arXiv subjects

Oleg Y. Gnedin

Publications and source records attributed to Oleg Y. Gnedin.

119 records · Page 7Linked to original sources

arXiv1999

Cosmological Neutrino Background Revisited

We solve the Boltzmann equation for cosmological neutrinos around the epoch of the electron-positron annihilation in order to verify the freeze-out approximation and to compute accurately the cosmological neutrino distribution function. We find the radiation energy density to be about 0.3% higher than the one predicted by the freeze-out approximation. As a result, the spectrum of the Cosmic Microwave Background anisotropies changes by 0.3-05%, depending on the angular scale, and the amplitude of the mass fluctuations on scales below about 100 h^{-1} Mpc decreases by about 0.2-0.3%.

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arXiv1999

Effects of Tidal Shocks on the Evolution of Globular Clusters

We present new Fokker-Planck models of the evolution of globular clusters, including gravitational tidal shocks. We extend our calculations beyond the core collapse by adopting three-body binary heating. Effects of the shocks are included by adding the tidal shock diffusion coefficients to the ordinary Fokker-Planck equation: the first order heating term, , and the second order energy dispersion term, . As an example, we investigate the evolution of models for the globular cluster NGC 6254. Using the Hipparcos proper motions, we are now able to construct orbits of this cluster in the Galaxy. Tidal shocks accelerate significantly both core collapse and the evaporation of the cluster and shorten the destruction time from 24 Gyr to 18 Gyr. We examine various types of adiabatic corrections and find that they are critical for accurate calculation of the evolution. Without adiabatic corrections, the destruction time of the cluster is twice as short. We examine cluster evolution for a wide range of the concentration and tidal shock parameters, and determine the region of the parameter space where tidal shocks dominate the evolution. We present fitting formulae for the core collapse time and the destruction time, covering all reasonable initial conditions. In the limit of strong shocks, the typical value of the core collapse time decreases from 10 t_{rh} to 3 t_{rh} or less, while the destruction time is just twice that number. The effects of tidal shocks are rapidly self-limiting: as clusters lose mass and become more compact, the importance of the shocks diminishes. This implies that tidal shocks were more important in the past.

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arXiv1999

On the Self-Consistent Response of Stellar Systems to Gravitational Shocks

We study the reaction of a globular star cluster to a time-varying tidal perturbation (gravitational shock) using self-consistent N-body simulations and address two questions. First, to what extent is the cluster interior protected by adiabatic invariants. Second, how much further energy change does the postshock evolution of the cluster potential produce and how much does it affect the dispersion of stellar energies. We introduce the adiabatic correction} as ratio of the energy change, , to its value in the impulse approximation. When the potential is kept fixed, the numerical results for the adiabatic correction for stars with orbital frequency ωcan be approximated as (1 + ω^2 τ^2)^{-γ}. For shocks with the characteristic duration of the order the half-mass dynamical time of the cluster, τ< t_{dyn,h}, the exponent γ= 5/2. For more prolonged shocks, τ> 4 t_{dyn,h}, the adiabatic correction is shallower, γ= 3/2. When we allow for self-gravity and potential oscillations which follow the shock, the energy of stars in the core changes significantly, while the total energy of the system is conserved. Paradoxically, the postshock potential fluctuations reduce the total amount of energy dispersion, . The effect is small but real and is due to the postshock energy change being statistically anti-correlated with the shock induced heating. These results are to be applied to Fokker-Planck models of the evolution of globular clusters.

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arXiv1998

Dynamics of Galaxies in Clusters

Recent observations by the Hubble Space Telescope suggest that a strong dynamical and morphological evolution of disk galaxies has occurred in clusters of galaxies. I present the result of the self-consistent high-resolution simulations of galactc dynamics in the three cosmologically formed clusters. The disks of large galaxies survive but thicken by a factor of two and cease new star formation. The dark matter halos are truncated in agreement with analytical models. The overall effect is the morphological transformation of spirals into S0 galaxies.

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arXiv1997

Evolution of Globular Clusters: Effects of Tidal Shocks

The semi-analytic theory of tidal shocks proves to be a powerful tool to study tidal interactions of star clusters and satellite galaxies with their massive hosts. New models of the globular cluster evolution employ a combination of analytic estimates, solutions of the Fokker-Planck equation and direct N-body simulations. The models predict large destruction rates for the Galactic globular clusters. Those on the highly eccentric orbits around the Galactic center are much more likely to be disrupted than the ones on nearly circular orbits. The destruction rates are largely increased near the bulge. Disruption of the low-mass clusters changes the Luminosity Function of the Globular Cluster System, shifting the peak of the Luminosity Function to the brighter end.

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arXiv1997

Emission Lines from Companion Galaxies of QSOs

Anticipating new spectroscopic studies of quasar companions, we calculate expected emission line spectra for the companion galaxies. Under the assumption of isotropic photoionizing radiation, we predict equivalent widths for Halpha, Hbeta, [O III], [O II], [Ne III], and Mg II lines. We consider 6 quasars, two of which (PKS 2135-14 and PKS 2349-014) have very close companions with projected separation < 4 h^{-1} kpc. For these companions the calculated emission lines should be easily observable, with W_lambda > 20 AA for the [O III], [O II], and Halpha lines. Presence of strong emission lines from the companions occupying only a tiny solid angle would suggest that 1) quasar radiation is isotropic; 2) companion galaxies have measurable amount of gas, which can be assessed using the photoionization models; 3) host galaxies, if present, have been depleted of gas or do not interact with the companions.

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arXiv1997

Dynamics of ``Small Galaxies'' in the Hubble Deep Field

We have previously found in the Hubble Deep Field a significant angular correlation of faint, high color-redshift objects on scales below one arcsecond, or several kiloparsecs in metric size. We examine the correlation and nearest neighbor statistics to conclude that 38% of these objects in the HDF have a companion within one arcsecond, three times the number expected in a random distribution with the same number of objects. We examine three dynamical scenarios for these object multiplets: 1) the objects are star-forming regions within normal galaxies, whose disks have been relatively dimmed by K-correction and surface brightness dimming; 2) they are fragments merging into large galaxies; 3) they are satellites accreting onto normal L_* galaxies. We find that hypothesis 1 is most tenable. First, large galaxies in the process of a merger formation would have accumulated too much mass in their centers (5e12 M_sun inside 2 kpc) to correspond to present day objects. Second, accretion by dynamical friction occurs with a predictable density vs. radius slope, not seen among the faint HDF objects. Since the dynamical friction time is roughly (1 Gyr), a steady-state should have been reached by redshift z < 5. Star-forming regions within galaxies clearly present no dynamical problems. Since large spirals would still appear as such in the HDF, we favor a scenario in which the faint compact sources in the HDF are giant starforming regions within small normal galaxies, such as Magellanic irregulars. Finally we checked that reduction in mass-to-light from induced star-formation cannot alone explain the luminosity overdensity.

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arXiv1996

The Globular Cluster Luminosity Function as a Distance Indicator: Dynamical Effects

The dynamical evolution of the globular cluster systems in galaxies is predicted, based on the standard dynamical theory normalized to the example of the Milky Way. The major processes varying with the galactocentric distance are the tidal shocks and dynamical friction. Our simple model explains, on a quantitative basis, the observed differences of the inner and outer populations of globular clusters. We can thus calculate corrections for dynamical evolution for the luminosity function of globular clusters with the assumption that the initial luminosity function is identical in all galaxies (and we can test this assumption as well, in certain cases). Then we can compute the expected distribution of absolute magnitudes and compare it with the observed distribution of apparent magnitudes to estimate the distance moduli for M31 and M87. Using this new method we find dm(M31)=24.05 +- 0.23, dm(M87)=30.83 +- 0.17, as compared to current best estimates using other methods of dm(M31)=24.30 +- 0.20, dm(M87)=31.0 +- 0.1. As a check on the method we compute, and compare with observations, the differences between the inner and outer globular clusters in all three galaxies. This new method, coupled with HST observations, promises to provide an independent method of estimating distances to galaxies with recession velocities < 10,000 km/s, or D < 100 Mpc.

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arXiv1996

On the Evolution of the Globular Cluster Luminosity Function: The Differences

We consider the observational signature of the dynamical effects on the luminosity function of globular clusters. For the three best studied systems, in Milky Way, M31, and M87, there is a statistically significant difference between the inner and outer population of globular clusters. In all cases the inner clusters are on average brighter than the outer clusters ($0.26 < Δm_0 < 0.84$) and have a smaller dispersion in magnitudes ($0.04 < Δσ< 0.53$), with the larger differences for the local, better observed samples. The differences are of the type that would be expected if the inner population had been depleted by tidal shocks. The results suggest that the inner population suffers substantial evolution from its initial distribution and cannot not be used as a standard candle without correction for dynamical evolution.

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arXiv1996

Destruction of the Galactic Globular Cluster System

We investigate the dynamical evolution of the Galactic Globular Cluster System in considerably greater detail than has been done hitherto, finding that destruction rates are significantly larger than given by previous estimates. More than half of the present clusters (52%-58% for the OC galactic model, and 75%-86% for the BSS model) will be destroyed in the next Hubble time. For the evolution of individual clusters we use a Fokker-Planck code including two-body relaxation, tidal truncation of clusters, compressive gravitational shocks while clusters pass through the Galactic disk, and tidal shocks due to passage close to the bulge. Gravitational shocks are treated comprehensively, using a recent result by Kundic & Ostriker (1995) that the shock-induced relaxation term, driving an additional dispersion of energies, is generally more important than the usual energy shift term. We discuss some implications for a past history of the Globular Cluster System, and the initial distribution of the destruction times raising the possibility that the current population is but a very small fraction of the initial population with the remnants of the destroyed clusters constituting presently a large fraction of the spheroid (bulge+halo) stellar population.

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arXiv1995

MEASURING SPIRAL ARM TORQUES: RESULTS FOR M100

Spiral arms, if they are massive, exert gravitational torques that transport angular momentum radially within galactic disks. These torques depend not on the pattern speed or permanence of the arms but only on the nonaxisymmetric mass distribution. Hence the torques can be measured from photometry. We demonstrate this using $gri$ CCD data for M100 (NGC 4321). Since we find consistency among the three bands, we believe that dust and young stars in the arms do not seriously bias our results. If the present epoch is representative, the timescale for redistribution of angular momentum in M100 is $5-10$ Gyr, the main uncertainty being the mass-to-light ratio of the disk.

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