arXiv subjects

Chung-Ming Ko

Publications and source records attributed to Chung-Ming Ko.

At least 19 recordsLinked to original sources

arXiv2026

The Baryonic Faber-Jackson Relation and Fundamental Plane of Galaxy Groups, Elliptical Galaxies, and Dwarf Galaxies

The baryonic Faber-Jackson relation (BFJR) links the baryonic mass of pressure-supported systems to their mean velocity dispersion. For elliptical galaxies, the BFJR is thought to be a projection of the fundamental plane (FP), which includes the stellar half-mass radius as a third variable. We study the BFJR and FP across eight orders of magnitude in baryonic mass, encompassing galaxy groups, ellipticals, dwarf ellipticals, and dwarf spheroidals. We compile and homogenize data for 1400 pressure-supported systems and measure their mean internal baryonic acceleration $\langle g_\mathrm{bar}\rangle$. We find that the properties of the BFJR and FP systematically depend on the internal acceleration of the sampled systems, with a transition around the acceleration scale $a_0\simeq 1.2\times10^{10}$ m s$^{-2}$. For low-acceleration systems with $\langle g_\mathrm{bar}\rangle < 0.6\,a_0$ (dwarf galaxies and galaxy groups), the BFJR relation takes the form $\log_{10}(M_\mathrm{bar}/M_{\odot}) = (4.19 \pm 0.10) \log_{10}(\sigma_{\rm los}/\rm{km s}^{-1}) + (2.55^{+0.16}_{-0.16})$. The FP expected from the Newtonian virial theorem is followed by high-acceleration systems (massive ellipticals with $\langle g_\mathrm{bar}\rangle \gtrsim 6 \,a_0$), whereas low-acceleration systems deviate from the FP at both low masses (dwarf galaxies) and high masses (galaxy groups). Our results generally agree with the expectations of modified Newtonian dynamics (MOND): high-acceleration systems follow the Newtonian virial theorem in which a radial variable explicitly appears (the FP), while low-acceleration systems follow the MOND virial theorem in which the radial dependence disappears (the BFJR). On average, the MOND external field effect seems to play a secondary role in dwarf galaxies in galaxy groups and clusters.

astro-ph.GA↗

arXiv2024

Sources and Radiations of the Fermi Bubbles

Two enigmatic gamma-ray features in the Galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release (e.g., by tidal disruption events in the Galactic center, GC), generates a cavity with a shock that expands into the local ambient medium of the Galactic halo. A decade or so ago, a phenomenological model of the FBs was suggested as a result of routine star disruptions by the supermassive black hole in the GC which might provide enough energy for large-scale structures, like the FBs. In 2020, analytical and numerical models of the FBs as a process of routine tidal disruption of stars near the GC were developed, which can provide enough cumulative energy to form and maintain large scale structures like the FBs. The disruption events are expected to be ten to hundred events per million years, providing the average power of energy release from the GC into the halo of 3E41 erg/s, which is needed to support the FBs. Analysis of the evolution of superbubbles in exponentially stratified disks concluded that the FB envelope would be destroyed by the Rayleigh-Taylor (RT) instabilities at late stages. The shell is composed of a swept-up gas of the bubble, whose thickness is much thinner in comparison to the size of the envelope. We assume that hydrodynamic turbulence is excited in the FB envelope by the RT instability. In this case, the universal energy spectrum of turbulence may be developed in the inertial range of wavenumbers of fluctuations (the Kolmogorov-Obukhov spectrum). From our model we suppose the power of the FBs is transformed partly into the energy of hydrodynamic turbulence in the envelope. If so, hydrodynamic turbulence may generate MHD-fluctuations, which accelerate cosmic rays there and generate gamma-ray and radio emission from the FBs. We hope that this model may interpret the observed nonthermal emission from the bubbles.

astro-ph.HE↗

arXiv2024

A Distinct Radial Acceleration Relation across Brightest Cluster Galaxies and Galaxy Clusters

Recent studies reveal a radial acceleration relation (RAR) in galaxies, which illustrates a tight empirical correlation connecting the observational acceleration and the baryonic acceleration with a characteristic acceleration scale. However, a distinct RAR has been revealed on BCG-cluster scales with a seventeen times larger acceleration scale by the gravitational lensing effect. In this work, we systematically explored the acceleration and mass correlations between dynamical and baryonic components in 50 Brightest Cluster Galaxies (BCGs). To investigate the dynamical RAR in BCGs, we derived their dynamical accelerations from the stellar kinematics using the Jeans equation through Abel inversion and adopted the baryonic mass from the SDSS photometry. We explored the spatially resolved kinematic profiles with the largest integral field spectroscopy (IFS) data mounted by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. Our results demonstrate that the dynamical RAR in BCGs is consistent with the lensing RAR on BCG-cluster scales as well as a larger acceleration scale. This finding may imply that BCGs and galaxy clusters have fundamental differences from field galaxies. We also find a mass correlation, but it is less tight than the acceleration correlation.

astro-ph.GA↗

arXiv2023

Measuring galaxy cluster mass profiles into the low acceleration regime with galaxy kinematics

We probe the dynamical mass profiles of 10 galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) using galaxy kinematics. We numerically solve the spherical Jeans equation, and parameterize the dynamical mass profile and the galaxy velocity anisotropy profile using two general functions to ensure that our results are not biased towards any specific model. The mass-velocity anisotropy degeneracy is ameliorated by using two "virial shape parameters" that depend on the fourth moment of velocity distribution. The resulting velocity anisotropy estimates consistently show a nearly isotropic distribution in the inner regions, with an increasing radial anisotropy towards large radii. We compare our derived dynamical masses with those calculated from X-ray gas data assuming hydrostatic equilibrium, finding that massive and rich relaxed clusters generally present consistent mass measurements, while unrelaxed or low-richness clusters have systematically larger total mass than hydrostatic mass by an average of 50\%. This might help alleviate current tensions in the measurement of $σ_8$, but it also leads to cluster baryon fractions below the cosmic value. Finally, our approach probes accelerations as low as $10^{-11}$ m s$^{-2}$, comparable to the outskirts of individual late-type galaxies. We confirm that galaxy clusters deviate from the radial acceleration relation defined by galaxies.

astro-ph.CO↗

arXiv2022

The Effect of Adiabatic Compression on Dark Matter Halos and the Radial Acceleration Relation

We use a semi-empirical model to investigate the radial acceleration relation (RAR) in a cold dark matter (CDM) framework. Specifically, we build 80 model galaxies covering the same parameter space as the observed galaxies in the SPARC database, assigning them to dark matter halos using abundance matching and halo mass-concentration relations. We consider several abundance matching relations, finding some to be a better match to the kinematic data than others. We compute the unavoidable gravitational interactions between baryons and their dark matter halos, leading to an overall compression of the original NFW halos. Before halo compression, high-mass galaxies approximately lie on the observed RAR whereas low-mass galaxies display up-bending "hooks" at small radii due to DM cusps, making them deviate systematically from the observed relation. After halo compression, the initial NFW halos become more concentrated at small radii, making larger contributions to rotation curves. This increases the total accelerations, moving all model galaxies away from the observed relation. These systematic deviations suggest that the CDM model with abundance matching alone cannot explain the observed RAR. Further effects (e.g., feedback) would need to counteract the compression with precisely the right amount of halo expansion, even in high mass galaxies with deep potential wells where such effects are generally predicted to be negligible.

astro-ph.GA↗

arXiv2021

Mass-Velocity Dispersion Relation in MaNGA Brightest Cluster Galaxies

We investigate a kinematic scaling relation between the baryonic mass and the flat velocity dispersion, i.e. mass-velocity dispersion relation (MVDR), from the brightest cluster galaxies (BCGs) to the galaxy clusters. In our studies, the baryonic mass of BCGs is mainly estimated by photometry. The velocity dispersion profiles are explored with the integrated field unit (IFU) by Mapping Nearby Galaxies at Apache Point Observatory (MaNGA). For the first time, we reveal two significant results with 54 MaNGA BCGs: (1) the flat velocity dispersion profiles; (2) a tight empirical relation on the BCG-cluster scale together with cluster samples, i.e., MVDR, $\log(M_\mathrm{bar}/M_\odot)=4.1^{+0.1}_{-0.1}\log(σ_{\mathrm{los}}/\mathrm{km}\,\mathrm{s}^{-1})+1.6^{+0.3}_{-0.3}$, with a tiny lognormal intrinsic scatter of $10^{+2}_{-1}\%$. This slope is identical to the acceleration relation in galaxy clusters, which is reminiscent of the spiral galaxies, albeit at a larger characteristic acceleration scale. The residuals of the MVDR represent a Gaussian distribution, displaying no correlations with four properties: baryonic mass, scale length, surface density, and redshift. Notably, the MVDR on the BCG-cluster scale provides a strict test, which disfavors the general prediction of the slope of three in the dark matter model.

astro-ph.GA↗

arXiv2021

Mass-Velocity Dispersion Relation in HIFLUGCS Galaxy Clusters

We investigate the mass-velocity dispersion relation (MVDR) in 29 galaxy clusters in the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS). We measure the spatially resolved, line-of-sight velocity dispersion profiles of these clusters, which we find to be mostly flat at large radii, reminiscent of the rotation curves of galaxies. We discover a tight empirical relation between the baryonic mass $M_\mathrm{bar}$ and the flat velocity dispersion $σ$ of the member galaxies, i.e. MVDR, $\log(M_\mathrm{bar}/M_\odot)=4.1^{+0.4}_{-0.4}\,\log(σ/\mathrm{km}\,\mathrm{s}^{-1})+1.6^{+1.0}_{-1.3}$, with the lognormal intrinsic scatter of $12^{+3}_{-3}\%$. The residuals of the MVDR are uncorrelated with other cluster properties like temperature, cluster radius, baryonic mass surface density, and redshift. These characteristics are reminiscent of the MVDR for individual galaxies, albeit about ten times larger characteristic acceleration scale. The cluster baryon fraction falls short of the cosmic value, exposing a problem: the discrepancy increases systematically for clusters of lower mass and lower baryonic acceleration.

astro-ph.GA↗

arXiv2020

The Radial Acceleration Relation in CLASH Galaxy Clusters

The radial acceleration relation (RAR) in galaxies describes a tight empirical scaling law between the total acceleration $g_\mathrm{tot}(r)=GM_\mathrm{tot}(<r)/r^2$ observed in galaxies and that expected from their baryonic mass $g_\mathrm{bar}(r)=GM_\mathrm{bar}(<r)/r^2$, with a characteristic acceleration scale of $g_\dagger\simeq 1.2\times 10^{-10}$ms$^{-2}$. Here, we examine if such a correlation exists in galaxy clusters using weak-lensing, strong-lensing, and X-ray data sets available for 20 high-mass clusters targeted by the CLASH survey. By combining our CLASH data with stellar mass estimates for the brightest cluster galaxies (BCGs) and accounting for the stellar baryonic component in clusters, we determine, for the first time, an RAR on BCG--cluster scales. The resulting RAR is well described by a tight power-law relation, $g_\mathrm{tot}\propto g_\mathrm{bar}^{0.51^{+0.04}_{-0.05}}$, with lognormal intrinsic scatter of $14.7^{+2.9}_{-2.8}\%$. The slope is consistent with the low acceleration limit of the RAR in galaxies, $g_\mathrm{tot}=\sqrt{g_\dagger\,g_\mathrm{bar}}$, whereas the intercept implies a much higher acceleration scale of $g_\ddagger = (2.02\pm0.11)\times 10^{-9}$ms$^{-2}$, indicating that there is no universal RAR that holds on all scales from galaxies to clusters. We find that the observed RAR in CLASH clusters is consistent with predictions from a semi-analytical model developed in the standard $Λ$CDM framework. Our results also predict the presence of a baryonic Faber--Jackson relation ($σ_v^4\propto M_\mathrm{bar}$) on cluster scales.

astro-ph.CO↗

arXiv2019

Halo Acceleration Relation

Recently, from the new Spitzer Photometry and Accurate Rotation Curves (SPARC) data, McGaugh et al. (2016) reported a tight Radial Acceleration Relation (RAR) between the observed total acceleration and the acceleration produced by baryons in spiral galaxies. The relation can be fitted by different functions. However, these functions can be discerned if we express the data in the form of halo acceleration relation (HAR). The data reveals a maximum in the halo acceleration. We examined NFW (cusp) and Burkert (core) profiles in the context of dark matter and different parameter families of the interpolating function in the framework of Modified Newtonian Dynamics (MOND).

astro-ph.GA↗

arXiv2017

Mass Discrepancy-Acceleration Relation in Einstein Rings

We study the Mass Discrepancy-Acceleration Relation (MDAR) of 57 elliptical galaxies by their Einstein rings from the Sloan Lens ACS Survey (SLACS). The mass discrepancy between the lensing mass and the baryonic mass derived from population synthesis is larger when the acceleration of the elliptical galaxy lenses is smaller. The MDAR is also related to surface mass density discrepancy. At the Einstein ring, these lenses belong to high-surface-mass density galaxies. Similarly, we find that the discrepancy between the lensing and stellar surface mass density is small. It is consistent with the recent discovery of dynamical surface mass density discrepancy in disk galaxies where the discrepancy is smaller when surface density is larger. We also find relativistic modified Newtonian dynamics (MOND) can naturally explain the MDAR and surface mass density discrepancy in 57 Einstein rings. Moreover, the lensing mass, the dynamical mass and the stellar mass of these galaxies are consistent with each other in relativistic MOND.

astro-ph.GA↗

arXiv2017

Origin of the fundamental plane of elliptical galaxies in the Coma Cluster without fine-tuning

After thirty years of the discovery of the fundamental plane, explanations to the tilt of the fundamental plane with respect to the virial plane still suffer from the need of fine-tuning. In this paper, we try to explore the origin of this tilt from the perspective of modified Newtonian dynamics (MOND) by applying the 16 Coma galaxies available in Thomas et al.[1]. Based on the mass models that can reproduce de Vaucouleurs' law closely, we find that the tilt of the traditional fundamental plane is naturally explained by the simple form of the MONDian interpolating function, if we assume a well motivated choice of anisotropic velocity distribution, and adopt the Kroupa or Salpeter stellar mass-to-light ratio. Our analysis does not necessarily rule out a varying stellar mass-to-light ratio.

astro-ph.GA↗

arXiv2016

Dynamics of Elliptical Galaxies with Planetary Nebulae in Modified Newtonian Dynamics

The dynamics of an elliptical galaxy within a couple of effective radii can be probed effectively by stars. However, at larger distances planetary nebulae (PNe) replace stars as the tracer of the dynamics. Making use of the motion of PNe, Romanowsky et al. (2003) measured the dynamics of three luminous elliptical galaxies (NGC821, NGC3379, and NGC4494) at large distances from the galactic center. They found that little dark matter is needed up to 6 effective radii. Milgrom & Sanders (2003) showed that this result can be understood in the framework of MOdified Newtonian Dynamics (MOND). As more data are available in the past decade, we revisit this problem. We combine PNe data (up to 6{8 effective radii) and stellar data from SAURON of 7 elliptical galaxies, including those 3 galaxies in Romanowsky et al. (2003) with updated data and 4 other galaxies which have not been analyzed before. We conclude that the dynamics of these galaxies can be well explained by MOND.

astro-ph.GA↗

arXiv2016

On the problem of deformed spherical systems in Modified Newtonian Dynamics

Based on Newtonian dynamics, observations show that the luminous masses of astrophysical objects that are the size of a galaxy or larger are not enough to generate the measured motions which they supposedly determine. This is typically attributed to the existence of dark matter, which possesses mass but does not radiate (or absorb radiation). Alternatively, the mismatch can be explained if the underlying dynamics is not Newtonian. Within this conceptual scheme, Modified Newtonian Dynamics (MOND) is a successful theoretical paradigm. MOND is usually expressed in terms of a nonlinear Poisson equation, which is difficult to analyse for arbitrary matter distributions. We study the MONDian gravitational field generated by slightly non-spherically symmetric mass distributions based on the fact that both Newtonian and MONDian fields are conservative (which we refer to as the compatibility condition). As the non-relativistic version of MOND has two different formulations (AQUAL and QuMOND) and the compatibility condition can be expressed in two ways, there are four approaches to the problem in total. The method involves solving a suitably defined linear deformation potential, which generally depends on the choice of MOND interpolation function. However, for some specific form of the deformation potential, the solution is independent of the interpolation function.

gr-qc↗

arXiv2015

Analysis of the magneto-rotational instability with the effect of cosmic-ray diffusion

We present the results obtained from linear stability analysis and 2.5-dimensional magnetohydrodynamic (MHD) simulations of the magnetorotational instability (MRI), including the effects of cosmic rays (CRs). We took into account of the CR diffusion along the magnetic field but neglect the cross-field-line diffusion. Two models are considered in this paper: shearing box model and differentially rotating cylinder model. We studied how MRI is affected by the initial CR pressure (i.e., energy) distribution. In the shearing box model, the initial state is uniform distribution. Linear analysis shows that the growth rate of MRI does not depend on the value of CR diffusion coefficient. In the differentially rotating cylinder model, the initial state is a constant angular momentum polytropic disk threaded by weak uniform vertical magnetic field. Linear analysis shows that the growth rate of MRI becomes larger if the CR diffusion coefficient is larger. Both results are confirmed by MHD simulations. The MHD simulation results show that the outward movement of matter by the growth of MRI is not impeded by the CR pressure gradient, and the centrifugal force which acts to the concentrated matter becomes larger. Consequently, the growth rate of MRI is increased. On the other hand, if the initial CR pressure is uniform, then the growth rate of the MRI barely depends on the value of the CR diffusion coefficient.

astro-ph.HE↗

arXiv2013

Hubble Constant, Lensing, and Time Delay in Relativistic MOND

Time delay in galaxy gravitational lensing systems has been used to determine the value of Hubble constant. As in other dynamical phenomena at the scale of galaxy, dark matter is often invoked in gravitational lensing to account for the "missing mass" (the apparent discrepancy between the dynamical mass and the luminous mass). Alternatively, modified gravity can be used to explain the discrepancy. In this paper we adopt the Tensor-Vector-Scalar gravity (TeVeS), a relativistic version of MOdified Newtonian Dynamics (MOND), to study gravitational lensing phenomena and derive the formulae needed to evaluate the Hubble constant. We test our method on quasar lensing by elliptical galaxies in the literature. We focus on double-image systems with time delay measurement. Three candidates are suitable for our study: HE 2149-2745, FBQ J0951+2635 and SBS 0909+532. The Hubble constant obtained is consistent with the value used in fitting the CMB result in neutrino cosmological model.

astro-ph.CO↗

arXiv2012

The Information Of The Milky Way From 2MASS Whole Sky Star Count: The Bimodal Color Distributions

The J-Ks color distribution (CD) with a bin size of 0.05 magnitude for the entire Milky Way has been carried out by using the Two Micron All Sky Survey Point Source Catalog (2MASS PSC). The CDs are bimodal, which has a red peak at 0.8 < J-Ks < 0.85 and a blue peak at 0.3 < J-Ks < 0.4. The colors of the red peak are more or less the same for the whole sky, but that of the blue peak depend on Galactic latitude, (J-Ks ~ 0.35 at low Galactic latitudes and 0.35 < J-Ks < 0.4 for other sky areas). The blue peak dominates the bimodal CDs at low Galactic latitudes and becomes comparable with the red peak in other sky regions. In order to explain the bimodal distribution and the global trend shown by the all sky 2MASS CDs, we assemble an empirical HR diagram, which is composed by observational-based near infrared HR diagrams and color magnitude diagrams, and incorporate a Milky Way model. In the empirical HR diagram, the main sequence stars turnoff the thin disk is relatively bluer, (J-Ks)0 = 0.31, when we compare with the thick disk which is (J-Ks)0 = 0.39. The age of the thin/thick disk is roughly estimated to be around 4-5/8-9 Gyr according to the color-age relation of the main sequence turnoff. In general, the 2MASS CDs can be treated as a tool to census the age of stellar population of the Milky Way in a statistical manner and to our knowledge this is a first attempt to measure the age.

astro-ph.GA↗

arXiv2011

MHD Simulations of Parker Instability Undergoing Cosmic-Ray Diffusion

Parker instability arises from the presence of magnetic fields in a plasma in a gravitational field such as the interstellar medium (ISM), wherein the magnetic buoyant pressure expels the gas and causes the gas to move along the field lines. The process of mixing of this instability in the ISM near the Galactic plane is investigated. The initial ISM is assumed to consist of two fluids: plasma gas and cosmic-ray particles, in hydrostatic equilibrium, coupled with a uniform, azimuthally-aligned magnetic field. The evolution of the instability is explored in two models: an isothermal exponential-declining density model and a two-layered, hyperbolic tangent temperature model. After a small perturbation, the unstable gas aggregates at the bottom of the magnetic loops and forms dense blobs. The growth rate of the instability decreases as the coupling between the cosmic rays and the plasma becomes stronger (meaning a smaller CR diffusion coefficient). The mixing is enhanced by the cosmic-ray diffusion, while the shape of the condensed gas depends sensitively on the initial equilibrium conditions. The hyperbolic tangent temperature model produces a more concentrated and round shape of clumps at the foot points of rising magnetic arches, like the observed giant molecular cloud, whereas the exponential density model gives rise to a filamentary morphology of the clumpy structure. When considering a minimum perpendicular or cross field diffusion of cosmic rays, which is often substantially smaller than the parallel coefficient $κ_{\|}$, around $2%-4%$ of $κ_{\|}$, the flow speed is significantly increased such that the magnetic loops extend to a greater altitude. We speculate that the galactic wind flow perpendicular to the galactic disk may be facilitated by Parker instabilities through the cross field diffusion of cosmic rays.

astro-ph.HE↗

arXiv2011

The Information Of The Milky Way From 2MASS Whole Sky Star Count: The Structure Parameters

The Ks band differential star count of the Two Micron All Sky Survey (2MASS) is used to derive the global structure parameters of the smooth components of the Milky Way. To avoid complication introduced by other fine structures and significant extinction near and at the Galactic plane, we only consider Galactic latitude |b| > 30 degree data. The star count data is fitted with a threecomponent model: double exponential thin disk and thick disk, and a power law decay oblate halo. Using maximum likelihood the best-fit local density of thin disk is n0 = 0.030 +- 0.002 stars/pc^3. The best-fit scale-height and length of the thin disk are Hz1 = 360+-10 pc and Hr1 = 3.7+-1.0 kpc, and those of the thick disk are and Hz2 = 1020+-30 pc and Hr2 = 5.0+-1.0 kpc, the local thick-to-thin disk density ratio is f2 = 7+-1%. The best-fit axis ratio, power law index and local density ratio of the oblate halo are kappa = 0.55+-0.15, p = 2.6+-0.6 and fh = 0.20+-0:10%, respectively. Moreover, we find some degeneracy among the key parameters (e.g. n0,Hz1, f2 and Hz2). Any pair of these parameters are anticorrelated to each other. The 2MASS data can be well-fitted by several possible combinations of parameters. This is probably the reason that there is a wide range of values for the structure parameters in literature similar to this study. Since only medium and high Galactic latitude data are analyzed, the fitting is very insensitive to the scale-lengths of the disks.

astro-ph.GA↗