arXiv subjects

HongSheng Zhao

Publications and source records attributed to HongSheng Zhao.

At least 37 records · Page 2Linked to original sources

arXiv2009

N-body simulations for testing the stability of triaxial galaxies in MOND

We perform a stability test of triaxial models in MOdified Newtonian Dynamics (MOND) using N-body simulations. The triaxial models considered here have densities that vary with $r^{-1}$ in the center and $r^{-4}$ at large radii. The total mass of the model varies from $10^8\Msun$ to $10^{10}\Msun$, representing the mass scale of dwarfs to medium-mass elliptical galaxies, respectively, from deep MOND to quasi-Newtonian gravity. We build triaxial galaxy models using the Schwarzschild technique, and evolve the systems for 200 Keplerian dynamical times (at the typical length scale of 1.0 kpc). We find that the systems are virial overheating, and in quasi-equilibrium with the relaxation taking approximately 5 Keplerian dynamical times (1.0 kpc). For all systems, the change of the inertial (kinetic) energy is less than 10% (20%) after relaxation. However, the central profile of the model is flattened during the relaxation and the (overall) axis ratios change by roughly 10% within 200 Keplerian dynamical times (at 1.0kpc) in our simulations. We further find that the systems are stable once they reach the equilibrium state.

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arXiv2009

The Galactic potential and the asymmetric distribution of hypervelocity stars

In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic center and then propagated in the Galactic potential up to their current position. The recent survey for candidate HVSs show an asymmetry in the kinematics of candidate HVSs (position and velocity vectors), where more outgoing stars than ingoing stars (i.e. positive Galactocentric velocities vs. negative ones) are observed. We show that such kinematic asymmetry, which is likely due to the finite lifetime of the stars and Galactic potential structure, could be used in a novel method to probe and constrain the Galactic potential, identify the stellar type of the stars in the survey and estimate the number of HVSs. Kinematics-independent identification of the stellar types of the stars in such surveys (e.g. spectroscopic identification) could further improve these results. We find that the observed asymmetry between ingoing and outgoing stars favors specific Galactic potential models. It also implies a lower limit of ~54+-8 main sequence HVSs in the survey sample (>=648+-96 in the Galaxy), assuming that all of the main sequence stars in the survey originate from the Galactic center. The other stars in the survey are likely to be hot blue horizontal branch stars born in the halo rather than stars ejected from the Galactic center.

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arXiv2009

Physics of galactic colliders: high speed satellites in LCDM vs MONDian cosmology

The statistics of high speed satellite galaxies, as reported in the recent literature, can be a powerful diagnosis of the depth of the potential well of the host halo, and hence discriminate between competing gravitational theories. Naively one expects that high speed satellites are more common in Modified Newtonian Dynamics (MOND) than in cold dark matter (CDM) since an isolated MONDian system has an infinite potential well, while CDM halos have finite potential wells. In this \textit{Letter} we report on an initial test of this hypothesis in the context of the first generation of cosmological simulations utilising a rigorous MONDian Poisson solver. We find that such high speed encounters are approximately a factor of four more common in MOND than in the concordance $Λ$CDM model of cosmic structure formation.

astro-ph.CO↗

arXiv2008

An ecological approach to problems of Dark Energy, Dark Matter, MOND and Neutrinos

Modern astronomical data on galaxy and cosmological scales have revealed powerfully the existence of certain dark sectors of fundamental physics, i.e., existence of particles and fields outside the standard models and inaccessible by current experiments. Various approaches are taken to modify/extend the standard models. Generic theories introduce multiple de-coupled fields A, B, C, each responsible for the effects of DM (cold supersymmetric particles), DE (Dark Energy) effect, and MG (Modified Gravity) effect respectively. Some theories use adopt vanilla combinations like AB, BC, or CA, and assume A, B, C belong to decoupled sectors of physics. MOND-like MG and Cold DM are often taken as opposite frameworks, e.g. in the debate around the Bullet Cluster. Here we argue that these ad hoc divisions of sectors miss important clues from the data. The data actually suggest that the physics of all dark sectors is likely linked together by a self-interacting oscillating field, which governs a chameleon-like dark fluid, appearing as DM, DE and MG in different settings. It is timely to consider an interdisciplinary approach across all semantic boundaries of dark sectors, treating the dark stress as one identity, hence accounts for several "coincidences" naturally.

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arXiv2008

The diffuse Galactic gamma-rays from dark matter annihilation

The diffuse Galactic $γ$-rays from EGRET observation shows excesses above 1 GeV in comparison with the expectations from conventional Galactic cosmic ray (CR) propagation model. In the work we try to solve the "GeV excess" problem by dark matter (DM) annihilation in the frame of supersymmetry (SUSY). Compared with previous works, there are three aspects improved in this work: first, the direction-independent "boost factor" for diffuse $γ$-rays from dark matter annihilation (DMA) is naturally reproduced by taking the DM substructures into account; second, there is no need for renormalization of the diffuse $γ$-ray background produced by CRs; last but not the least, in this work our new propagation model can give consistent results of both diffuse $γ$-rays and antiprotons, by directly adding the signals from DMA to the diffuse $γ$-ray background. This is a self-consistent model among several possible scenarios at present, and can be tested or optimized by the forthcoming experiments such as GLAST, PAMELA and AMS02.

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arXiv2008

Is Gravitational Lensing by Intercluster Filaments Always Negligible?

Intercluster filaments negligibly contribute to the weak lensing signal in general relativity (GR), $γ_{N}\sim 10^{-4}-10^{-3}$. In the context of relativistic modified Newtonian dynamics (MOND) introduced by Bekenstein, however, a single filament inclined by $\approx 45^\circ$ from the line of sight can cause substantial distortion of background sources pointing towards the filament's axis ($κ=γ=(1-A^{-1})/2\sim 0.01$); this is rigorous for infinitely long uniform filaments, but also qualitatively true for short filaments ($\sim 30$Mpc), and even in regions where the projected matter density of the filament is equal to zero. Since galaxies and galaxy clusters are generally embedded in filaments or are projected on such structures, this contribution complicates the interpretation of the weak lensing shear map in the context of MOND. While our analysis is of mainly theoretical interest providing order-of-magnitude estimates only, it seems safe to conclude that when modeling systems with anomalous weak lensing signals, e.g. the "bullet cluster" of Clowe et al., the "cosmic train wreck" of Abell 520 from Mahdavi et al., and the "dark clusters" of Erben et al., filamentary structures might contribute in a significant and likely complex fashion. On the other hand, our predictions of a (conceptual) difference in the weak lensing signal could, in principle, be used to falsify MOND/TeVeS and its variations.

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arXiv2008

An analytic model for non-spherical lenses in covariant MOdified Newtonian Dynamics

Strong gravitational lensing by galaxies in MOdified Newtonian Dynamics (MOND) has until now been restricted to spherically symmetric models. These models were able to account for the size of the Einstein ring of observed lenses, but were unable to account for double-imaged systems with collinear images, as well as four-image lenses. Non-spherical models are generally cumbersome to compute numerically in MOND, but we present here a class of analytic non-spherical models that can be applied to fit double-imaged and quadruple-imaged systems. We use them to obtain a reasonable MOND fit to ten double-imaged systems, as well as to the quadruple-imaged system Q2237+030 which is an isolated bulge-disc lens producing an Einstein cross. However, we also find five double-imaged systems and three quadruple-imaged systems for which no reasonable MOND fit can be obtained with our models. We argue that this is mostly due to the intrinsic limitation of the analytic models, even though the presence of small amounts of additional dark mass on galaxy scales in MOND is also plausible.

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arXiv2008

Cosmological Structure Formation under MOND: a new numerical solver for Poisson's equation

We present a novel solver for an analogue to Poisson's equation in the framework of modified Newtonian dynamics (MOND). This equation is highly non-linear and hence standard codes based upon tree structures and/or FFT's in general are not applicable; one needs to defer to multi-grid relaxation techniques. After a detailed description of the necessary modifications to the cosmological N-body code AMIGA (formerly known as MLAPM) we utilize the new code to revisit the issue of cosmic structure formation under MOND. We find that the proper (numerical) integration of a MONDian Poisson's equation has some noticable effects on the final results when compared against simulations of the same kind but based upon rather ad-hoc assumptions about the properties of the MONDian force field. Namely, we find that the large-scale structure evolution is faster in our revised MOND model leading to an even stronger clustering of galaxies, especially when compared to the standard LCDM paradigm.

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arXiv2008

The Bulge Radial Velocity Assay (BRAVA): I. Sample Selection and a Rotation Curve

Results from the Bulge Radial Velocity Assay (BRAVA) are presented. BRAVA uses M giant stars, selected from the 2MASS catalog to lie within a bound of reddening corrected color and luminosity, as targets for the Cerro Tololo Inter-American Observatory 4-m Hydra multi-object spectrograph. Three years of observations investigate the kinematics of the Galactic bulge major and minor axes with ~3300 radial velocities from 32 bulge fields and one disk field. We construct a longitude-velocity plot for the bulge stars and find that, contrary to previous studies, the bulge does not rotate as a solid body; for |l|<4 degree the rotation curve has a slope of roughly 100 km/s/kpc and flattens considerably at greater l, reaching a maximum rotation of 75 km/s. We compare our rotation curve and velocity dispersion profile to both the self-consistent model of Zhao (1996) and to N-body models; neither satisfactorily fit both measured rotation and dispersion. We place the bulge on the plot of (Vmax/sigma) vs. epsilon and find that the bulge lies near the oblate rotator line, and very close to the parameters of NGC 4565, an edge-on spiral galaxy with a bulge similar to that of the Milky Way. We find that our summed velocity distribution of bulge stars appears to be sampled from a Gaussian distribution, with sigma=116+/-2 km/s for our summed bulge fields.

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arXiv2008

Galaxy Bulges As Tests of CDM vs MOND in Strong Gravity

The tight correlation between galaxy bulges and their central black hole masses likely emerges in a phase of rapid collapse and starburst at high redshift, due to the balance of gravity on gas with the feedback force from starbursts and the wind from the black hole; the average gravity on per unit mass of gas is ~ 2 x 10^-10 m/sec^2 during the star burst phase. This level of gravity could come from the real r^{-1} cusps of Cold Dark Matter (CDM) halos, but the predicted gravity would have a large scatter due to dependence on cosmological parameters and formation histories. Better agreement is found with the gravity from the scalar field in some co-variant versions of MOND, which can create the mirage of a Newtonian effective dark halo of density Pi r^{-1} near the center, where the characteristic surface density Pi=130alpha^{-1} Msun pc^{-2} and alpha is a fundamental constant of order unity fixed by the Lagrangian of the co-variant theory if neglecting environmental effects. We show with a toy analytical model and a hydrodynamical simulation that a constant background gravity due to MOND/TeVeS scalar field implies a critical pressure synchronizing starbursts and the formation of galaxy bulges and ellipticals. A universal threshold for the formation of the brightest regions of galaxies in a MONDian universe suggests that the central BHs, bulges and ellipticals would respect tight correlations like the M_{bulge}-M_{BH}-sigma relations. In general MOND tends to produce tight correlations in galaxy properties because its effective halo has less freedom and scatter than CDM halos.

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arXiv2008

Reinterpreting MOND: coupling of Einsteinian gravity and spin of cosmic neutrinos?

Several rare coincidences of scales in standard particle physics are needed to explain (i) why neutrinos have mass, (ii) why the negative pressure of the cosmological dark energy (DE) coincides with the positive pressure of random motion of dark matter (DM) in bright galaxies, (iii) why Dark Matter in galaxies seems to have a finite phase-space density, and to follow the Tully-Fisher-Milgrom relation of galaxy rotation curves. The old idea of self-interacting DM is given a new spin: we propose that the neutrino spin-gravity coupling could lead to a cosmic neutrino dark fluid with a an internal energy density varying as function of the local acceleration of the neutrino fluid with respect the CMB background. We link the Tully-Fisher-Milgrom relation of spiral galaxies (or MOND) with the relativistic pressure of the neutrino dark fluid without modifying Einsteinian gravity.

gr-qc↗

arXiv2008

Testing Alternative Theories of Dark Matter with the CMB

We propose a method to study and constrain modified gravity theories for dark matter using CMB temperature anisotropies and polarization. We assume that the theories considered here have already passed the matter power-spectrum test of large-scale structure. With this requirement met, we show that a modified gravity theory can be specified by parametrizing the time evolution of its dark-matter density contrast, which is completely controlled by the dark matter stress history. We calculate how the stress history with a given parametrization affects the CMB observables, and a qualitative discussion of the physical effects involved is supplemented with numerical examples. It is found that, in general, alternative gravity theories can be efficiently constrained by the CMB temperature and polarization spectra. There exist, however, special cases where modified gravity cannot be distinguished from the CDM model even by using both CMB and matter power spectrum observations, nor can they be efficiently restricted by other observables in perturbed cosmologies. Our results show how the stress properties of dark matter, which determine the evolutions of both density perturbations and the gravitational potential, can be effectively investigated using just the general conservation equations and without assuming any specific theoretical gravitational theory within a wide class.

gr-qc↗

arXiv2008

Dark Fluid: Towards a unification of empirical theories of galaxy rotation, Inflation and Dark Energy

Empirical theories of Dark Matter like MOND gravity and of Dark Energy like f(R) gravity were motivated by astronomical data. But could these theories be branches rooted from a more general hence natural framework? Here we propose the natural Lagrangian of such a framework based on simple dimensional analysis and co-variant symmetry requirements, and explore various outcomes in a top-down fashion. Our framework preserves the co-variant formulation of GR, but allows the expanding physical metric be bent by a single new species of Dark Fluid flowing in space-time. Its non-uniform stress tensor and current vector are simply functions of a vector field of variable norm, resembling the 4-vector electromagnetic potential description for the photon fluid, but is dark (e.g., by very early decoupling from the baryon-radiation fluid). The Dark Fluid framework naturally branches into a continuous spectrum of theories with Dark Energy and Dark Matter effects, including the $f(R)$ gravity, TeVeS-like theories, Einstein-Aether and $νΛ$ theories as limiting cases. When the vector field degenerates into a pure Higgs-like scalar field, we obtain the physics for inflaton and quintessence. In this broad setting we emphasize the non-constant dynamical field behind the cosmological constant effect, and highlight plausible corrections beyond the classical MOND predictions. Choices of parameters can be made to pass BBN, PPN, and causality constraints. The Dark Fluid is inspired to unify/simplify the astronomically successful ingredients of previous constructions: the desired effects of inflaton plus quintessence plus Cold DM particle fields or MOND-like scalar field(s) are shown largely achievable by one vector field only.

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arXiv2008

Milky Way potentials in CDM and MOND. Is the Large Magellanic Cloud on a bound orbit?

We compute the Milky Way potential in different cold dark matter (CDM) based models, and compare these with the modified Newtonian dynamics (MOND) framework. We calculate the axis ratio of the potential in various models, and find that isopotentials are less spherical in MOND than in CDM potentials. As an application of these models, we predict the escape velocity as a function of the position in the Galaxy. This could be useful in comparing with future data from planned or already-underway kinematic surveys (RAVE, SDSS, SEGUE, SIM, GAIA or the hypervelocity stars survey). In addition, the predicted escape velocity is compared with the recently measured high proper motion velocity of the Large Magellanic Cloud (LMC). To bind the LMC to the Galaxy in a MOND model, while still being compatible with the RAVE-measured local escape speed at the Sun's position, we show that an external field modulus of less than $0.03 a_0$ is needed.

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arXiv2008

An Uneven Vacuum Energy Fluid as $Λ$, Dark Matter, MOND and Lens

Various TeVeS-inspired and f(R)-inspired theories of gravity have added an interesting twist to the search for dark matter and vacuum energy, modifying the landscape of astrophysics day by day. These theories can be together called a {\bf N}on-{\bf u}niform Dark Energy fluid (a Nu-Lambda fluid or a ${\mathbf VΛ}$ fluid); a common thread of these theories, according of an up-to-date summary by HZL \cite{Halle}, is a non-uniform vector field, describing an uneven vacuum energy fluid. The so-called "alternative" gravity theories are in fact in the standard GR gravity framework except that the cosmological "constant" is replaced by a non-trivial non-uniform vacuum energy, which couples the effects of Dark Matter and Dark Energy together by a single field. Built initially bottom-up rather than top-down as most gravity theories, TeVeS-inspired theories are healthily rooted on empirical facts. Here I attempt a review of some sanity checks of these fast-developing theories from galaxy rotation curves, gravitational lensing and cosmic acceleration. I will also discuss some theoretical aspects of the vacuum energy, and point out some analogies with electromagnetism and the Casimir effect.

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arXiv2008

Perturbations In A Non-Uniform Dark Energy Fluid: Equations Reveal Effects of Modified Gravity and Dark Matter

We propose a unified single-field description of the galactic Dark Matter and various uniform scalar fields for the inflation and cosmological constant. The two types of effects could originate from a fluid of both spatially and temporally varying Vacuum Energy if the vacuum has an uneven pressure caused by a photon-like vector field (of perhaps an unstable massive boson). We propose a most general Lagrangian with a {\bf N}on-{\bf u}niform Cosmological Constant for this vacuum fluid (dubbed as a Nu-Lambda fluid), working within the framework of Einsteinian gravity. This theory includes a continuous spectrum of plausible dark energy theories and gravity theories, e.g., inflation, quintessence, k-essence, f(R), Generalized Einstein-Aether f(K), MOND, TeVeS, BSTV etc. theories. It also suggests new models such as a certain f(K+R) model, which suggests intriguing corrections to MOND depending of redshift and density. Some specific constructions of the Nu-Lambda fluid (e.g., Zhao's V-$Λ$ model) closely resemble the $Λ$CDM cosmology on large scale, but fit galaxy rotation curves as good as MOND. Perturbed Einstein Equations in a simple $f(K_4)$ model are solvable and show effects of a DM coupled to DE. Incorporating the perturbation equations here into standard simulations for cosmological structure growth offers a chance to falsify examples of the Nu-Lambda theories.

astro-ph↗

arXiv2008

Self-consistent models of triaxial galaxies in MOND gravity

The Bekenstein-Milgrom gravity theory with a modified Poisson equation is tested here for the existence of triaxial equilibrium solutions. Using the non-negative least square method, we show that self-consistent triaxial galaxies exist for baryonic models with a mild density cusp $ρ\sim {Σ\over r}$. Self-consistency is achieved for a wide range of central concentrations, $Σ\sim 10-1000\mathrm{M_{\odot}pc^{-2}}$, representing low-to-high surface brightness galaxies. Our results demonstrate for the first time that the orbit superposition technique is fruitful for constructing galaxy models beyond Newtonian gravity, and triaxial cuspy galaxies might exist without the help of Cold dark Matter.

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arXiv2007

Mapping the dark trajectory of the Bullet Cluster

We model the electrons/positrons produced by dark matter annihilations in the colliding galaxy cluster system 1E0657-56 (the bullet cluster). These charged particles, confined by the Magnetic field, clearly trace the path of the bullet, which passes through the main cluster with a speed of 3000-5000 km/s. Adding the effect of dark matter substructure (subhalos) in each cluster we find the annihilation rate is enhancedgreatly and the density of positrons in the trail is similar to that within the bullet cluster. This opens the door to unambiguous detection of the annihilation signal through the SZ effect, at significant separation from the thermal electrons.

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