Category Archives: Highlights on physics papers

Properties of self gravitating quasi-stationary states

On a possibile solution of the core-cusp problem and the fundamental properties of self gravitating quasi-stationary states.

One of the crucial problems for the galaxy formation theory is to explain the cusp-core paradox: while the simulations of the theoretical models give rise to a diverging density profile at a small distance (1 / r) from the center,  observations of galaxy profiles show that these  are constant.

To understand the origin of these profiles it is necessary to understand the fundamental properties of the quasi-equilibrium states of gravitational systems and in particular what kind of dynamical path they followed during their formation.

We have found that, depending on the initial conditions, these stationary states can exhibit a small-scale divergent profile or a flat profile. This difference corresponds to a different dynamical nature underlying their origin.

Coming back to  galaxies, the profiles indicate that the dynamic that gave them origin is a violent type of relaxation. That is, a dynamical process different from the “standard” one of cold dark matter models.

arXiv:2009.11624

15 pages, 24 figures, Accepteb for publication in Astronomy and Astrophysics

by Francesco Sylos Labini, Roberto Capuzzo-Dolcetta

Continue reading Properties of self gravitating quasi-stationary states

Formation of disk galaxies with long-lived and non-stationary spiral arms

This work represents a development of the study of the collapses of purely self-gravitating systems  (see https://physics.aps.org/articles/v12/s19) to the case in which a dissipational gas component is also present. These latter systems show much richer morphological and kinematical structures that may have important observational implications to understand the kinematic and dynamics of the Milky Way as revealed by ongoing surveys such as the Gaia Mission. 

 by Francesco Sylos Labini, Luis Diego Pinto, Roberto Capuzzo-Dolcetta 

Continue reading Formation of disk galaxies with long-lived and non-stationary spiral arms

Gaia-DR2 extended kinematical maps. Part III: Rotation curves analysis, dark matter, and Modified Newtonian dynamics tests

Ž. Chrobáková, M. López-Corredoira, F. Sylos Labini, H.-F. Wang, R. Nagy

Recent statistical deconvolution methods have produced extended kinematical maps in a range of heliocentric distances that are a factor of two to three larger than those analysed in the Gaia Collaboration based on the same data. In this paper, we use such maps to derive the rotation curve both in the Galactic plane and in off-plane regions and to analyse the density distribution. By assuming stationary equilibrium and axisymmetry, we used the Jeans equation to derive the rotation curve. Then we fit it with density models that include both dark matter and predictions of the MOND (Modified Newtonian dynamics) theory. Since the Milky Way exhibits deviations from axisymmetry and equilibrium, we also considered corrections to the Jeans equation. To compute such corrections, we ran N-body experiments of mock disk galaxies where the departure from equilibrium becomes larger as a function of the distance from the centre. The rotation curve in the outer disk of the Milky Way that is constructed with the Jeans equation exhibits very low dependence on R and z and it is well-fitted both by dark matter halo and MOND models. The application of the Jeans equation for deriving the rotation curve, in the case of the systems that deviate from equilibrium and axisymmetry, introduces systematic errors that grow as a function of the amplitude of the average radial velocity. In the case of the Milky Way, we can observe that the amplitude of the radial velocity reaches ∼10% that of the azimuthal one at R≈20 kpc. Based on this condition, using the rotation curve obtained from the Jeans equation to calculate the mass may overestimate its measurement.

Comments: 11 pages, 8 figures, accepted to be published in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2007.14825 [astro-ph.GA]
  (or arXiv:2007.14825v1 [astro-ph.GA] for this version)

Dynamics in the Galactic disk explaining radial and vertical velocities

Context: In our Paper I, by using statistical deconvolution methods, extended kinematics maps of Gaia-DR2 data have been produced in a range of heliocentric distances that are a factor of two to three larger than those analyzed previously by the Gaia Collaboration with the same data. It added the range of Galactocentric distances between 13 kpc and 20 kpc to the previous maps.
Aims: Here, we investigate the dynamical effects produced by different mechanisms that can explain the radial and vertical components of these extended kinematic maps, including a decomposition of bending and breathing of the vertical components. This paper as a whole tries to be a compendium of different dynamical mechanisms whose predictions can be compared to the kinematic maps.
Methods: Using analytical methods or simulations, we are able to predict the main dynamical factors and compare them to the predictions of the extended kinematic maps of Gaia-DR2.
Results: The gravitational influence of Galactic components that are different from the disk, such as the long bar or bulge, the spiral arms, or a tidal interaction with Sagittarius dwarf galaxy, may explain some features of the velocity maps, especially in the inner parts of the disk. However, they are not sufficient in explaining the most conspicuous gradients in the outer disk. Vertical motions might be dominated by external perturbations or mergers, although a minor component may be due to a warp whose amplitude evolves with time. Here, we show with two different methods, which analyze the dispersion of velocities, that the mass distribution of the disk is flared. Despite these partial explanations, the main observed features can only be explained in terms of out-of-equilibrium models, which are either due to external perturbers or to the fact that the disk has not had time to reach equilibrium since its formation.

Comments: 15 pages, 14 figures, accepted to be published in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2001.05455 [astro-ph.GA]
(or arXiv:2001.05455v1 [astro-ph.GA] for this version)

Galactic Spirals May Form Spontaneously

Synopsis: Galactic Spirals May Form Spontaneously

Spiral galaxies could be transient, nonequilibrium structures originating from the collapse of clouds of matter interacting solely through self-gravity.   (see https://physics.aps.org/synopsis-for/10.1103/PhysRevE.99.022125)

Synopsis figure

D. Benhaiem et al., Phys. Rev. E (2019)

Continue reading Galactic Spirals May Form Spontaneously

Una nuova ipotesi per il mistero delle galassie a spirale

Schermata 2019-02-04 alle 17.34.58

 

Comunicato stampa – Le braccia che avvolgono il nucleo di questa interessante formazione celeste infrangono la terza legge di Keplero per cui la velocità orbitale decresce con la distanza dal centro. Per spiegare questo fenomeno si ipotizzano la materia oscura o una correzione della seconda legge di Newton.
Un team internazionale composto da ricercatori dell’Isc-Cnr e del Laboratoire de Physique Nucleaire et de Hautes Energies di Parigi apre la strada a ipotesi diverse, dimostrando come sia possibile simulare al computer la nascita di una galassia a spirale.

 

Roma, 22 gennaio 2018 – Hanno la forma di un disco composto da un nucleo con alcune braccia che gli si avvolgono intorno. Sono le galassie a spirale, uno degli oggetti più suggestivi e interessanti dell’universo visibile rivelati dall’astronomia. Francesco Sylos Labini, ricercatore presso l’Istituto dei sistemi complessi del Consiglio nazionale delle ricerche (Isc-Cnr) e del Centro Fermi, ha recentemente pubblicato sulla rivista The Astrophysical Journal una ricerca sul tema in collaborazione con il Laboratoire de Physique Nucleaire et de Haute Energies (Lpnhe) di Parigi.

Continue reading Una nuova ipotesi per il mistero delle galassie a spirale

Formation of spiral arms

Here is a gif animated of a simulation with 1 million particles. The initial condition was an isolated, uniform,  prolate ellipsoid with a some rigid rotation. The system is evolved for 50 dynamical times. This is a projection on the XY plane where rotation is around the Z axis. The color code is proportional to the log of the density.

More: Long-lived transient structure in collisionless self-gravitating systems David Benhaiem, Francesco Sylos Labini, and Michael Joyce, Phys. Rev. E 99, 022125, 2019 

s29_movie