arXiv subjects

Nicolas Estrada

Publications and source records attributed to Nicolas Estrada.

5 recordsLinked to original sources

arXiv2026

Origin of geometric cohesion in non-convex granular materials: interplay between interdigitation and rotational constraints enhancing frictional stability

We present a series of experiments investigating the local microstructure of cylindrical piles composed of highly concave particles. By systematically varying particle geometry -- from spheres to strongly non-convex polypods -- as well as frictional properties and the number of branches, we explore how these parameters, together with the preparation protocol, shape the internal structure of the system. Using X-ray tomography combined with a dedicated image-analysis pipeline, we accurately extract the position, orientation, and contacts of every particle in each pile. This allows us to quantify the evolution of key structural observables as a function of particle geometry and preparation method. In particular, we measure the distributions of local packing fraction, coordination number, number of neighbors, and contact locations, along with particle-particle positional and orientational correlations. More importantly, we construct a new stability indicator that correlates perfectly with the observed pile stabilities, enabling us to identify the fundamental mechanisms responsible for \textit{geometrically induced cohesion} in granular systems composed of non-interlocking particle shapes: interdigitation, rotational constraint, friction-mediated cohesion, and the ability of a pile to re-stabilize.

cond-mat.soft↗

arXiv2025

Galaxies at the edges: a complete census of MACS J0416.1-2403 cluster

Numerous studies have established that the environment influences the physical properties of a galaxy. While gas inflows supply the fuel for SF, high density and temperature conditions suppress SF activity through various quenching processes. Investigations into large scale structures, such as filaments and overdense regions in the cluster outskirts, have focused on the low z. To move to intermediate z and explore galaxy pathways combined with environmental effects, it is crucial to join wide field spectroscopy and deep photometry. Our primary objective is to spectroscopically analyze the photometric overdensities observed by Estrada et al.(2023) in the outskirts of massive cluster MACS J0416.1-2403 (z=0.397), interpreted as evidence of ongoing group infall, i.e. the pre processing scenario, and to investigate the behavior of galaxies in the outskirts about their g-r color, Mstar, and local density, emphasizing the influence of the environment on galaxy evolution. We conducted a spectroscopic analysis out to 5.5R200, using the AAOmega spectrograph. The large FoV and depth allowed us to explore galaxies up to the cluster periphery and across a wide Mstar range, reaching down to the limit of dwarf galaxies. Redshifts were obtained through independent but comparable methods: Redrock, EZ, and Redmost. We identified 148 new spectroscopic cluster members from a sample of 1236 objects. We found 81 galaxies located in filamentary and overdense regions, supporting the role of filamentary infall in the cluster mass assembly history. Our analysis revealed that galaxies in high density regions are more massive, redder, and more passive, compared to galaxies in low density regions that appear to be bluer, less massive, and more SF. These findings underscore the significance of environmental effects and the role of pre processing in shaping galaxy properties before cluster infall.

astro-ph.GA↗

arXiv2022

VST-GAME: Galaxy Assembly as a function of Mass and Environment with VST. Photometric assessment and density field of MACSJ0416

Observational studies have widely demonstrated that galaxy physical properties are strongly affected by the surrounding environment. On one side, gas inflows provide galaxies with new fuel for star formation. On the other side, the high temperatures and densities of the medium are expected to induce quenching in the star formation. Observations of large structures, in particular filaments at the cluster outskirts (r>2r$_{200}$), are currently limited to the low redshift Universe. We present a multi-band dataset for the cluster MACS J0416.1-2403 (z=0.397), observed in the context of the Galaxy Assembly as a function of Mass and Environment with VST (VST-GAME) survey. The project aims at gathering deep ($r$<24.4) and wide (20x20Mpc$^2$) observations at optical wavelengths for six massive galaxy clusters at 0.2<z<0.6, complemented with near infrared data. This work describes the photometric analysis of the cluster, defines a density field and studies galaxy properties in the cluster outskirts. We extract sources paying particular attention to recover the faintest ones. We combine all the extractions in a multi-band catalog and compute photometric redshifts. We then define cluster memberships up to 5r$_{200}$ from the cluster core and measure the density field, comparing galaxy properties in different environments. We found that the $g-r$ colors show bimodal behaviours in all the environments, but the peak of the distribution of red galaxies shifts toward redder colors with increasing density and the fraction of galaxies in the blue cloud increases with decreasing density. We also found 3 overdense regions in the cluster outskirts at r$\sim$5r$_{200}$. The color of galaxies suggests the presence of evolved galaxy populations, an insight for pre-processing phenomena over these substructures. We release the multi-band catalog, down to the completeness limit $r$<24.4 mag.

astro-ph.GA↗

arXiv2020

Modelling the spread of Covid19 in Italy using a revised version of the SIR model

In this paper, we present a model to predict the spread of the Covid-19 epidemic and apply it to the specific case of Italy. We started from a simple Susceptible, Infected, Recovered (SIR) model and we added the condition that, after a certain time, the basic reproduction number $R_0$ exponentially decays in time, as empirically suggested by world data. Using this model, we were able to reproduce the real behavior of the epidemic with an average error of 5\%. Moreover, we illustrate possible future scenarios, associated to different intervals of $R_0$. This model has been used since the beginning of March 2020, predicting the Italian peak of the epidemic in April 2020 with about 100.000 detected active cases. The real peak of the epidemic happened on the 20th of April 2020, with 108.000 active cases. This result shows that the model had predictive power for the italian case.

physics.soc-ph↗

arXiv2011

Identification of rolling resistance as a shape parameter in sheared granular media

Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

cond-mat.soft↗