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Spiral galaxies are well-known astrophysical structures, but how they form is not fully understood. This paper describes simulations showing that they could be transient, nonequilibrium structures originating from the collapse of clouds of matter interacting solely through self-gravity.

Schermata 2019-02-15 alle 13.31.21


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

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 


Radial velocities in the outermost disk toward the anticenter

d15_coo_2000-cyl-vrWe measure the mean Galactocentric radial component of the velocity of stars (vR) in the disk at 8 kpc<R<28 kpc in the direction of the anticenter. For this, we use the Apache Point Galactic Evolution Experiment (APOGEE). Furthermore, we compare the result with HI maps along the same line of sight. We find an increase in positive (expansion) vR at R913 kpc, reaching a maximum of 6 km/s, and a decrease at large values of R reaching a negative (contraction) value of 10 km/s for R>17 kpc. Negative velocities are also observed in 21 cm HI maps, possibly dominated by local gas emission. Among the possible dynamical causes for these non-zero vR, factors such as the effect of the Galactic bar, streams, or mergers do not seem appropriate to explain our observations. An explanation might be the gravitational attraction of overdensities in a spiral arm. As a matter of fact, we see a change of regime from positive to negative velocities around R15 kpc, in the position where we cross the Outer spiral arm in the anticenter. The mass in spiral arms necessary to produce these velocities would be about 3\% of the mass of the disk, consistent with our knowledge of the spiral arms. Another scenario that we explore is a simple class of out-of-equilibrium systems in which radial motions are generally created by the monolithic collapse of isolated self-gravitating overdensities.


M. Lopez-Corredoira, F. Sylos Labini, P. M. W. Kalberla, C. Allende Prieto

Comments: 21 pages, 16 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Journal reference: The Astronomical Journal, Volume 157, Number 1, 2019
Cite as: arXiv:1901.01300 [astro-ph.GA]
(or arXiv:1901.01300v1 [astro-ph.GA] for this version)