arXiv subjects

HongSheng Zhao

Publications and source records attributed to HongSheng Zhao.

106 records · Page 6Linked to original sources

arXiv1998

Detecting Halo Streams with GAIA

We investigate what the proposed ESA astrometric satellite GAIA will reveal by observing the halo of the Milky Way. Specifically, we look for halo streams which are the signatures left by the merging/accretion events experienced by a typical galaxy like the Milky Way. We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to generate artificial GAIA halo catalogs. We recover the streams by searching for peaks in angular momentum space.

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arXiv1998

The survival of the Sgr dwarf galaxy and the flatness of the rotation curve of the Galaxy

How has the ``fluffy'' core of the Sgr dwarf galaxy survived multiple strong shocks from the tidal force of the Galactic halo and disc since the formation of the core a Hubble time ago? A scenario that Sgr was deflected to its current orbit by the Magellanic Clouds after a rendezvous on the north Galactic pole 2-3 Gyrs ago is examined. It is shown that the conditions of the collision fix both the sense of circulation of Sgr and the LMC around the Galaxy and the slope of the Galactic rotation curve. The model argues that the two orthogonal polar circles traced by a dozen or so Galactic halo dwarf galaxies and globular clusters (LMC-SMC-Magellanic Stream-Draco-Ursa Minor along $l \approx 270^o$ and M54-Ter 7-Ter 8-Arp 2-NGC 2419-Pal 15 along $l \approx 0^o$) are streams of tidal relics from two ancient galaxies which was captured on two intersecting polar rosette orbits by the Galaxy. Our results favor the interpretation of microlensing towards the LMC being due to source or lens stars in tidal features of the Magellanic Clouds. We discuss direct and indirect observations to test the collision scenario.

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arXiv1998

The microlensing rate and mass function vs. dynamics of the Galactic bar

With the steady increase of the sample size of observed microlenses towards the central regions of the Galaxy, the main source of the uncertainty in the lens mass will shift from the simple Poisson noise to the intrinsic non-uniqueness of our dynamical models of the inner Galaxy, particularly, the Galactic bar. We use a set of simple self-consistent bar models to investigate how the microlensing event rate varies as a function of axis ratio, bar angle and velocity distribution. The non-uniqueness of the velocity distribution of the bar model adds a significant uncertainty (by about a factor of 1.5) to any prediction of the lens mass. Kinematic data and self-consistent models are critical to lift the non-uniqueness. We discuss the implications of these results for the interpretation of microlensing observations of the Galactic bulge. In particular we show that Freeman bar models scaled to the mass of the Galactic bulge/bar imply a typical lens mass of around $0.8M_\odot$, a factor of 3-5 times larger than the value from other models.

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arXiv1997

The Galactic Bar

We summarize recent work on the structure and dynamics of the Galactic bar and inner disk. Current work focusses on constructing a quantitative model which integrates NIR photometry, source count observations, gas kinematics, stellar dynamical observations, and microlensing. Some avenues for future research are discussed.

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arXiv1997

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 $10^{-7}-10^{-6}$, 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.

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arXiv1997

Density cusps: restrictions on non-axisymmetric models

Galactic nuclei are now generally thought to have density cusps in their centres, and the evidence is mounting that as a consequence they are unlikely to be triaxial. Self-consistent stellar dynamical models of non-axisymmetric cusps would be an interesting counter-argument to this conclusion. We consider 2-d analogues of triaxial cusps: a sequence of non-axisymmetric, cuspy discs first described by Sridhar & Touma (1997). Scale-free models with potential {}$Φ\propto r^α$ are examined in detail. It is shown analytically for $0<α\la 0.43$ that self-consistent models with positive phase-space density do not exist. Numerical solutions of the combined Vlasov and Poisson equations suggest that the whole sequence of models with $0<α<1$ are also unphysical. Along with existing work on cusps, we conclude on purely theoretical grounds that galactic nuclei are not expected to be triaxial.

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arXiv1997

Origin of non-unique deprojection of the Galactic bar

Non-uniqueness of deprojecting the integrated light distribution of a nearby or faraway triaxial body is reviewed in the context of deriving the volume density of the Galactic bar from the COBE/DIRBE maps of the Galactic plane. The exact origin of this non-uniqueness is studied. One can write down a sequence of triaxial bar models which appear identical in integrated light from the Sun's perspective, and the whole sequence is mapped out as a function of the Galactocentric distance of the observer from galactic to extragalactic distance scales. While mirror symmetries and perspective effects are compatible with any orientation of the bar in the sequence, weak upper and lower bounds can still be placed on the angles and axis ratios of the bar by positivity and other general requirements. Star count data of bulge giants are ideal for selecting a unique model from the sequence.

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arXiv1996

Possible Implications of Sagittarius Type Dwarf Galaxy on Microlensing Towards the Large Magellanic Cloud

The discovery of the Sgr dwarf galaxy (Ibata, Gilmore, Irwin 1994) shows that the Galactic halo contains large clumps which are not full viralized. Stars in such a clump can lense a background extragalactic source. I use parameters of Sgr to demonstrate that microlensing by a disintegrated dwarf galaxy has a probability (optical depth) $10^{-7}-10^{-6}$, which is comparable to that of an isothermal Galactic dark halo of massive compact objects (MACHOs). This may change implications of observed microlensing events towards the LMC. If more than one such clumps are hidden in the Galactic halo, then given their enormous angular size ($\sim 5^o \times 20^o$), there is a significant chance for {\sl any} line of sight, say the LMC direction, to intersect one of them. As an alternative to the half-MACHO dark halo models (Alcock et al. 1996a,c), I discuss a non-MACHO model with lenses of the observed events being faint stars in a hidden Sgr-like disintegrated dwarf in the line-of-sight path to the LMC.

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arXiv1996

On the Microlensing Optical Depth of the Galactic Bar

The microlensing probability (optical depth $τ$) toward the Galactic center carries information about the mass distribution of the Galactic bulge/bar, so can be used to constrain the very uncertain shape parameters of the bar. We find $tau$ depends on the bar mass, radial profile, angle, axis scale lengths and boxyness by a few simple analytical formulae, which shows: (1) $τ$ is proportional to the mass of the bar, $M$. (2) $τ$ falls along the minor axis with a strong gradient. (3) An oblate bulge can have more optical depth than a triaxial bar if the bar angle $α>45$ degress. (4) $τ$ is the largest if the angle $α$ and the axis ratio $y_0/x_0$ conspires so that $y_0/x_0=\tan α$. (5) At a fixed field on the minor axis but away from the center, boxy bars with a flat density profile tend to give a larger optical depth than ellipsoidal bars with a steep profile. (6) Main sequence sources should have a significantly lower (20-50\% lower) optical depth than red clump giants if main sequence stars are not observed as deep as the bright clump giants. An application to four COBE-constrained models (Dwek et al. 1994) shows most models produce optical depth $2σ$ lower than MACHO and OGLE observed values even with both a massive bar $2.8\times 10^{10} M_\odot$ and a full disk. The high $τ$ argues for a massive ($> 2\times 10^{10}M_\odot$) boxy bar with $y_0/x0\approx \tanα$ and $α<20$ deg and with a flat radial profile up to corotation.

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arXiv1996

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.

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arXiv1995

Analytical Models For Galactic Nuclei

I present a general family of dynamical models with simple analytical potential-density pairs suited to model galactic bulges and nuclei with double power-law radial density profiles and an optional central black hole. Analytical expressions for the potential and velocity dispersion of these models are given in the spherical case. Many previously known analytical spherical models, including also the recent $γ/ η$-models by Dehnen (1993) and Tremaine et al. (1994), are special cases of this family. This family also forms a complete set for constructing general galaxy models or solving Poisson's equation in the non-spherical case. In particular, a generalized Clutton-Brock (1973) and Hernquist-Ostriker (1992) orthogonal basis set is given.

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arXiv1995

A Consistent Microlensing Model for the Galactic Bar

We compute a microlensing map for the Galactic bar. The predicted event rate and event duration distribution are consistent with the $55$ events recently reported by the MACHO and OGLE collaborations. Most of the events are due to lensing by stars in the near end of the bar. Lens mass functions with about 30-60\% of lens mass as brown dwarfs are rejected at $2-6σ$ levels. To make our model useful for other workers, we tabulate the bar's optical depth and average event duration (scaled to $1M_\odot$ lenses) on a grid of Galactic coordinates. The distance and the proper motions of the lens and the source are derived from a consistent dynamical model of the stellar bar, which has originally been built to fit data on the stellar light and stellar/gas kinematics of the bar. We explore several alternative models and we find that our standard model best matches observations.

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arXiv1995

New Parametrizations of Non-Gaussian Line-of-sight Velocity Distribution

A five-parameter fitting formula for the line-of-sight stellar velocity distributions of steady state systems is proposed. It can faithfully reproduce velocity distributions of theoretical models ranging from nearly Gaussian profiles to strongly skewed or mildly double-peaked profiles. In contrast to van der Marel and Franx (1993) and Kuijken and Merrifield (1993), the line profiles are required to have neither multi-peaks nor negative velocity wings. Information of the profile is mostly specified by five physically meaningful and nearly orthogonal fitting parameters.

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arXiv1995

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.

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arXiv1995

A Self-Consistent Dynamical Model for the {\sl COBE} Observed Galactic Bar and Its Application to Microlensing

A self-consistent stellar dynamical model for the Galactic bar is constructed from about 500 numerically computed orbits with an extension of the Schwarzschild technique. The model fits the {\sl COBE} found asymmetric boxy light distribution and the observed stellar kinematics of the bulge. The model potential is also consistent with the non-circular motions of the HI and CO velocity maps of the inner Galaxy. We also use the stellar bar model to construct a N-body model to study stability and a microlensing map towards the bulge, which can account for the observed optical depth and the event duration by the MACHO and OGLE collaborations. The technique used here can be applied to interpret light and velocity data of external bulges/bars and galactic nuclei.

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arXiv1994

Signatures of Bulge Triaxiality from Kinematics in Baade's Window/

We study a sample of 62 Baade's Window (1,-4) K giants that have published proper motions, radial velocity and metallicity. We construct the velocity ellipsoids for the metal rich stars and metal poor stars. We find a vertex deviation characteristic of a non-axisymmetric system. The long axes of the velocity ellipsoids of two samples point to two nearly perpendicular directions (lv,bv)=(-65+-9,+14+-9) and (25+-14,-11+-14) respectively. The vertex deviations of the velocity ellipsoids cannot be consistently explained by ANY oblate model. We populate orbits in a realistic bar potential with a Gaussian velocity distribution, allowing us to simulate and interpret observations. We conclude that the data are consistent with a triaxial bulge pointing towards (l,b) with l<0 and b=0 as suggested by earlier work. We also predict that low latitude abs(b)<4 deg bulge fields should show the vertex deviation more strongly and would therefore be the best locations for future proper motion studies. The metal rich and metal poor stars appear to occupy the direct boxy and retrograde loop orbits. The correlations between the metallicity and the orbit families can develop if the Bulge forms dissipatively on a sufficiently long time scale. However, it is difficult to reconcile the existence of such correlations with a violent history for the potential of the inner Galaxy.

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