arXiv subjects

Hongsheng Zhao

Publications and source records attributed to Hongsheng Zhao.

At least 37 records · Page 2Linked to original sources

arXiv2019

Does the galaxy NGC1052-DF2 falsify Milgromian dynamics?

A great challenge in present-day physics is to understand whether the observed internal dynamics of galaxies is due to dark matter matter or due to a modification of the law of gravity. Recently, van Dokkum et al. reported that the ultra-diffuse dwarf galaxy NGC1052-DF2 lacks dark matter, and they claimed that this would -- paradoxically -- be problematic for modified gravity theories like Milgromian dynamics (MOND). However, NGC1052-DF2 is not isolated, so that a valid prediction of its internal dynamics in MOND cannot be made without properly accounting for the external gravitational fields from neighbouring galaxies. Including this external field effect following Haghi et al. shows that NGC1052-DF2 is consistent with MOND.

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arXiv2019

Toomre stability of disk galaxies in quasi-linear MOND

We consider disk stability in the quasi-linear formulation of MOND (QUMOND), the basis for some $N$-body integrators. We generalize the Toomre criterion for the stability of disks to tightly wound, axisymmetric perturbations. We apply this to a family of thin exponential disks with different central surface densities. By numerically calculating their QUMOND rotation curves, we obtain the minimum radial velocity dispersion required for stability against local self-gravitating collapse. MOND correctly predicts much higher rotation speeds in low surface brightness galaxies (LSBs) than does Newtonian dynamics without dark matter. Newtonian models thus require putative very massive halos, whose inert nature implies they would strongly stabilize the disk. MOND also increases the stability of galactic disks, but in contradistinction to Newtonian gravity, this extra stability is limited to a factor of 2. MOND is thus rather more conducive to the formation of bars and spiral arms. Therefore, observation of such features in LSBs could be problematic for Newtonian galaxy models. This could constitute a crucial discriminating test. We quantitatively account for these facts in QUMOND. We also compare numerical QUMOND rotation curves of thin exponential disks to those predicted by two algebraic expressions commonly used to calculate MOND rotation curves. For the choice that best approximates QUMOND, we find the circular velocities agree to within 1.5% beyond $\approx 0.5$ disk scale lengths, regardless of the central surface density. The other expression can underestimate the rotational speed by up to 12.5% at one scale length, though rather less so at larger radii.

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arXiv2019

Predicted MOND velocity dispersions for a catalog of ultra-diffuse galaxies in group environments

The possibility that ultra-diffuse galaxies lacking dark matter has recently stimulated interest to check the validity of Modified Newton Dynamics (MOND) predictions on the scale of such galaxies. It has been shown that the External Field Effect (EFE) induced by the close-by galaxy can suppress the velocity dispersion of these systems, so that they appear almost dark matter free in the Newtonian context. Here, following up on this, we are making a priori predictions for the velocity dispersion of 22 ultra-diffuse galaxies in the nearby Universe. This sample can be used to test MOND and the EFE with future follow-up measurements. We construct a catalog of nearby ultra-diffuse galaxies in galaxy group environments, and set upper and lower limits for the possible velocity dispersion allowed in MOND, taking into account possible variations in the mass-to-light ratio of the dwarf and in the distance to the galaxy group. The prediction for the velocity dispersion is made as a function of the three dimensional separation of the dwarf to its host. In 17 out of 22 cases, the EFE plays a crucial role in the prediction.

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arXiv2018

A common Milgromian acceleration scale in nature

A central problem of contemporary physics is whether the law of gravity is non-Newtonian on galaxy scales. Rodrigues et al. argue that Milgromian gravitation, which solves the flat rotation curve problem without the need for dark matter particles, is ruled out at > 10σ significance. To a large extent, this conclusion relies on galaxies with very uncertain distances and/or nearly edge-on orientations, where dust obscuration often becomes significant. Applying appropriate quality cuts to the data leaves only a handful of outliers to the predictions of Milgromian gravitation according to the analysis of Rodrigues et al., but even these outliers can be explained with Milgromian gravitation.

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arXiv2018

Origin of the Local Group satellite planes

We attempt to understand the planes of satellite galaxies orbiting the Milky Way (MW) and M31 in the context of Modified Newtonian Dynamics (MOND), which implies a close MW-M31 flyby occurred ${\approx 8}$ Gyr ago. Using the timing argument, we obtain MW-M31 trajectories consistent with cosmological initial conditions and present observations. We adjust the present M31 proper motion within its uncertainty in order to simulate a range of orbital geometries and closest approach distances. Treating the MW and M31 as point masses, we follow the trajectories of surrounding test particle disks, thereby mapping out the tidal debris distribution. Around each galaxy, the resulting tidal debris tends to cluster around a particular orbital pole. We find some models in which these preferred spin vectors align fairly well with those of the corresponding observed satellite planes. The radial distributions of material in the simulated satellite planes are similar to what we observe. Around the MW, our best-fitting model yields a significant fraction (0.22) of counter-rotating material, perhaps explaining why Sculptor counter-rotates within the MW satellite plane. In contrast, our model yields no counter-rotating material around M31. This is testable with proper motions of M31 satellites. In our best model, the MW disk is thickened by the flyby 7.65 Gyr ago to a root mean square height of 0.75 kpc. This is similar to the observed age and thickness of the Galactic thick disk. Thus, the MW thick disk may have formed together with the MW and M31 satellite planes during a past MW-M31 flyby.

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arXiv2017

A Plane of High Velocity Galaxies Across the Local Group

We recently showed that several Local Group (LG) galaxies have much higher radial velocities (RVs) than predicted by a 3D dynamical model of the standard cosmological paradigm. Here, we show that 6 of these 7 galaxies define a thin plane with root mean square thickness of only 101 kpc despite a widest extent of nearly 3 Mpc, much larger than the conventional virial radius of the Milky Way (MW) or M31. This plane passes within ${\sim 70}$ kpc of the MW-M31 barycentre and is oriented so the MW-M31 line is inclined by $16^\circ$ to it. We develop a toy model to constrain the scenario whereby a past MW-M31 flyby in Modified Newtonian Dynamics (MOND) forms tidal dwarf galaxies that settle into the recently discovered planes of satellites around the MW and M31. The scenario is viable only for a particular MW-M31 orbital plane. This roughly coincides with the plane of LG dwarfs with anomalously high RVs. Using a restricted $N$-body simulation of the LG in MOND, we show how the once fast-moving MW and M31 gravitationally slingshot test particles outwards at high speeds. The most distant such particles preferentially lie within the MW-M31 orbital plane, probably because the particles ending up with the highest RVs are those flung out almost parallel to the motion of the perturber. This suggests a dynamical reason for our finding of a similar trend in the real LG, something not easily explained as a chance alignment of galaxies with an isotropic or mildly flattened distribution (probability $= {0.0015}$).

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arXiv2017

Galaxy Clusters in the Context of Superfluid Dark Matter

It has recently been proposed, by assuming that dark matter is a superfluid, that MOND-like effects can be achieved on small scales whilst preserving the success of $Λ$CDM on large scales. Here we aim to provide the first set of spherical models of galaxy clusters in the context of superfluid dark matter. We first outline the theoretical structure of the superfluid core and the surrounding "normal phase" dark halo of quasi-particles in thermal equlibrium. The latter should encompass the largest part of galaxy clusters. Here, we set the SfDM transition at the radius where the density and pressure of the superfluid and normal phase coincides, neglecting the effect of phonons in the suprefluid core. We then apply the theory to a sample of galaxy clusters, and directly compare the SfDM predicted mass profiles to data. We find that the superfluid formulation can reproduce the X-ray dynamical mass profile of clusters. The SfDM fits however display slight under-predictions of the gravity in the central regions which might be partly related to our neglecting of the effect of phonons in these regions. We conclude that this superfluid formulation is successful in describing galaxy clusters, but further work will be needed to determine whether the parameter choice is consistent with galaxies. Our model could be made more realistic by exploring non-sphericity and the SfDM transition condition we impose.

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arXiv2017

Galaxy Cluster A1689 in Modified MOND, MOG and Emergent Gravity

We model the cluster A1689 in two modified MOND frameworks (EMOND and what we call generalised MOND or GMOND) with the aim of determining whether it is possible to explain the inferred acceleration profile, from gravitational lensing, without the aid of dark matter. We also compare our result to predictions from MOG/STVG and Emergent Gravity. By using a baryonic mass model, we determine the total gravitational acceleration predicted by the modified gravitational equations and compare the result to NFW profiles of dark matter studies on A1689 from the literature. Theory parameters are inferred empirically, with the aid of previous work. We are able to reproduce the desired acceleration profile of A1689 for GMOND, EMOND and MOG, but not Emergent Gravity. There is much more work which needs to be conducted with regards to understanding how the GMOND parameters behave in different environments. Furthermore, we show that the exact baryonic profile becomes very important when undertaking modified gravity modelling rather than ΛCDM.

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arXiv2017

Are Over-massive Haloes of Ultra Diffuse Galaxies Consistent with Extended MOND?

A sample of Coma cluster ultra-diffuse galaxies (UDGs) are modelled in the context of Extended Modified Newtonian Dynamics (EMOND) with the aim to explain the large dark matter-like effect observed in these cluster galaxies. We first build a model of the Coma cluster in the context of EMOND using gas and galaxy mass profiles from the literature. Then assuming the dynamical mass of the UDGs satisfies the fundamental manifold of other ellipticals, and that the UDG stellar mass-to-light matches their colour, we can verify the EMOND formulation by comparing two predictions of the baryonic mass of UDGs. We find that EMOND can explain the UDG mass, within the expected modelling errors, if they lie on the fundamental manifold of ellipsoids, however, given that measurements show one UDG lying off the fundamental manifold, observations of more UDGs are needed to confirm this assumption.

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arXiv2017

Dynamical History Of The Local Group In $Λ$CDM $\rm{II}$ $-$ Including External Perturbers In 3D

We attempt to fit the observed radial velocities (RVs) of ${\,{\sim}\,}30$ Local Group (LG) galaxies using a 3D dynamical model of it and its immediate environment within the context of the standard cosmological paradigm, $Λ$CDM. This extends and confirms the basic results of our previous axisymmetric investigation of the LG (MNRAS, 459, 2237). We find that there remains a tendency for observed RVs to exceed those predicted by our best-fitting model. The typical mismatch is slightly higher than in our 2D model, with a root mean square value of ${\,{\sim}\,}50$ km/s. Our main finding is that including the 3D distribution of massive perturbing dark matter halos is unlikely to help greatly with the high velocity galaxy problem. Nonetheless, the 2D and 3D results differ in several other ways such as which galaxies' RVs are most problematic and the preferred values of parameters common to both models. The anomalously high RVs of several LG dwarfs may be better explained if the Milky Way (MW) and Andromeda (M31) were once moving much faster than in our models. This would allow LG dwarfs to gain very high RVs via gravitational slingshot encounters with a massive fast-moving galaxy. Such a scenario is possible in some modified gravity theories, especially those which require the MW and M31 to have previously undergone a close flyby. In a $Λ$CDM context, however, this scenario is not feasible as the resulting dynamical friction would cause a rapid merger.

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arXiv2017

Generalizing MOND to explain the missing mass in galaxy clusters

Modified Newtonian Dynamics (MOND) is a gravitational framework designed to explain the astronomical observations in the Universe without the inclusion of particle dark matter. Modified Newtonian Dynamics, in its current form, cannot explain the missing mass in galaxy clusters without the inclusion of some extra mass, be it in the form of neutrinos or non-luminous baryonic matter. We investigate whether the MOND framework can be generalized to account for the missing mass in galaxy clusters by boosting gravity in high gravitational potential regions. We examine and review Extended MOND (EMOND), which was designed to increase the MOND scale acceleration in high potential regions, thereby boosting the gravity in clusters. We seek to investigate galaxy cluster mass profiles in the context of MOND with the primary aim at explaining the missing mass problem fully without the need for dark matter. Using the assumption that the clusters are in hydrostatic equilibrium, we can compute the dynamical mass of each cluster and compare the result to the predicted mass of the EMOND formalism. We find that EMOND has some success in fitting some clusters, but overall has issues when trying to explain the mass deficit fully. We also investigate an empirical relation to solve the cluster problem, which is found by analysing the cluster data and is based on the MOND paradigm. We discuss the limitations in the text.

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arXiv2016

Effects of Lens Motion and Uneven Magnification on Image Spectra

Counter to intuition, the images of an extended galaxy lensed by a moving galaxy cluster should have slightly different spectra in any metric gravity theory. This is mainly for two reasons. One relies on the gravitational potential of a moving lens being time-dependent (the $\text{Moving}$ $\text{Cluster}$ $\text{Effect}$, $\text{MCE}$). The other is due to uneven magnification across the extended, rotating source (the $\text{Differential}$ $\text{Magnification}$ $\text{Effect}$, $\text{DME}$). The time delay between the images can also cause their redshifts to differ because of cosmological expansion. This Differential Expansion Effect is likely to be small. Using a simple model, we derive these effects from first principles. One application would be to the Bullet Cluster, whose large tangential velocity may be inconsistent with the $ΛCDM$ paradigm. This velocity can be estimated with complicated hydrodynamic models. Uncertainties with such models can be avoided using the MCE. We argue that the MCE should be observable with ALMA. However, such measurements can be corrupted by the DME if typical spiral galaxies are used as sources. Fortunately, we find that if detailed spectral line profiles were available, then the DME and MCE could be distinguished. It might also be feasible to calculate how much the DME should affect the mean redshift of each image. Resolved observations of the source would be required to do this accurately. The DME is of order the source angular size divided by the Einstein radius times the redshift variation across the source. Thus, it mostly affects nearly edge-on spiral galaxies in certain orientations. This suggests that observers should reduce the DME by careful choice of target, a possibility we discuss in some detail.

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arXiv2016

Dynamical History of the Local Group in $Λ$CDM

The positions and velocities of galaxies in the Local Group (LG) measure the gravitational field within it. This is mostly due to the Milky Way (MW) and Andromeda (M31). We constrain their masses using distance and radial velocity (RV) measurements of 32 LG galaxies. To do this, we follow the trajectories of many simulated particles starting on a pure Hubble flow at redshift 9. For each observed galaxy, we obtain a trajectory which today is at the same position. Its final velocity is the model prediction for the velocity of that galaxy. Unlike previous simulations based on spherical symmetry, ours are axisymmetric and include gravity from Centaurus A. We find the total LG mass is ${4.33^{+0.37}_{-0.32}\times{10}^{12}M_\odot}$, with $0.14 \pm 0.07$ of this being in the MW. We approximately account for IC 342, M81, the Great Attractor and the Large Magellanic Cloud. No plausible set of initial conditions yields a good match to the RVs of our sample of LG galaxies. Observed RVs systematically exceed those predicted by the best-fitting $Λ$CDM model, with a typical disagreement of ${45.1^{+7.0}_{-5.7}}$ km/s and a maximum of ${110 \pm 13}$ km/s for DDO 99. Interactions between LG dwarf galaxies can't easily explain this. One possibility is a past close flyby of the MW and M31. This arises in some modified gravity theories but not in $Λ$CDM. Gravitational slingshot encounters of material in the LG with either of these massive fast-moving galaxies could plausibly explain why some non-satellite LG galaxies are moving away from us even faster than a pure Hubble flow.

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arXiv2015

Comment on "Evidence for dark matter in the inner Milky Way"

This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.

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arXiv2014

On radial anisotropy limits in stellar systems

Following earlier authors, we re-examine constraints on the radial velocity anisotropy of generic stellar systems using arguments for phase space density positivity, stability, and separability. It is known that although the majority of commonly used systems have an maximum anisotropy of less than half of the logarithmic density slope \emph{i.e.} $β< γ/2$, there are exceptions for separable models with large central anisotropy. Here we present a new exceptional case with above-threshold anisotropy locally but with an isotropic center nevertheless. These models are non-separable and we maintain positivity. Our analysis suggests that regions of above-threshold anisotropy are more related to regions of possible secular instability, which might be observed in self-consistent galaxies in a short-lived phase. s an equilibrium solution for a collisionless system.

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arXiv2014

Stirring N-body systems II: Necessary conditions for the dark matter attractor

We study the evolution of the phase-space of collisionless N-body systems under repeated stirrings or perturbations, which has been shown to lead to a convergence towards a limited group of end states. This so-called attractor was previously shown to be independent of the initial system and environmental conditions. However the fundamental reason for its appearance is still unclear. It has been suggested that the origin of the attractor may be either radial infall (RI), the radial orbit instability (ROI), or energy exchange which, for instance, happens during violent relaxation. Here we examine the effects of a set of controlled perturbations, referred to as `kicks', which act in addition to the standard collisionless dynamics by allowing pre-specified instantaneous perturbations in phase-space. We first demonstrate that the attractor persists in the absence of RI and ROI by forcing the system to expand. We then consider radial velocity kicks in a rigid potential and isotropic velocity kicks, since there are no energy exchanges in these two recipes of kicks. We find that these kicks do not lead to the attractor, indicating that the energy exchange in a dynamic potential is indeed the physical mechanism responsible for the attractor.

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arXiv2013

Gravity theories, Transverse Doppler and Gravitational Redshifts in Galaxy Clusters

There is growing interest in testing alternative gravity theories using the subtle gravitational redshifts in clusters of galaxies. However, current models all neglect a transverse Doppler redshift of similar magnitude, and some models are not self-consistent. An equilibrium model would fix the gravitational and transverse Doppler velocity shifts to be about 6sigma^2/c and 3sigma^2/2c in order to fit the observed velocity dispersion sigma self-consistently. This result comes from the Virial Theorem for a spherical isotropic cluster, and is insensitive to the theory of gravity. A gravitational redshift signal also does not directly distinguish between the Einsteinian and f(R) gravity theories, because each theory requires different dark halo mass function to keep the clusters in equilibrium. When this constraint is imposed, the gravitational redshift has no sensitivity to theory. Indeed our N-body simulations show that the halo mass function differs in f(R), and that the transverse Doppler effect is stronger than analytically predicted due to non-equilibrium.

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arXiv2012

A New Model for the Milky Way Bar

We use Schwarzschild's orbit-superposition technique to construct self-consistent models of the Galactic bar. Using $χ^2$ minimisation, we find that the best-fit Galactic bar model has a pattern speed $Ω_{\rm p}=60 \rm{km s^{-1} kpc^{-1}}$, disk mass $\rm{M_{\rm d}=1.0\times10^{11}M_{\odot}}$ and bar angle $θ_{\rm bar}=20^{\circ}$ for an adopted bar mass $\rm{M_{\rm bar}=2\times10^{10}M_{\odot}}$. The model can reproduce not only the three-dimensional and projected density distributions but also velocity and velocity dispersion data from the BRAVA survey. We also predict the proper motions in the range $l=[-12^{\circ},12^{\circ}]$, $b=[-10^{\circ},10^{\circ}]$, which appear to be higher than observations in the longitudinal direction. The model is stable within a timescale of 0.5 Gyr, but appears to deviate from steady-state on longer timescales. Our model can be further tested by future observations such as those from GAIA.

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