976 research outputs found

    Microlensing of tidal debris on the Magellanic great circle

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    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 10−7−10−610^{-7}-10^{-6}, 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

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    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

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    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

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    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

    Extinction bias of microlensed stars towards the LMC and the fraction of machos in the halo

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    We study the effect of reddening on microlensed stars towards the LMC. If lenses are in the LMC disk, then the source stars should be at the far side or behind the LMC disk. Thus they should experience more reddening and extinction by dust in the LMC disk than typical stars in the immediate neighbouring lines of sight. We simulate this effect in a variety of models for the LMC stars and dust. We stress that the optical depth is not one number, but a function of the reddening of the survey stars. We discuss how these effects could be used to constrain the fraction of machos in the dark halo. The effect of patchiness of dust can be controled by working with faint stars in the smallest patches of the sky around the microlensed stars. This can be done most effectively with the Hubble Space Telescope in the ultra-violet. The non-detection of the reddening bias would be strongly in favor of machos in the Galactic halo.Comment: A longer Introduction and minor changes with text and figure format. 18 pages with 9 postscript figures. ApJ submitte