Gravitational clustering in N-body simulations

In this talk we discuss some of the main theoretical problems in the understanding of the statistical properties of gravity. By means of N-body simulations we approach the problem of understanding the r\^ole of gravity in the clustering of a finite set of N-interacting particles which samples a portion of an infinite system. Through the use of the conditional average density, we study the evolution of the clustering for the system putting in evidence some interesting and not yet understood features of the process.Comment: 5 pages, 1 figur

Clustering in N-Body gravitating systems

Self-gravitating systems have acquired growing interest in statistical mechanics, due to the peculiarities of the 1/r potential. Indeed, the usual approach of statistical mechanics cannot be applied to a system of many point particles interacting with the Newtonian potential, because of (i) the long range nature of the 1/r potential and of (ii) the divergence at the origin. We study numerically the evolutionary behavior of self-gravitating systems with periodical boundary conditions, starting from simple initial conditions. We do not consider in the simulations additional effects as the (cosmological) metric expansion and/or sophisticated initial conditions, since we are interested whether and how gravity by itself can produce clustered structures. We are able to identify well defined correlation properties during the evolution of the system, which seem to show a well defined thermodynamic limit, as opposed to the properties of the ``equilibrium state''. Gravity-induced clustering also shows interesting self-similar characteristics.Comment: 6 pages, 5 figures. To be published on Physica

On the kinematic detection of accreted streams in the Gaia era: a cautionary tale

The $\Lambda$CDM cosmological scenario predicts that our Galaxy should contain hundreds of stellar streams at the solar vicinity, fossil relics of the merging history of the Milky Way and more generally of the hierarchical growth of galaxies. Because of the mixing time scales in the inner Galaxy, it has been claimed that these streams should be difficult to detect in configuration space but can still be identifiable in kinematic-related spaces like the energy/angular momenta spaces, E-Lz and Lperp-Lz, or spaces of orbital/velocity parameters. By means of high-resolution, dissipationless N-body simulations, containing between 25$\times10^6$ and 35$\times10^6$ particles, we model the accretion of a series of up to four 1:10 mass ratio satellites then up to eight 1:100 satellites and we search systematically for the signature of these accretions in these spaces. In all spaces considered (1) each satellite gives origin to several independent overdensities; (2) overdensities of multiple satellites overlap; (3) satellites of different masses can produce similar substructures; (4) the overlap between the in-situ and the accreted population is considerable everywhere; (5) in-situ stars also form substructures in response to the satellite(s) accretion. These points are valid even if the search is restricted to kinematically-selected halo stars only. As we are now entering the 'Gaia era', our results warn that an extreme caution must be employed before interpreting overdensities in any of those spaces as evidence of relics of accreted satellites. Reconstructing the accretion history of our Galaxy will require a substantial amount of accurate spectroscopic data, that, complemented by the kinematic information, will possibly allow us to (chemically) identify accreted streams and measure their orbital properties. (abridged)Comment: Accepted on A&A. A high-resolution version of the paper is available at http://aramis.obspm.fr/~paola/ELZ/Elz.pd

A topological approach to neural complexity

Considerable efforts in modern statistical physics is devoted to the study of networked systems. One of the most important example of them is the brain, which creates and continuously develops complex networks of correlated dynamics. An important quantity which captures fundamental aspects of brain network organization is the neural complexity C(X)introduced by Tononi et al. This work addresses the dependence of this measure on the topological features of a network in the case of gaussian stationary process. Both anlytical and numerical results show that the degree of complexity has a clear and simple meaning from a topological point of view. Moreover the analytical result offers a straightforward algorithm to compute the complexity than the standard one.Comment: 6 pages, 4 figure

Le tecniche di indagine NDT nel processo di restauro e consolidamento di edifici soggetti a tutela

Le attività di restauro e consolidamento di edifici soggetti a tutela sono fortemente ancorate alla conoscenza del bene, alla consistenza geometrico-strutturale e a quella materica, nonché all’analisi del degrado del manufatto e del suo stato di conservazione in tutti i suoi elementi. Altri aspetti tecnici da portare in conto sono ovviamente quelli legati al monitoraggio da attuare eventualmente in fase di diagnosi e obbligatoriamente in fase di esercizio, al fine di disporre di una robusta mole di dati per governare l’attività manutentiva. Un valido supporto all’insieme di queste problematiche è offerto oggi dalle tecniche non distruttive (NDT), le quali si inseriscono sia nel processo di screening conoscitivo del manufatto che nel processo di progettazione, di realizzazione degli interventi e di vita utile del bene. In questo contesto, la presente nota illustra una panoramica di alcune delle moderne tecniche di indagine NDT impiegate nelle attività di rilievo, di valutazione dello stato dei materiali e nelle attività di monitoraggio sia propedeutiche alla redazione del progetto, sia nel corso dell’esercizio. In riferimento a quest’ultimo aspetto, le tecniche illustrate mirano ad essere parte integrante di una metodologia di controllo "routinaria" da esercitare in continuo e con la possibilità di essere interrogabile anche in tempo reale. Alcuni esempi applicativi qui riportati, seppur in forma sintetica, consentiranno di apprezzare i benefici derivanti dall'utilizzo di tecniche NDT, il cui punto di forza risiede soprattutto nel loro essere non invasive e autodiagnosticanti

Yb-1 oncoprotein controls pi3k/akt pathway by reducing pten protein level

YB-1 is a multifunctional protein overexpressed in many types of cancer. It is a crucial oncoprotein that regulates cancer cell progression and proliferation. Ubiquitously expressed in human cells, YB-1 protein functions are strictly dependent on its subcellular localization. In the cytoplasm, where YB-1 is primarily localized, it regulates mRNA translation and stability. However, in response to stress stimuli and activation of PI3K and RSK signaling, YB-1 moves to the nucleus acting as a prosurvival factor. YB-1 is reported to regulate many cellular signaling pathways in different types of malignancies. Furthermore, several observations also suggest that YB-1 is a sensor of oxidative stress and DNA damage. Here we show that YB-1 reduces PTEN intracellular levels thus leading to PI3K/Akt pathway activation. Remarkably, PTEN reduction mediated by YB-1 overexpression can be observed in human immortalized keratinocytes and HEK293T cells and cannot be reversed by proteasome inhibition. Real-time PCR data indicate that YB-1 silencing up-regulates the PTEN mRNA level. Collectively, these observations indicate that YB-1 negatively controls PTEN at the transcript level and its overexpression could confer survival and proliferative advantage to PTEN proficient cancer cells

Galaxy number counts and Fractal correlations

We report the correlation analysis of various redshift surveys which shows that the available data are consistent with each other and manifest fractal correlations (with dimension $D \simeq 2$) up to the present observational limits (\approx 150 \hmp) without any tendency towards homogenization. This result points to a new interpretation of the number counts that represents the main subject of this letter. We show that an analysis of the small scale fluctuations allows us to reconcile the correlation analysis and the number counts in a new perspective which has a number of important implications.Comment: Latex file 7 pages and 3 Postscipt figures. Accepted for publication in Europhysics Letters. This paper is also available at http://www.phys.uniroma1.it/DOCS/PIL/pil.htm