# 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)

# Gravitational structure via violent relaxation

Talk given at the workshop

# The secular evolution of self-gravitating systems over cosmic ages

Abstract.

Isolated, initially cold and spherically symmetric self-gravitating systems may give rise to a virial equilibrium state which is far from spherically symmetric, and typically triaxial, and with no-zero angular momentum. We discuss the main features of the dynamical mechanism that gives rise to such a quasi-stationary configuration stressing the potential interest from an observational point of view.

Slides here

Video here

# The secular evolution of self-gravitating systems over cosmic ages

I will participate to the  meeting

# The secular evolution of self-gravitating systems over cosmic ages

These are the title and abstract of my talk:

Gravitational structure formation via violent relaxation

Isolated, initially cold and spherically symmetric self-gravitating systems may give rise to a virial equilibrium state which is far from spherically symmetric, and typically triaxial, and with no-zero angular momentum. We discuss the main features of the dynamical mechanism that gives rise to such a quasi-stationary configuration stressing the potential interest from an observational point of view.

# On the origin of the angular momentum of galaxies

The angular momentum is a conserved quantity and the usual theoretical interpretation of the origin of the angular momentum of galaxies is that this is originated by from tidal interactions of the galaxy with its neighborhoods. We propose in this paper (in print in Astronomy and Astrophysics) a new mechanism for its origin based that can be efficient also for the case of an isolated object (or for an object with small tidal interactions). The new mechanism works as follows : during the violent relaxation of an isolated  self-gravitating system a significant fraction of its mass may be ejected. If the time varying gravitational field also breaks spherical symmetry this mass can potentially carry angular momentum. Thus starting initial configurations with zero angular momentum can in principle lead to a bound virialized system with non-zero angular momentum even though the overall angular momentum is conserved. A simple picture of this mechanism is illustrated in the following figure. The astrophysical and cosmological implications of such a fundamental physical process will be subject of a forthcoming work.

(For more details see http://lanl.arxiv.org/abs/1505.03371 now in press in Astronomy and Astrophysics)