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Evgeny Griv

Bio: Evgeny Griv is an academic researcher from Ben-Gurion University of the Negev. The author has contributed to research in topics: Galaxy & Spiral galaxy. The author has an hindex of 15, co-authored 92 publications receiving 851 citations. Previous affiliations of Evgeny Griv include Academia Sinica Institute of Astronomy and Astrophysics & Academia Sinica.


Papers
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Journal ArticleDOI
TL;DR: In this article, the reaction between almost aperiodically growing Jeans-unstable gravity perturbations and stars of a rotating and spatially inhomogeneous disk of flat galaxies is analyzed.
Abstract: We analyse the reaction between almost aperiodically growing Jeans-unstable gravity perturbations and stars of a rotating and spatially inhomogeneous disk of flat galaxies. A mathematical formalism in the approx- imation of weak turbulence (a quasi-linearization of the Boltzmann collisionless kinetic equation) is developed. A diusion equation in conguration space is derived which describes the change in the main body of equilibrium distribution of stars. The distortion in phase space resulting from such a wave{star interaction is studied. The theory, applied to the Solar neighborhood, accounts for the observed Schwarzschild shape of the velocity ellipsoid, the increase in the random stellar velocities with age, and the essential radial migration of the Sun from its birth-place in the inner part of the Galaxy outwards during its lifetime.

38 citations

Journal ArticleDOI
TL;DR: In this paper, a modified theory of the Lin-Shu density wave, studied in connection with the problem of spiral pattern of rapidly and differentially rotating disc galaxies, is presented for both the axisymmetric and non-axisymetric structures in highly flattened galaxies resulted from the classical Jeans instability of small gravity perturbations.
Abstract: A modified theory of the Lin–Shu density waves, studied in connection with the problem of spiral pattern of rapidly and differentially rotating disc galaxies, is presented for both the axisymmetric and non-axisymmetric structures in highly flattened galaxies resulted from the classical Jeans instability of small gravity perturbations (e.g. those produced by a spontaneous disturbance). A new method is provided for the analytical solution of the self-consistent system of the gas-dynamic equations and the Poisson equation describing the stability of a three-dimensional galactic disc composed of stars or gaseous clouds. In order to apply the method, the modifications introduced for the properties of the gravitationally unstable, that is to say, amplitude-growing density waves are considered by removing the often used assumptions that the gravity perturbations are axisymmetric and the disc is infinitesimally thin. In contrast to previous studies, in this paper these two effects – the non-axial symmetry effect and the finite thickness effect – are simultaneously taken into account. We show that nonaxisymmetric perturbations developing in a differentially rotating disc are more unstable than the axisymmetric ones. We also show that destabilizing self-gravity is far more ‘dangerous’ in thin discs than in thick discs. The primary effect of small but finite thickness is a reduction of the growth rate of the gravitational Jeans instability and a shift in the threshold of instability towards a longer wavelength (and larger wavelength will include more mass). The results of this paper are in qualitative agreement with previous analytical and numerical estimations of the effects. The extent to which our results on the disc’s stability can have a bearing on observable spiral galaxies, including the Milky Way, is also discussed.

38 citations

Journal ArticleDOI
TL;DR: In this article, N-body simulations of the firehose-type bending instability are presented for the spiral starburst galaxy NGC 2146 with the Very Large Array at an angular resolution of 2''.
Abstract: As the observations of highly flattened galaxies, including the Milky Way, and gravitational N-body simulations show, the central parts of these systems at distances of, say, r Fg. The latter condition is none other than the condition of the so-called firehose electromagnetic instability in collisionless plasmas. The source of free energy in the instability is the intrinsic anisotropy of a velocity dispersion ("temperature"). It seems reasonable that this is a natural mechanism for building a snake-shaped radio structure, which has recently been observed by Zhao and coworkers in the central region of the spiral starburst galaxy NGC 2146 with the Very Large Array at an angular resolution of 2''. In the current paper, in order to investigate the dynamics of the central region, N-body simulations of the firehose-type bending instability are presented for this galaxy. Use of concurrent computers has enabled us to make long simulation runs using a sufficiently large number of particles in the direct summation code, N ~ 20,000. In contrast to all previous N-body simulations of bending instabilities, we show how bending structures may be longer lived in real starburst galaxies than in the computer models. The simulations clearly confirm the qualitative picture and, moreover, are in fair quantitative agreement with the theory. A theoretical prediction is confirmed that the instability is driven by an excess of plane kinetic energy of random motions of stars, when the ratio of the dispersion of radial velocities of stars in the plane cr to the velocity dispersion in the perpendicular direction cz is large enough, cr 0.6cz, in other words, if the thickness of the stellar disk h ∝ cz is small enough. The extent to which our results on the stability of the disk can have a bearing on observable spiral galaxies with a high star formation rate in the central parts is discussed.

37 citations

Journal ArticleDOI
TL;DR: In this article, a self-consistent timing analysis with data covering a timescale of 1849 epochs was performed, and the results gave an orbital decay rate = −0.02890795 ± 0.00772547 s year−1.
Abstract: Motivated by the previously reported high orbital decay rate of the planet WASP-43b, we have obtained and present eight newly transiting light curves. Together with other data in the literature, we perform a self-consistent timing analysis with data covering a timescale of 1849 epochs. The results give an orbital decay rate = −0.02890795 ± 0.00772547 s year−1, which is one order smaller than previous values. This slow decay rate corresponds to a normally assumed theoretical value of the stellar tidal dissipation factor. In addition, through the frequency analysis, the transit timing variations presented here are unlikely to be periodic, but could be signals of a slow orbital decay.

29 citations

Journal ArticleDOI
TL;DR: Drift-resonant Landau excitation of spiral density waves in a stellar disk of flat galaxies is proposed as a mechanism for the formation of structural features such as spiral arms and the slow dynamical relaxation of galaxies in a regime of hydrodynamical Jeans-type stability.
Abstract: Drift-resonant Landau excitation of spiral density waves in a stellar disk of flat galaxies is proposed. This excitation of waves is suggested as a mechanism for the formation of structural features such as spiral arms and the slow dynamical relaxation of galaxies in a regime of hydrodynamical Jeans-type stability.

25 citations


Cited by
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Journal ArticleDOI
TL;DR: In the far future, evolution will mostly be secular, the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos as mentioned in this paper.
Abstract: ▪ Abstract The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and...

1,767 citations

Journal ArticleDOI
TL;DR: In this paper, the authors study numerical simulations of satellite galaxy disruption in a potential resembling that of the Milky Way and assess whether a merger origin for the stellar halo would leave observable fossil structure in the phase space distribution of nearby stars.
Abstract: We study numerical simulations of satellite galaxy disruption in a potential resembling that of the Milky Way. Our goal is to assess whether a merger origin for the stellar halo would leave observable fossil structure in the phase-space distribution of nearby stars. We show how mixing of disrupted satellites can be quantified using a coarse-grained entropy. Although after 10 Gyr few obvious asymmetries remain in the distribution of particles in configuration space, strong correlations are still present in velocity space. We give a simple analytic description of these effects, based on a linearized treatment in action-angle variables, which shows how the kinematic and density structure of the debris stream changes with time. By applying this description we find that a single dwarf elliptical-like satellite of current luminosity 10 8 L⊙ disrupted 10 Gyr ago from an orbit circulating in the inner halo (mean apocentre � 12 kpc) would contribute about � 30 kinematically cold streams with internal velocity dispersions below 5 kms −1 to the local stellar halo. If the whole stellar halo were built by such disrupted satellites, it should consist locally of 300 500 such streams. Clear detection of all these structures would require a sample of a few thousand stars with 3-D velocities accurate to better than 5 kms −1 . Even with velocity errors several times worse than this, the expected clumpiness should be quite evident. We apply our formalism to a group of stars detected near the North Galactic Pole, and derive an order of magnitude estimate for the initial properties of the progenitor system.

412 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present an updated model for the structure of the Milky Way and incorporate it into the previously published parallax-based distance estimation program for sources associated with spiral arms.
Abstract: We compile and analyze ~200 trigonometric parallaxes and proper motions of molecular masers associated with very young high-mass stars. These measurements strongly suggest that the Milky Way is a four-arm spiral. Fitting log-periodic spirals to the locations of the masers, allows us to significantly expand our view of the structure of the Milky Way. We present an updated model for its spiral structure and incorporate it into our previously published parallax-based distance-estimation program for sources associated with spiral arms. Modeling the three-dimensional space motions yields estimates of the distance to the Galactic center, Ro = 8.15 +/- 0.15 kpc, the circular rotation speed at the Sun's position, To = 236 +/- 7 km/s, and the nature of the rotation curve. Our data strongly constrain the full circular velocity of the Sun, To + Vsun = 247 +/- 4 km/s, and its angular velocity, (To + Vsun)/Ro = 30.32 +/- 0.27 km/s/kpc. Transforming the measured space motions to a Galactocentric frame which rotates with the Galaxy, we find non-circular velocity components typically about 10 km/s. However, near the Galactic bar and in a portion of the Perseus arm, we find significantly larger non-circular motions. Young high-mass stars within 7 kpc of the Galactic center have a scale height of only 19 pc and, thus, are well suited to define the Galactic plane. We find that the orientation of the plane is consistent with the IAU-defined plane to within +/-0.1 deg., and that the Sun is offset toward the north Galactic pole by Zsun = 5.5 +/- 5.8 pc. Accounting for this offset places the central supermassive black hole, Sgr A*, in the midplane of the Galaxy. Using our improved Galactic parameters, we predict the Hulse-Taylor binary pulsar to be at a distance of 6.54 +/- 0.24 kpc, assuming its orbital decay from gravitational radiation follows general relativity.

247 citations

Journal ArticleDOI
25 Feb 2005-Science
TL;DR: Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem have produced many new findings, including new saturnian moons; refined orbits of new and previously known moons; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings.
Abstract: Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem (ISS) during the first 9 months of Cassini operations at Saturn have produced many new findings. These include new saturnian moons; refined orbits of new and previously known moons; narrow diffuse rings in the F-ring region and embedded in gaps within the main rings; exceptionally fine-scale ring structure in moderate– to high–optical depth regions; new estimates for the masses of ring-region moons, as well as ring particle properties in the Cassini division, derived from the analysis of linear density waves; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings.

233 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a simple one-zone model of protostellar accretion disks with high-mass infall rates, which combines a selfconsistent calculation of disk temperatures with an approximate treatment of angular momentum transport via two mechanisms.
Abstract: Most analytic work to date on protostellar disks has focused on those in isolation from their environments. However, observations are now beginning to probe the earliest, most embedded phases of star formation, during which disks are rapidly accreting from their parent cores and cannot be modeled in isolation. We present a simple, one-zone model of protostellar accretion disks with high-mass infall rates. Our model combines a self-consistent calculation of disk temperatures with an approximate treatment of angular momentum transport via two mechanisms. We use this model to survey the properties of protostellar disks across a wide range of stellar masses and evolutionary times and make predictions for disks' masses, sizes, spiral structure, and fragmentation that will be directly testable by future large-scale surveys of deeply embedded disks. We define a dimensionless accretion-rotation parameter that, in conjunction with the disk's temperature, controls the disk evolution. We track the dominant mode of angular momentum transport and demonstrate that for stars with final masses greater than roughly one solar mass, gravitational instabilities are the most important mechanism as most of the mass accumulates. We predict that binary formation through disk fission, fragmentation of the disk into small objects, and spiral arm strength all increase in importance to higher stellar masses.

215 citations