887 research outputs found
Microlensing of tidal debris on the Magellanic great circle
Increasing evidences suggest that the Galactic halo is lumpy on kpc scales
due to the accretion of at least a dozen small galaxies (LMC/SMC, Sgr, Fornax
etc.). Faint stars in such lumpy structures can significant microlense a
background star with an optical depth , which is comparable to
the observed value to the LMC (Alcock et al. 1996c). The observed several
microlensing events towards the LMC can be explained by a tidal debris tail
lying in the Magellanic plane if the progenitor of the Magellanic Clouds and
Stream and other satellite galaxies in the Magellanic plane has a mass about
twice that of the disc of the LMC. The LMC stars can either lense stars in the
debris tail behind the LMC, or be lensed by stars in the part of debris tail in
front of the LMC. The models are consistent with an elementary particle
dominated Galactic halo without massive compact halo objects (MACHOs). They
also differ from Sahu's (1994) LMC-self-lensing model by predicting a higher
optical depth and event rate and less concentration of events to the LMC
center.Comment: 8 pages no figures in uuencoded compressed tar file. References
updated, text reformatted, some corrections made in content. Resubmitted to
MNRA
Dynamical limits on galactic winds, halo machos and intergalactic globular clusters
We argue that any violent galactic winds following early epoch of star bursts
would significantly weaken the potentials of galaxies, and leave lasting
signatures such as a lowered dark halo density and preferentially
radial/escaping orbits for halo tracers such as globular clusters. A galaxy is
disintegrated if more than half of its dynamical mass is blown off. The
presence of dense halos in galaxies and the absence of intergalactic/escaping
globulars should imply an upper limit on the amount of baryons lost in galactic
winds around 4% of the total mass of the galaxy. This translates to limits on
the baryons participating the early star bursts and baryons locked in stellar
remnents, such as white dwarfs. The amount of halo white dwarfs claimed in
recent proper motion searches and microlensing observations in the Galactic
halo are too high to be consistent with our dynamical upper limits. Similar
arguments also imply upper limits for the amount of neutron stars and stellar
black holes, in galaxy halos. Nevertheless, a milder outflow is desirable,
especially in dwarf galaxies, both for lowering their cold dark matter central
density and for injecting metals to the intergalactic medium.Comment: 18p with 5 figs, MNRAS in pres
Building galaxy models with Schwarzschild method and spectral dynamics
Tremendous progress has been made recently in modelling the morphology and
kinematics of centers of galaxies. Increasingly realistic models are built for
central bar, bulge, nucleus and black hole of galaxies, including our own. The
newly revived Schwarzschild method has played a central role in these
theoretical modellings. Here I will highlight some recent work at Leiden on
extending the Schwarzschild method in a few directions. After a brief
discussion of (i) an analytical approach to include stochastic orbits (Zhao
1996), and (ii) the ``pendulum effect'' of loop and boxlet orbits (Zhao,
Carollo, de Zeeuw 1999), I will concentrate on the very promising (iii)
spectral dynamics method, with which not only can one obtain semi-analytically
the actions of individual orbits as previously known, but also many other
physical quantities, such as the density in configuration space and the
line-of-sight velocity distribution of a superposition of orbits (Copin, Zhao &
de Zeeuw 1999). The latter method also represents a drastic reduction of
storage space for the orbit library and an increase in accuracy over the
grid-based Schwarzschild method.Comment: 11 pages including 3 ps figures, Contribution to IAU Colloquium 172
on ``Impact of Modern Dynamics in Astronomy'', July 6-11, 1998, Namur,
Belgium; ed. S. Ferraz-Mello (Dordrecht:Kluwer
A Self-Consistent Dynamical Model for the COBE Detected Galactic Bar
A 3D steady state stellar dynamical model for the Galactic bar is constructed
with 485 orbit building blocks using an extension of Schwarzschild technique.
The weights of the orbits are assigned using non-negative least square method.
The model fits the density profile of the COBE light distribution, the observed
solid body stellar rotation curve, the fall-off of minor axis velocity
dispersion and the velocity ellipsoid at Baade's window. We show that the model
is stable. Maps and tables of observable velocity moments are made for easy
comparisons with observation. The model can also be used to set up equilibrium
initial conditions for N-body simulations to study stability. The technique
used here can be applied to interpret high quality velocity data of external
bulges/bars and galactic nuclei.Comment: submitted to MNRAS; 37 page AAS latex file with 2 tables and no
figures; complete uuencoded compressed PS file with 9 figs is available at
ftp://ibm-1.mpa-garching.mpg.de/pub/hsz/cobe_bar_dynamics.uu Hardcopies are
available by reques
Analytical Dynamical Models for Double-Power-Law Galactic Nuclei
Motivated by the finding that the observed surface brightness profile of many
galactic nuclei are well fit by double-power-laws, we explore a range of
spherical self-consistent dynamical models with this light profile.
We find that the corresponding deprojected volume density profile, phase
space density and line-of-sight velocity distribution of these models are well
fit by simple analytic approximations. We illustrate the application of these
models by fitting a sample of about 25 galactic nuclei observed by Hubble Space
Telescope. We give the derived volume density, phase space density, velocity
dispersion and line profile parameters in tables. The models are useful for
predicting kinematic properties of these galaxies for comparison with future
observations. They can also be easily applied to seed N-body simulations of
galactic nuclei with realistic density profiles for studying the evolution of
these systems.Comment: 25 pages, 6 figures uuencoded compressed tar file
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