18 research outputs found
Image construction from the IRAS survey and data fusion
The IRAS survey data can be used successfully to produce images of extended objects. The major difficulty, viz. non-uniform sampling, different response functions for each detector, and varying signal-to-noise levels for each detector for each scan, were resolved. The results of three different image construction techniques are compared: co-addition, constrained least squares, and maximum entropy. The maximum entropy result is superior. An image of the galaxy M51 with an average spatial resolution of 45 arc seconds, is presented using 60 micron survey data. This exceeds the telescope diffraction limit of 1 minute of arc, at this wavelength. Data fusion is a proposed method for combining data from different instruments, with different spatial resolutions, at different wavelengths. Direct estimates of the physical parameters, temperature, density and composition, can be made from the data without prior images (re-)construction. An increase in the accuracy of these parameters is expected as the result of this more systematic approach
Accretion of a satellite onto a spherical galaxy. II. Binary evolution and orbital decay
We study the dynamical evolution of a satellite orbiting outside of a
companion spherical galaxy. The satellite is subject to a back-reaction force
resulting from the density fluctuations excited in the primary stellar system.
We evaluate this force using the linear response theory developed in Colpi and
Pallavicini (1997). The force is computed in the reference frame comoving with
the primary galaxy and is expanded in multipoles. To capture the relevant
features of the physical process determining the evolution of the detached
binary, we introduce in the Hamiltonian the harmonic potential as interaction
potential among stars. The dynamics of the satellite is computed
self-consistently. We determine the conditions for tidal capture of a satellite
from an asymptotic free state. If the binary comes to existence as a bound
pair, stability against orbital decay is lost near resonance. The time scale of
binary coalescence is computed as a function of the eccentricity and mass
ratio. In a comparison with Weinberg's perturbative technique we demonstrate
that pinning the center of mass of the galaxy would induce a much larger torque
on the satellite.Comment: 13 pages, Tex,+ 10 .ps figures Submitted to The Astrophysical Journa
Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response
The evolution of a small satellite inside a more massive truncated isothermal
spherical halo is studied using both the Theory of Linear Response for
dynamical friction and N-Body simulations. The analytical approach includes the
effects of the gravitational wake, of the tidal deformation and the shift of
the barycenter of the primary, so unifying the local versus global
interpretation of dynamical friction. Sizes, masses, orbital energies and
eccentricities are chosen as expected in hierarchical clustering models. We
find that in general the drag force in self-gravitating backgrounds is weaker
than in uniform media and that the orbital decay is not accompanied by a
significant circularization. We also show that the dynamical friction time
scale is weakly dependent on the initial circularity. We provide a fitting
formula for the decay time that includes the effect of mass and angular
momentum loss. Live satellites with dense cores can survive disruption up to an
Hubble time within the primary, notwithstanding the initial choice of orbital
parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A
and Fornax, have already suffered mass stripping and, with their present
masses, the sinking times exceed 10 Gyr even if they are on very eccentric
orbits.Comment: 27 pages including 9 figures. Accepted for publication in the
Astrophysical Journal. Part 2, issue November 10 1999, Volume 52
Galaxies in N-body simulations: overcoming the overmerging problem
We present analysis of the evolution of dark matter halos in dense
environments of groups and clusters in dissipationless cosmological
simulations. The premature destruction of halos in such environments, known as
the overmerging, reduces the predictive power of N-body simulations and makes
difficult any comparison between models and observations. We analyze the
possible processes that cause the overmerging and assess the extent to which
this problem can be cured with current computer resources and codes. Using both
analytic estimates and high resolution numerical simulations, we argue that the
overmerging is mainly due to the lack of numerical resolution. We find that the
force and mass resolution required for a simulated halo to survive in galaxy
groups and clusters is extremely high and was almost never reached before: ~1-3
kpc and 10^8-10^9 Msun, respectively. We use the high-resolution Adaptive
Refinement Tree (ART) N-body code to run cosmological simulations with the
particle mass of \approx 2x10^8/h Msun} and the spatial resolution of \approx
1-2/h kpc, and show that in these simulations the halos do survive in regions
that would appear overmerged with lower force resolution. Nevertheless, the
halo identification in very dense environments remains a challenge even with
the resolution this high. We present two new halo finding algorithms developed
to identify both isolated and satellite halos that are stable (existed at
previous moments) and gravitationally bound. To illustrate the use of the
satellite halos that survive the overmerging, we present a series of halo
statistics, that can be compared with those of observed galaxies. (Abridged)Comment: Accepted for publication in ApJ, substantional revisions after the
first version, LaTeX 23 pages, 18 figs. (uses emulateapj.sty),
Full-resolution version of Fig.9 is available upon reques
Drag on a satellite moving across a spherical galaxy. I. Tidal and frictional forces in shortlived encounters
We derive a formalism, within the theory of linear response, for the analysis
of the interaction of a satellite (the perturber) with a spherical galaxy whose
equilibrium is described by a one-particle distribution function. We compute
the formal expression of the force on the satellite including the self-gravity
of the stars and the shift of the stellar center of mass. We apply the
perturbative technique to the case of a satellite moving at high speed across a
stellar system and find a natural decomposition of the force into a global
component resulting from the tidal interaction and a component that is related
to dynamical friction. When the satellite orbits outside the galaxy, we derive
the force in the impulse approximation. In penetrating shortlived encounters,
the wake excited in the density field is responsible for most of the
deceleration. We find that dynamical friction rises from a memory effect
involving only those stars perturbed along the path. The force can be written
in terms of an effective Coulomb logarithm which now depends on the dynamical
history. It is derived for two simple equilibrium density distributions. In the
case of a homogeneous cloud, we compute the total energy loss: Tides excited by
the satellite in the galaxy reduce the value of the energy loss by friction.Comment: 22 pages, Tex + 5 .ps figures Submitted to The Astrophysical Journa