arXiv subjects

HongSheng Zhao

Publications and source records attributed to HongSheng Zhao.

At least 91 records · Page 5Linked to original sources

arXiv2001

Are dwarf spheroidal galaxies dark matter dominated or remnants of disrupted larger satellite galaxies? -- A possible test

The failure of standard cosmolocical models in accounting for the statistics of dwarf satellites and the rotation curve of gas-rich dwarf galaxies in detail has led us to examine whether earlier non-equilibrium models of dwarf spheroidal satellites without any dark matter should be reconsidered in more detail. Such models can explain the high dispersion of the dwarf spheroids by the projection of disrupted tidal debris. We show in the case of Milky Way satellites, that these models predict a significant spread in the apparent magnitude of horizontal branch stars which is correlated with sky position and velocity. In particular, the models produce a strong correlation of radial velocity with the long axis of the dwarf. Current data do not set strong enough constraints on the models, but we suggest that photometric and spectroscopic surveys of extra-tidal stars of nearby dwarf spheroids in the Milky Way and Andromeda can falsify these models without dark matter.

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arXiv2001

LCDM-based models for the Milky Way and M31 I: Dynamical Models

We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard LCDM cosmology. The resulting models score remarkably well when confronted with the broad range of observational data available for the Milky Way and M31 galaxies, giving a Milky Way virial mass of 1-2x10^12Msun and concentration C=12-17. We consider two types of models, in which: (A) baryons conserve angular momentum and (B) some of the angular momentum of the baryons is transferred to the dark matter. Type-A models produce good agreement with observed rotation curves and obey constraints in the solar neighborhood, but may have too much dark matter in the center to allow a fast rotating bar. The type-B models with angular momentum transport have a slightly more massive disk and less dark matter in the central part, allowing a fast rotating bar to persist. Both classes of models probably have sufficient baryonic mass in the central 3.5kpc to reproduce recent observational values of the optical depth to microlensing events towards the Galactic center. All models require that about 1/2 of all baryons expected inside the virial radius must not be in the disk or bulge. We investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function, and find a range of parameter space that is allowed by this constraint. We conclude that rotation curves and dynamical properties of ``normal'' high surface brightness spiral galaxies appear to be consistent with standard LCDM.

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arXiv2001

A Dark-Matter Spike at the Galactic Center?

The past growth of the central black hole (BH) might have enhanced the density of cold dark matter halo particles at the Galactic center. We compute this effect in realistic growth models of the present (2-3)*10**6 solar mass BH from a low-mass seed BH, with special attention to dynamical modeling in a realistic galaxy environment with merger and orbital decay of a seed BH formed generally outside the exact center of the halo. An intriguing ``very-dense spike'' of dark matter has been claimed in models of Gondolo and Silk with density high enough to contradict with experimental upper bounds of neutralino annihilation radiation. This ``spike'' disappears completely or is greatly weakened when we include important dynamical processes neglected in their idealized/restrictive picture with cold particles surrounding an at-the-center zero-seed adiabaticly-growing BH. For the seed BH to spiral in and settle to the center within a Hubble time by dynamical friction, the seed mass must be at least a significant fraction of the present BH. Any subsequent at-the-center growth of the BH and steepening of the central Keplerian potential well can squeeze the halo density distribution only mildly, whether the squeezing happens adiabatically or instantaneously.

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arXiv2000

Systematic uncertainties in gravitational lensing models: a semi-analytical study of PG1115+080

While the Hubble constant can be derived from observable time delays between images of lensed quasars, the result is often highly sensitive to assumptions and systematic uncertainties in the lensing model. Unlike most previous authors we explore a broad class of models with non-parametric lens radial profile and allow for non-elliptical lens potentials. The axis ratio and position angle of the lens can be determined from the image positions of quadruple gravitational lensed systems by solving the lens equation semi-analytically, independent of the radial profile. We give simple equations for estimating the power-law slope of the lens density directly from the image positions and for estimating the time delay ratios. Our method greatly simplifies the numerics for fitting observations and is fast in exploring the model parameter space. As an illustration we apply the model to PG1115+080. We show that the measured image positions and time delays do not uniquely determine the Hubble constant. The mass-to-light ratio of the lens galaxy increases with radius, higher at image C than at image B.

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arXiv2000

Dynamics of the Galactic Bulge using Planetary Nebulae

Evidence for a bar at the center of the Milky Way triggered a renewed enthusiasm for dynamical modelling of the Galactic bar-bulge. Our goal is to compare the kinematics of a sample of tracers, planetary nebulae, widely distributed over the bulge with the corresponding kinematics for a range of models of the inner Galaxy. Three of these models are N-body barred systems arising from the instabilities of a stellar disk (Sellwood, Fux and Kalnajs), and one is a Schwarzschild system constructed to represent the 3D distribution of the COBE/DIRBE near-IR light and then evolved as an N-body system for a few dynamical times (Zhao). For the comparison of our data with the models, we use a new technique developed by Saha (1998). The procedure finds the parameters of each model, i.e. the solar galactocentric distance R_o in model units, the orientation angle phi, the velocity scale (in km/s per model unit), and the solar tangential velocity which best fit the data.

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arXiv2000

Magnitude bias of microlensed sources towards the Large Magellanic Cloud

There are lines of evidence suggesting that some of the observed microlensing events in the direction of the Large Magellanic Cloud (LMC) are caused by ordinary star lenses as opposed to dark Machos in the Galactic halo. Efficient lensing by ordinary stars generally requires the presence of one or more additional concentrations of stars along the line of sight to the LMC disk. If such a population behind the LMC disk exists, then the source stars (for lensing by LMC disk objects) will be drawn preferentially from the background population and will show systematic differences from LMC field stars. One such difference is that the (lensed) source stars will be farther away than the average LMC field stars, and this should be reflected in their apparent baseline magnitudes. We focus on red clump stars: these should appear in the color-magnitude diagram at a few tenths of a magnitude fainter than the field red clump. Suggestively, one of the two near-clump confirmed events, MACHO-LMC-1, is a few tenths of magnitude fainter than the clump.

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arXiv1999

Microlensing by the Galactic Bar

We compute the predicted optical depth and duration distribution of microlensing events towards Baade's window in a model composed of a Galactic disk and a bar. The bar model is a self-consistent dynamical model built out of individual orbits that has been populated to be consistent with the COBE maps of the Galaxy and kinematic observations of the Bulge. We find most of the lenses are at 6.25 kpc in BW. The microlensing optical depth of a 2E10 Msun bar plus a truncated disk is (2.2+-0.3)E-6, consistent with the very large optical depth (3.2+-1.2)E-6 found by Udalski et al. (1994). This prediction doubles that of axisymmetric models by Kiraga and Paczynski (1994) since the bar is elongated along the line-of-sight. The large Einstein radius and small transverse velocity dispersion also predict a longer event duration in the bar model than the KP model. The event rate and duration distribution also depend on the lower mass cutoff of the lens mass function. With a 0.1Msun cutoff, 5-7 events of typical 20 days duration are expected for the OGLE experiment according to our model, while Udalski et al. observed 9 events with durations from 8-62 days. On the other hand, if most of the lenses are brown dwarfs, our model predicts too many short duration events; a KS test finds only 7% probability for the model with 0.01Msun cutoff to be consistent with current data.

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arXiv1999

Deprojection of light distributions of nearby systems: perspective effect and non-uniqueness

Deriving the 3-dimensional volume density distribution from a 2-dimensional light distribution of a system yields generally non-unique results. The case for nearby dust-free systems is studied, taking into account the extra constraints from the perspective effect. It is shown analytically that a new form of non-uniqueness exists. We can design a Phantom Spheroid (PS) for a nearby system which preserves the intrinsic mirror symmetry and projected surface brightness of the system while changing the shape and the major-axis orientation of the system. A family of analytical models are given as functions of the distance ($D_0$) to the object and the amount ($γ$) of the superimposed PS density. The PS density, different from the well-known konuses of extragalactic systems, makes the luminosity of the system vary slightly with the distance $D_0$. The physical ranges for $γ$ and the major axis angles are constrained analytically by requiring a positive volume density everywhere. These models suggest that observations other than surface brightness maps are required to lift the degeneracy in the tilt angles and axis ratio of the central bar of the Milky Way.

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arXiv1999

Reddening of microlensed LMC stars vs. the location of the lenses

We propose an observational test that can break the degeneracy of two main classes of microlensing models to the Magellanic Clouds: (a) the lenses are located in the Galactic halo, and (b) the lenses are located in the LMC disk. The source stars in the latter (self-lensing) models tend to be at the far side or behind the LMC disk, thus experience more reddening and extinction by dust in the LMC disk than ordinary stars in a nearby line of sight. Clearly such bias would not occur in the macho halo lensing models. We show that this reddening effect is at a level readily observable for the present 30 or so microlensing alerts fields, either with multi-band photometry from a good seeing site, or more definitively with ultra-violet (UV) spectroscopy with Space Telescope Imaging Spectrograph (STIS). Stars behind the LMC dust layer should stand out as UV-faint objects (by more than 1 mag than average stars in the LMC). HST can also resolve numerous faint neighbouring stars within a few arcsecs of a lensed source, hence remove blending in these crowded regions and build a reddening map to control the patchiness of dust.

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arXiv1999

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

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.

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arXiv1999

Evolution of the Galactic Potential and Halo Streamers with Future Astrometric Satellites

ESA's Global Astrometric Interferometer for Astrophysics (GAIA) holds the promise of mapping out the detailed phase space structure of the Galactic halo by providing unprecedented annual proper motion and parallax of $1-10\muas$ astrometric accuracy (Gilmore et al. 1998). Unlike NASA's Space Interferometry Mission (SIM), which will achieve similar accuracies but is a pointed instrument, GAIA will be able to construct a global catalogue of the halo. Here we study proper motions of giant branch stars in a tidal debris torn from a small satellite system in the halo. We follow the evolution of a cold stream on a polar orbit between 8-50 kpc in a variety of histories of the Galactic potential, and observe the bright ($V<18$mag) members of the debris tail with GAIA accuracy. We simulate effects due to the growing or flipping of the Galactic disk over the past 4 Gyrs or the perturbation from a massive accreted lump such as the progenitor of the Magellanic Clouds. Our simulations suggest that the results of Johnston, Zhao, Spergel & Hernquist (1999) and Helmi, Zhao & de Zeeuw (1999) for static Galactic potentials are likely to be largely generalizable to realistic time-dependent potentials: a tidal debris remains cold in spite of evolution and non-axial symmetry of the potential. GAIA proper motion measurements of debris stars might be used to probe both Galactic structure and Galactic history. We also study several other factors influencing our ability to identify streams, including accuracy of radial velocity and parallax data from GAIA, and contamination from random field stars. We conclude that nearby, cold streams could be detected with GAIA if these cousins of the Sagittarius stream exist.

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arXiv1999

The pendulum dilemma of fish orbits

The shape of a galaxy is constrained both by mechanisms of formation (dissipational vs. dissipationless) and by the available orbit families (the shape and amount of regular and stochastic orbits). It is shown that, despite the often very flattened shapes of banana and fish orbits, these boxlet orbits generally do not fit a triaxial galaxy in detail because, similar to loop orbits, they spend too little time at the major axis of the model density distribution. This constraint from the shape of fish orbits is relaxed at (large) radii where the density profile of a galaxy is steep.

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arXiv1999

A Radial Velocity Survey for LMC Microlensed Sources

We propose a radial velocity survey with the aim to resolve the current dispute on the LMC lensing: in the pro-macho hypothesis the lenses are halo white dwarfs or machos in general; in the pro-star hypothesis both the lenses and the sources are stars in various observed or hypothesized structures of the Magellanic Clouds and the Galaxy. Star-star lensing should prefer sources at the backside or behind the LMC disc because lensing is most efficient if the source is located a few kpc behind a dense screen of stars, here the thin disc of the LMC. This signature of self-lensing can be looked for by a radial velocity survey since kinematics of the stars at the back can be markedly different from that of the majority of stars in the cold, rapidly rotating disc of the LMC. Detailed simulations of effect together with optimal strategies of carrying out the proposed survey are reported here. Assuming that the existing 30 or so alerted stars in the LMC are truely microlensed stars, their kinematics can test the two lensing scenarios; the confidence level varies with the still very uncertain structure of the LMC. Spectroscopy of the existing sample and future events requires about two or three good-seeing nights per year at a 4m-8m class southern telescope, either during the amplification phase or long after.

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arXiv1999

What Could the Machos Be?

If the Universe has a significant baryonic dark component in the form of compact objects in galaxy halos (machos), then there is a minute chance (about $10^{-7}$) that one of the Galactic machos passes sufficiently close to our line of sight to a star out of some $10^7$ monitored stars in the Magellanic Clouds (MCs) that it brightens by more than 0.3 magnitude due to gravitational focusing. After a brief discussion of the current controversy over the interpretation of the observed events, i.e., whether the lensing is caused by halo white dwarfs or machos in general or by stars in various observed or hypothesized structures of the Clouds and the Galaxy, I propose a few observations to put ideas of the pro-macho camp and the pro-star camp to test. In particular, I propose a radial velocity survey towards the MCs.

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arXiv1999

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.

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arXiv1999

Can galactic nuclei be non-axisymmetric? --- The parameter space of power-law discs

The shape of a cusped galactic nucleus is constrained by the range of shapes of orbits in its gravitational potential. It is shown for scale-free non-axisymmetric discs that while a plausible elongated density model requires at least some orbits to spend more time near the major axis than anywhere else, both regular boxlets and tube orbits generally cross the major axis too fast for self-consistency. If galaxies host inner nuclear discs or flat bars with a cuspy surface light profile ($γ=|{d \log μ/ d \log r}|>0$), their ellipticity $1-{b / a}$ cannot be greater than about ${γ/ 2}$. Discs or bars with a shallow central profile ($γ\le 0.3$) should not be strongly elliptical.

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arXiv1999

Studying Evolution of the Galactic Potential and Halo Streamers with Future Astrometric Satellites

Future astrometric satellites, such as SIM (NASA's Space Interferometric Mission) and GAIA (ESA's Global Astrometric Interferometer for Astrophysics), hold the promise of mapping out the detailed phase space structure of the Galactic halo by providing unprecedented annual proper motion and parallax of $1-10μ$as astrometric accuracy. Here we show that proper motions of hundred or so giant branch stars in a tidal debris torn from a small satellite (a $10^{5-7}L_\odot$ Galactic dwarf galaxy or globular cluster) in the halo is sensitive to the current Galactic potential and its past evolution. We follow the evolution of a cold (velocity dispersion of 10 km/s) stream on a nearby (between 8-50 kpc) polar orbit in a variety of histories of the potential of the Galaxy, and observe the bright ($V<18$mag) members of the debris tail with GAIA accuracy. We simulate effects due to the growing or flipping of the Galactic disk over the past 4 Gyrs or the perturbation from a massive accreted lump such as the progenitor of the Magellanic Clouds. We study various factors influencing our ability to identify streams, including contamination from field stars, accuracy of radial velocity and distance data and evolution and non-axial symmetry of the potential. Our simulations suggest that nearby, cold streams could be detected with GAIA if these cousins of the Sagittarius stream exist. Results of Johnston, Zhao, Spergel & Hernquist (1999) and Helmi, Zhao & de Zeeuw (1999) for static Galactic potentials are likely to be largely generalizable to moderately time-evolving potentials. SIM and GAIA measurements of debris stars might be used to probe both Galactic structure and Galactic history.

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arXiv1998

Tidal Streams as Probes of the Galactic Potential

We explore the use of tidal streams from Galactic satellites to recover the potential of the Milky Way. Our study is motivated both by the discovery of the first lengthy stellar stream in the halo (\cite{it98}) and by the prospect of measuring proper motions of stars brighter than 20th magnitude in such a stream with an accuracy of $\sim 4μas/$yr, as will be possible with the Space Interferometry Mission (SIM). We assume that the heliocentric radial velocities of these stars can be determined from supporting ground-based spectroscopic surveys, and that the mass and phase-space coordinates of the Galactic satellite with which they are associated will also be known to SIM accuracy. Using results from numerical simulations as trial data sets, we find that, if we assume the correct form for the Galactic potential, we can predict the distances to the stars as a consequence of the narrow distribution of energy expected along the streams. We develop an algorithm to evaluate the accuracy of any adopted potential by requiring that the satellite and stars recombine within a Galactic lifetime when their current phase-space coordinates are integrated backwards. When applied to a four-dimensional grid of triaxial logarithmic potentials, with varying circular velocities, axis ratios and orientation of the major-axis in the disk plane, the algorithm can recover the parameters used for the Milky Way in a simulated data set to within a few percent using only 100 stars in a tidal stream.

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