Category Archives: Highlights on physics papers

Long-lived transient structure in collisionless self-gravitating systems

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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)

Nonaxisymmetric models of galaxy velocity maps

plot_NGC_3521_n_redone

Galaxy velocity maps often show the typical pattern of a rotating disk, consistent with the dynamical model where emitters rotate in circular orbits around the galactic center. The simplest template used to fit these maps consists in the rotating disk model (RDM) where the amplitude of circular velocities is fixed by the observed velocity profile along the kinematic axis. A more sophisticated template is the rotating tilted-ring model (RTRM) that takes into account the presence of warps and allows a radius-dependent orientation of the kinematic axis. In both cases, axisymmetry is assumed and residuals between the observed and the model velocity fields are interpreted as noncircular motions.

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Formazione di galassie dalla dinamica gravitazionale fuori dall’equilibrio

Un articolo divulgativo sulla nostra attività scientifica gratuitamente  disponibile a questo link

Il sistema solare si è formato circa cinque miliardi di anni fa: i diversi pianeti hanno dunque avuto il tempo di compiere più di un miliardo di rivoluzioni intorno al sole. In particolare Mercurio, che un periodo orbitale di 88 giorni, ne ha fatte circa 25 miliardi, la Terra 5 miliardi, mentre Nettuno, con un periodo orbitale di 160 anni, 250 milioni. Per questo motivo il sistema solare può essere considerato aver raggiunto una situazione di stabilità in cui i tutti pianeti, anche i più esterni, ruotano in orbite chiuse dove la forza gravitazionale di attrazione del sole è controbilanciata dalla forza centrifuga che ha ugual modulo di questa ma verso opposto. Per effetto della forza gravitazionale esercitata sui pianeti dal sole le velocità orbitali dei pianeti variano secondo la loro distanza dal sole: Mercurio si muove a 48 km/sec, la Terra a 30 km/sec e Nettuno a 5 km/sec. Questo equilibrio dinamico, una volta stabilito, rimane invariato e dura finché cause esterne non ne causano la rottura.

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Intervista su La Repubblica

Diapositiva1“In questi anni – spiega Francesco Sylos Labini, ricercatore del Cnr-Isc – la missione Gaia, un satellite dell’Agenzia spaziale europea progettato per indagare origine, evoluzione e struttura della Via Lattea, sta compiendo misurazioni astrometriche di altissima precisione, determinando la posizione di oltre un miliardo di stelle sulle quali è stato appena pubblicato il data release 2, il più grande e accurato censimento di informazioni quali posizioni, velocità e altre proprietà stellari. In particolare, è ora possibile esplorare lo spazio delle fasi (posizioni e velocità) di oltre sei milioni di stelle nel disco della Via Lattea. Le mappe delle velocità stellari pubblicate da Gaia coprono una distanza fino a 12 kilopaserc (kpc), unità di misura impiegata in astronomia per indicare la distanza fra oggetti celesti. Noi siamo stati in grado di estenderle fino a 20 kpc, tre volte in più rispetto alle mappe ufficiali, utilizzando una ricostruzione statistica della distanza”. – I ricercatori hanno quindi misurato deviazioni significative dalla circolarità nelle orbite medie delle stelle del disco della Via Lattea, insieme a un gradiente di velocità radiale di circa 40 km/s e di un gradiente di velocità verticale di 20 km/s.

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

46310467_10155856872102727_1616626076761456640_oLe 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

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

Fig8a
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

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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

 

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Transient spiral arms and galaxy rotation curves

Diapositiva126by 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.