419 research outputs found
Effect of the Environment on the Fundamental Plane of Elliptical Galaxies
We present an analysis of interacting E/S0 galaxies location on the
Fundamental Plane. Using the NEMO package, we performed N-body simulations of
close encounters and mergers between two spherical galaxies. We followed how
structural and dynamical parameters (central density, half-mass radius and
velocity dispersion)of galaxies are changed during the encounter. We analysed
the dependence of these changes on initial mass concentration and presence of
dark halo. The results of our simulations are used to discuss the Fundamental
Plane for interacting early-type galaxies.Comment: Poster presented at JENAM-2000 (Joint European and National
Astronomical meeting - S02. Morphology and dynamics of stellar systems: star
clusters, galactic arms and rings
Modulation of Circumstellar Extinction in a Young Binary System with a Low-Mass Companion in a Noncoplanar Orbit
The cyclic activity model of a young star with the low-mass secondary
component (q = M2/M1 <= 0.1) accreting a matter from circumbinary disk is
considered. It is assumed that the orbit is circular and the disk and orbital
planes are non-coplanar. Sets of hydrodynamics models of such a system have
been calculated by the SPH method and then the variations of the circumstellar
extinction and phase light curves were determined. The calculations showed that
depending on the model parameters and orientation of the system in regards to
an observer the different in shape and amplitude light curves can be observed.
An important property of the considered models is also the dependence of the
mass accretion rate onto the components on the phase of the orbital period. The
results of the calculation can be used for analysis of the cyclic activity of
UX Ori stars and young stars with the long-lasting eclipses.Comment: 14 pages, 7 figure
Estimating the Dark Halo Mass from the Relative Thickness of Stellar Disks
We analyze the relationship between the mass of a spherical component and the
minimum possible thickness of stable stellar disks. This relationship for real
galaxies allows the lower limit on the dark halo mass to be estimated (the
thinner the stable stellar disk is, the more massive the dark halo must be). In
our analysis, we use both theoretical relations and numerical N-body
simulations of the dynamical evolution of thin disks in the presence of
spherical components with different density profiles and different masses. We
conclude that the theoretical relationship between the thickness of disk
galaxies and the mass of their spherical components is a lower envelope for the
model data points. We recommend using this theoretical relationship to estimate
the lower limit for the dark halo mass in galaxies. The estimate obtained turns
out to be weak. Even for the thinnest galaxies, the dark halo mass within four
exponential disk scale lengths must be more than one stellar disk mass.Comment: 20 pages including 6 figures. To be published in Astronomy Letters
(v. 32, No. 10, pp. 649-660, 2006
New insights from old cosmic rays: A novel analysis of archival KASCADE data
Cosmic ray data collected by the KASCADE air shower experiment are
competitive in terms of quality and statistics with those of modern
observatories. We present a novel mass composition analysis based on archival
data acquired from 1998 to 2013 provided by the KASCADE Cosmic ray Data Center
(KCDC). The analysis is based on modern machine learning techniques trained on
simulation data provided by KCDC. We present spectra for individual groups of
primary nuclei, the results of a search for anisotropies in the event arrival
directions taking mass composition into account, and search for gamma-ray
candidates in the PeV energy domain.Comment: Proceedings of the 37th International Cosmic Ray Conference
(ICRC2021), 12-23 July 2021, Berlin, Germany - Onlin
Optimal Choice of the Softening Length and Time-Step in N-body Simulations
A criterion for the choice of optimal softening length and
time-step for -body simulations of a collisionless stellar system is
analyzed. Plummer and Hernquist spheres are used as models to follow how
changes in various parameters of an initially equilibrium stable model depend
on and . These dependences are used to derive a criterion for
choosing and . The resulting criterion is compared to Merritt's
criterion for choosing the softening length, which is based on minimizing the
mean irregular force acting on a particle with unit mass. Our criterion for
choosing and indicate that must be a factor of 1.5-2
smaller than the mean distance between particles in the densest regions to be
resolved. The time-step must always be adjusted to the chosen (the
particle must, on average, travel a distance smaller than during
one time-step). An algorithm for solving N-body problems with adaptive
variations of the softening length is discussed in connection with the task of
choosing , but is found not to be promising.Comment: To be published in Astonomy Reports, 11 pages, 5 figure
The Polar Stellar Ring and Dark Halo of NGC 5907
Numerical simulations of the disruption of a dwarf companion moving in the
polar plane of a massive galaxy are presented. The constructed model is
compared with observational data on the recently discovered
low-surface-brightness stellar ring around galaxy NGC 5907. Constraints on the
ring lifetime (<=1.5 gyr after the first approach of the galaxies), on the
structure of companion -- the ring presecursor, and on mass of dark halo of the
main galaxy in whose gravitational field the companion moves are provided. The
dark halo mass within 50 kpc of NGC 5907 center cannot exceed 3 or 4 "visible"
masses.Comment: 8 pages, 6 figures, article published in Astronomy Letters, Vol.26,
No.5, 2000, pp277-28
Mixed Morphology Pairs as a Breeding Ground for Active Nuclei
Mixed morphology pairs offer a simplification of the interaction equation
that involves a gas-rich fast rotator paired with a gas-poor slow rotator. In
past low resolution IRAS studies it was assumed that the bulk of the far
infrared emission originated in the spiral component. However our ISO studies
revealed a surprising number of early-type components with significant IR
emission some of which turned out to show active nuclei. This motivated us to
look at the current statistics of active nuclei in mixed pairs using the
FIR-radio continuum correlation as a diagnostic. We find a clear excess of
early-type components with radio continuum emission and an active nucleus. We
suggest that they arise more often in mixed pairs via cross fueling of gas from
the spiral companion. This fuel is more efficiently channeled into the nucleus
of the slow rotating receptor. In a sample of 112 mixed-morphology pairs from
the Karachentsev catalog we find that about 25-30% of detected mixed pairs show
a displacement from the radio-FIR relation defined by normal star forming
galaxies. The latter objects show excess radio continuum emission while others
extend the relation to unusually high radio and FIR flux levels. Many of the
outliers/extreme emitters involve an early-type component with an active
nucleus. The paired E/S0 galaxies in the sample exhibit a significant excess
detection fraction and a marginal excess luminosity distribution compared to
isolated unpaired E/S0 galaxies.Comment: 3 figure
Bending Instability of Stellar Disks: The Stabilizing Effect of a Compact Bulge
The saturation conditions for bending modes in inhomogeneous thin stellar
disks that follow from an analysis of the dispersion relation are compared with
those derived from -body simulations. In the central regions of
inhomogeneous disks, the reserve of disk strength against the growth of bending
instability is smaller than that for a homogeneous layer. The spheroidal
component (a dark halo, a bulge) is shown to have a stabilizing effect. The
latter turns out to depend not only on the total mass of the spherical
component, but also on the degree of mass concentration toward the center. We
conclude that the presence of a compact (not necessarily massive) bulge in
spiral galaxies may prove to be enough to suppress the bending perturbations
that increase the disk thickness. This conclusion is corroborated by our
-body simulations in which we simulated the evolution of almost equilibrium,
but unstable finite-thickness disks in the presence of spheroidal components.
The final disk thickness at the same total mass of the spherical component
(dark halo + bulge) has been found to be much smaller than that in the
simulations where a concentrated bulge is present.Comment: 27 pages including 10 figures. To be published in Astronomy Letters
(v.31, No. 1, pp. 15-29 2005
Mechanisms of the Vertical Secular Heating of a Stellar Disk
We investigate the nonlinear growth stages of bending instability in stellar
disks with exponential radial density profiles.We found that the unstable modes
are global (the wavelengths are larger than the disk scale lengths) and that
the instability saturation level is much higher than that following from a
linear criterion. The instability saturation time scales are of the order of
one billion years or more. For this reason, the bending instability can play an
important role in the secular heating of a stellar disk in the direction.
In an extensive series of numerical -body simulations with a high spatial
resolution, we were able to scan in detail the space of key parameters (the
initial disk thickness , the Toomre parameter , and the ratio of dark
halo mass to disk mass ). We revealed three distinct
mechanisms of disk heating in the direction: bending instability of the
entire disk, bending instability of the bar, and heating on vertical
inhomogeneities in the distribution of stellar matter.Comment: 22 pages including 8 figures. To be published in Astronomy Letters
(v.29, 2003
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