# CNR COMUNICATO STAMPA 96/2018: La Via Lattea non è in equilibrio

Le stelle della nostra galassia dovrebbero girare intorno al nucleo con un moto di rotazione in equilibrio dinamico. Un team internazionale, cui partecipano ricercatori del Cnr-Isc, analizzando i dati del satellite Gaia, ha ottenuto le più estese mappe di velocità delle stelle della nostra galassia, che mettono in discussione l’ipotesi che le stelle ruotino con soli moti circolari. Sono stati, infatti, rivelati moti radiali e verticali e differenze nella velocità di rotazione in diverse zone stellari. Lo studio, pubblicato su Astronomy and Astrophysics, induce a rivedere anche le stime sulla materia oscura

# Gaia-DR2 extended kinematical maps. Part I: Method and application

CONTEXT. The Gaia Collaboration has used Gaia-DR2 sources with six-dimensional (6D) phase space information to derive kinematical maps within 5 kpc of the Sun, which is a reachable range for stars with relative error in distance lower than 20%.
AIMS. Here we aim to extend the range of distances by a factor of two to three, thus adding the range of Galactocentric distances between 13 kpc and 20 kpc to the previous maps, with their corresponding error and root mean square values.
METHODS. We make use of the whole sample of stars of Gaia-DR2 including radial velocity measurements, which consists in more than seven million sources, and we apply a statistical deconvolution of the parallax errors based on the Lucy’s inversion method of the Fredholm integral equations of the first kind, without assuming any prior.
RESULTS. The new extended maps provide lots of new and corroborated information about the disk kinematics: significant departures of circularity in the mean orbits with radial Galactocentric velocities between -20 and +20 km/s and vertical velocities between -10 and +10 km/s; variations of the azimuthal velocity with position; asymmetries between the northern and the southern Galactic hemispheres, especially towards the anticenter that includes a larger azimuthal velocity in the south; and others.
CONCLUSIONS. These extended kinematical maps can be used to investigate the different dynamical models of our Galaxy, and we will present our own analyses in the forthcoming second part of this paper. At present, it is evident that the Milky Way is far from a simple stationary configuration in rotational equilibrium, but is characterized by streaming motions in all velocity components with conspicuous velocity gradients.
 Comments: 19 pages, 16 figures, accepted to be published in Astronomy and Astrophysics in the press; data of Figs. 8-12 and 16 publicly available Subjects: Astrophysics of Galaxies (astro-ph.GA) Cite as: arXiv:1810.13436 [astro-ph.GA] (or arXiv:1810.13436v1 [astro-ph.GA] for this version)

# Fractal universe and cosmic acceleration in a Lemaître-Tolman-Bondi scenario

Abstract: In this paper we attempt to answer to the question: can cosmic acceleration of the Universe have a fractal solution? We give an exact solution of a Lemaitre-Tolman-Bondi (LTB) Universe based on the assumption that such a smooth metric is able to describe, on average, a fractal distribution of matter. While the LTB model has a center, we speculate that, when the fractal dimension is not very different from the space dimension, this metric applies to any point of the fractal structure when chosen as center so that, on average, there is not any special point or direction. We examine the observed magnitude-redshift relation of type Ia supernovae (SNe Ia), showing that the apparent acceleration of the cosmic expansion can be explained as a consequence of the fractal distribution of matter when the corresponding space-time metric is modeled as a smooth LTB one and if the fractal dimension on scales of a few hundreds Mpc is D=2.9±0.02.
 Comments: 6 pages, 4 figures, accepted for publication in Classical and Quantum Gravity Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc) Cite as: arXiv:1810.06318 [astro-ph.CO] (or arXiv:1810.06318v1 [astro-ph.CO] for this version)

# Una nuova ipotesi per il mistero delle galassie a spirale: comunicato stampa CNR e articolo sul Corriere della Sera

Di seguito il comunicato stampa del CNR, l’articolo sul Corriere della Sera e su una serie di altri riviste, sul nostro lavoro sulla formazione delle galassie

# Transient spiral arms and galaxy rotation curves

by Francesco Sylos Labini (R)

We describe how a simple class of out of equilibrium mass distributions evolve under their self-gravity to produce a quasi-planar spiral structure surrounding a virialized core, qualitatively resembling a spiral galaxy. The spiral structure is transient, but can survive tens of dynamical times, and further reproduces qualitatively noted features of spiral galaxies as the predominance of trailing two-armed spirals and large pitch angles. The mechanism leads generically to a characteristic transition from predominantly rotational motion, in a region outside the core, to radial ballistic motion in the outermost parts. Such radial motions are excluded in our Galaxy up to 15 kpc, but could be detected at larger scales in the future by GAIA. We explore the apparent motions seen by external observers of the velocity distributions of our toy galaxies, and find that it is difficult to distinguish them from those of a rotating disc with sub-dominant radial motions at levels typically inferred from observations. These simple models illustrate the possibility that the observed apparent motions of spiral galaxies might be explained by non-trivial non-stationary mass and velocity distributions without invoking a dark matter halo or modification of Newtonian gravity. In this scenario the observed phenomenological relation between the centripetal and gravitational acceleration of the visible baryonic mass could have a simple explanation.

# Transient spiral arms from far out of equilibrium gravitational evolution

We describe how a simple class of out of equilibrium, rotating and asymmetrical mass distributions evolve under their self-gravity to produce a quasi-planar spiral structure surrounding a virialized core, qualitatively resembling a spiral galaxy. The spiral structure is transient, but can survive tens of dynamical times, and further reproduces qualitatively noted features of spiral galaxies as the predominance of trailing two-armed spirals and large pitch angles. As our models are highly idealized, a detailed comparison with observations is not appropriate, but generic features of the velocity distributions can be identified to be potential observational signatures of such a mechanism. Indeed, the mechanism leads generically to a characteristic transition from predominantly rotational motion, in a region outside the core, to radial ballistic motion in the outermost parts. Such radial motions are excluded in our Galaxy up to 15 kpc, but could be detected at larger scales in the future by GAIA. We explore the apparent motions seen by external observers of the velocity distributions of our toy galaxies, and find that it is difficult to distinguish them from those of a rotating disc with sub-dominant radial motions at levels typically inferred from observations. These simple models illustrate the possibility that the observed apparent motions of spiral galaxies might be explained by non-trivial non-stationary mass and velocity distributions without invoking a dark matter halo or modification of Newtonian gravity. In this scenario the observed phenomenological relation between the centripetal and gravitational acceleration of the visible baryonic mass could have a simple explanation.

 Comments: 14 pages, 9 figures, The Astrophysical Journal in press. Two movies of the simulation is available at this link: this http URL Subjects: Astrophysics of Galaxies (astro-ph.GA) Cite as: arXiv:1711.01913 [astro-ph.GA] (or arXiv:1711.01913v1 [astro-ph.GA] for this version)

# Particle number dependence in the non-linear evolution of N-body self-gravitating systems

Title: Particle number dependence in the non-linear evolution of N-body
self-gravitating systems
Authors: David Benhaiem, Michael Joyce, Francesco Sylos Labini and Tirawut
Worrakitpoonpon
Categories: astro-ph.CO
Comments: 8 pages, 5 figures; to appear in MNRAS

https://arxiv.org/abs/1709.06657

Simulations of purely self-gravitating N-body systems are often used in
astrophysics and cosmology to study the collisionless limit of such systems.
Their results for macroscopic quantities should then converge well for
sufficiently large N. Using a study of the evolution from a simple space of
spherical initial conditions – including a region characterised by so-called
“radial orbit instability” – we illustrate that the values of N at which such
convergence is obtained can vary enormously. In the family of initial
conditions we study, good convergence can be obtained up to a few dynamical
times with N $\sim 10^3$ – just large enough to suppress two body relaxation –
for certain initial conditions, while in other cases such convergence is not
attained at this time even in our largest simulations with N $\sim 10^5$. The
qualitative difference is due to the stability properties of fluctuations
introduced by the N-body discretisation, of which the initial amplitude depends
on N. We discuss briefly why the crucial role which such fluctuations can
potentially play in the evolution of the N-body system could, in particular,
constitute a serious problem in cosmological simulations of dark matter.

# Dynamics of self-gravitating systems

It was a long time that I wanted to fix the webpages about my research activity. Now I have done a first rough step in the organization of them… more is to come. This is the main one  while the sub-pages are the following:

# Formation of satellites from cold collapse

We study the collapse of an isolated, initially cold, irregular (but almost spherical) and (slightly) inhomogeneous cloud of self-gravitating particles. The cloud is driven towards a virialized quasi-equilibrium state by a fast relaxation mechanism, occurring in a typical time τc, whose signature is a large change in the particle energy distribution. Post-collapse particles are divided into two main species: bound and free, the latter being ejected from the system. Because of the initial system’s anisotropy, the time varying gravitational field breaks spherical symmetry so that the ejected mass can carry away angular momentum and the bound system can gain a non-zero angular momentum. In addition, while strongly bound particles form a compact core, weakly bound ones may form, in a time scale of the order of τc, several satellite sub-structures. These satellites have a finite lifetime that can be longer than τc and generally form a flattened distribution. Their origin and their abundance are related to the amplitude and nature of initial density fluctuations and to the initial cloud deviations from spherical symmetry, which are both amplified during the collapse phase. Satellites show a time dependent virial ratio that can be different from the equilibrium value b1: although they are bound to the main virialized object, they are not necessarily virially relaxed.

 Comments: 11 pages, 11 figures. Accepted for publication in Astronomy and Astrophysics Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO) Cite as: arXiv:1612.01283 [astro-ph.GA] (or arXiv:1612.01283v1 [astro-ph.GA] for this version)