48550/arXiv.2508.13765Focus to learn more

Breaking the Degeneracy between Warps and Radial Flows in External Galaxies

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The American Astronomical Society, find out more.

Francesco Sylos Labini, Giordano De Marzo, and Matteo Straccamore

Published 2025 July 17 • © 2025. The Author(s). Published by the American Astronomical Society. The Astrophysical Journal, Volume 988, Number 1

Citation Francesco Sylos Labini et al 2025 ApJ 988 122

DOI 10.3847/1538-4357/adc71c

Abstract

Observations of the line-of-sight (LOS) component of emitter velocities in galaxies are valuable for reconstructing their two-dimensional (2D) velocity fields, albeit requiring certain assumptions. A common one is that radial flows can be neglected in the outer regions of galaxies, while their geometry can be deformed by a warp. A specular approach assumes that galactic disks are flat but allows for the presence of radial flows. This approach enables the reconstruction of 2D velocity maps that encompass both the transversal and radial velocity fields. Through the study of velocity fields in toy disk models, we find that the presence of warps is manifested as a dipolar correlation between the two velocity components obtained by assuming a flat disk. This shows that the analysis of angular velocity anisotropies provides an effective tool for breaking the degeneracy between warps and radial flows. We have applied these findings to the analysis of velocity fields of the galaxies from the THINGS sample and M33. Many of these galaxies exhibit such a dipolar correlation, indicating the presence of warps. However, we have found that the warp alone cannot explain all variations in the velocity field, suggesting that intrinsic perturbations are common. Furthermore, we have observed that the spatial distribution of the LOS velocity dispersion may correlate with both velocity components, providing independent evidence of nontrivial velocity fields. These findings offer a robust approach to reconstructing the velocity fields of galaxies, allowing us to distinguish between the presence of warps and complex velocity structures by assessing their relative amplitude.

Breaking the degeneracy between warps and radial flows in external galaxies

Highlights on physics papers,

Francesco Sylos Labini, Giordano De Marzo, Matteo Straccamore

Observations of the line-of-sight component of emitter velocities in galaxies are valuable for reconstructing their 2D velocity fields, albeit requiring certain assumptions. A common one is that radial flows can be neglected in the outer regions of galaxies, while their geometry can be deformed by a warp. A specular approach assumes that galactic discs are flat but allows for the presence of radial flows. This approach enables the reconstruction of 2D velocity maps that encompass both the transversal and radial velocity fields. Through the study of velocity fields in toy disc models, we find that the presence of warps is manifested as a dipolar correlation between the two velocity components obtained by assuming a flat disc. This shows that the analysis of angular velocity anisotropies provides an effective tool for breaking the degeneracy between warps and radial flows. We have applied these findings to the analysis of velocity fields of the galaxies from the THINGS sample and M33. Many of these galaxies exhibit such a dipolar correlation, indicating the presence of warps. However, we have found that the warp alone cannot explain all variations in the velocity field, suggesting that intrinsic perturbations are common. Furthermore, we have observed that the spatial distribution of the line-of-sight velocity dispersion may correlate with both velocity components providing independent evidence of non-trivial velocity fields. These findings offer a robust approach to reconstructing the velocity fields of galaxies, allowing us to distinguish between the presence of warps and complex velocity structures assessing their relative amplitude.

Comments:69 pages, 56 figures, accepted for publication in The Astrophysical Journal
Subjects:Astrophysics of Galaxies (astro-ph.GA)
Cite as:arXiv:2503.22306 [astro-ph.GA]
 (or arXiv:2503.22306v1 [astro-ph.GA] for this version)
 https://doi.org/10.48550/arXiv.2503.22306Focus to learn more

Exploring the Dark Matter Disc Model in Dwarf Galaxies: Insights from the LITTLE THINGS Sample

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Francesco Sylos Labini, Roberto Capuzzo-Dolcetta, Giordano De Marzo, Matteo Straccamore

ABSTRACT We conducted an analysis of the velocity field of dwarf galaxies in the LITTLE THINGS sample, focusing on deriving 2D velocity maps that encompass both the transverse and radial velocity fields. Within the range of radial distances where velocity anisotropies are sufficiently small for the disc to be considered rotationally supported, and where the warped geometry of the disc can be neglected, we reconstructed the rotation curve while taking into account the effect of the asymmetric drift. To fit the rotation curves, we employed the standard halo model and the dark matter disc (DMD) model, which assumes that dark matter is primarily confined to the galactic discs and can be traced by the distribution of HI. Interestingly, our analysis revealed that the fits from the DMD model are statistically comparable to those obtained using the standard halo model, but the inferred masses of the galaxies in the DMD model are approximately 10 to 100 times smaller than the masses inferred in the standard halo model. In the DMD model, the inner slope of the rotation curve is directly related to a linear combination of the surface density profiles of the stellar and gas components, which generally exhibit a flat core. Consequently, the observation of a linear relationship between the rotation curve and the radius in the disc central regions is consistent with the framework of the DMD model.

Continue reading Exploring the Dark Matter Disc Model in Dwarf Galaxies: Insights from the LITTLE THINGS Sample →

Curve di rotazione generalizzate della Via Lattea dai dati del GAIA DR3: vincoli sui modelli di massa

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Uno dei grandi problemi dell’astrofisica è capire quanto sia e come sia distribuita la materia oscura. Nel caso della nostra galassia, la Via Lattea, per studiare questo problema si parte delle osservazioni della distribuzione delle stelle e del gas e dal loro campo di velocità.

Grazie ai dati del satellite Gaia è oggi possibile analizzare grandi campioni di stelle in cui è nota sia la posizione tridimensionale di ognuna, che le tre componenti della velocità. Da queste osservazioni si può dunque ricostruire il campo di velocità in tre dimensioni, cioè si può caratterizzare la cinematica della Via Lattea.

Continue reading Curve di rotazione generalizzate della Via Lattea dai dati del GAIA DR3: vincoli sui modelli di massa →

Generalized rotation curves of the Milky Way from the GAIA DR3 data-set: constraints on mass models

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https://doi.org/10.48550/arXiv.2410.14307

One of the major challenges in astrophysics is understanding how much dark matter exists and how it is distributed. In the case of our galaxy, the Milky Way, to study this problem, we start with observations of the distribution of stars and gas and their velocity field.

Thanks to data from the Gaia satellite, it is now possible to analyze large samples of stars for which both the three-dimensional position and the three components of velocity are known. From these observations, it is possible to reconstruct the velocity field in three dimensions, allowing us to characterize the kinematics of the Milky Way.

To connect kinematics to dynamics, we must assume that the galaxy has reached a steady equilibrium, where stars move in closed circular orbits and the entire system is stable and does not change over time.

Continue reading Generalized rotation curves of the Milky Way from the GAIA DR3 data-set: constraints on mass models →

Dark Matter in Galaxies

Highlights on physics papers

In astrophysics, to calculate the mass of a celestial body or a system of bodies, such as the Sun or our galaxy, there are essentially two methods: on one hand, one can measure the velocity that characterizes their motion, or on the other hand, one can measure the distortion of light they induce in distant sources, exploiting the gravitational lensing effect. In the first case, to find the mass from a velocity, it is necessary to make a key assumption: that the system is in a state of steady equilibrium.

Continue reading Dark Matter in Galaxies →