arXiv subjects

Man Ho Chan

Publications and source records attributed to Man Ho Chan.

At least 37 records · Page 2Linked to original sources

arXiv2022

Two mysterious universal dark matter-baryon relations in galaxies and galaxy clusters

Starting from the 1970s, some relations connecting dark matter and baryons were discovered, such as the Tully-Fisher relation. However, many of the relations found in galaxies are quite different from that found in galaxy clusters. Here, we report two new mysterious universal relations connecting dark matter and baryons in both galaxies and galaxy clusters. The first relation indicates that the total dynamical mass of a galaxy or a galaxy cluster $M_{500}$ has a power-law relation with its total baryonic mass $M_b$ within the `virial region': $M_{500} \propto M_b^a$, with $a \approx 3/4$. The second relation indicates that the enclosed dynamical mass $M_d$ is almost directly proportional to the baryonic mass for galaxies and galaxy clusters within the central baryonic region: $M_d \propto M_b$. The close relations between dark matter and baryons in both galaxies and galaxy clusters suggest that some unknown interaction or interplay except gravity might exist between dark matter and baryons.

astro-ph.GA↗

arXiv2022

The solar system test for the general modified gravity theories

In the past few decades, various versions of modified gravity theories were proposed to mimic the effect of dark matter. Compared with the conventional Newtonian or relativistic dynamics, these theories contain some extra apparent force terms in the dynamical equations to replace the role of dark matter. Generally speaking, the extra apparent force terms usually scale with radius so that the effect would be significant only on large scale to explain the missing mass in galaxies or galaxy clusters. Nevertheless, the apparent effect may still be observable in small structures like the solar system. In this article, we derive analytic general formulae to represent the contribution of the precession angle of the planets in the solar system due to the general modified gravity theories, in which the extra apparent force terms can be written in a power law of radius $r$ or an exponential function in $r$. We have tested three popular modified gravity theories, the Modified Newtonian Dynamics (MOND), the Emergent Gravity (EG), and the Modified Gravity (MOG). In particular, based on the solar system data, we have constrained the parameters involved for two popular general interpolating functions used in MOND. Our results can be generally applied to both of the modified inertia and modified gravity versions of MOND.

gr-qc↗

arXiv2022

Gravitational scattering of the sterile neutrino halo dark matter

A recent study shows that gravitational scattering of dark matter, in the form of massive objects with mass $m \sim 10^3-10^4M_{\odot}$, could provide a possible solution to alleviate the small-scale structure problems of cold dark matter. The scattering cross section is velocity-dependent so that this scenario can explain why self-interaction of dark matter is significant in dwarf galaxies, but not in massive galaxies and galaxy clusters. In this Letter, we show that this kind of dark massive objects could be made of sterile neutrinos with a possible rest mass range $m_ν \sim 7.6$ keV $-$ 71 MeV. This mass range generally satisfies most of the current observational constraints. The entire structure of the sterile neutrino halos can be simply predicted from standard physics.

astro-ph.GA↗

arXiv2022

Improved Lemaitre-Tolman model and the mass and turn-around radius in group of galaxies II: the role of dark energy

In this paper, we extend our previous study \cite{DelPopolo2021} on the Lemaitre-Tolman (LT) model showing how the prediction of the model changes when the equation of state parameter ($w$) of dark energy is modified. In the previous study, it was considered that dark energy was merely constituted by the cosmological constant. In this paper, as in the previous study, we also took into account the effect of angular momentum and dynamical friction ($Jη$ LT model) that modifies the evolution of a perturbation, initially moving with the Hubble flow. As a first step, solving the equation of motion, we calculated the relationship between mass, $M$, and the turn-around radius, $R_0$. If one knows the value of the turn-around radius $R_0$, it is possible to obtain the mass of the studied objects. As a second step, we build up, as in the previous paper, a relationship between the velocity, $v$, and radius, $R$. The relation was fitted to data of groups and clusters. Since the relationship $v-R$ depends on the Hubble constant and the mass of the object, we obtained optimized values of the two parameters of the objects studied. The mass decreases of a factor of maximum 25\% comparing the $Jη$ LT results (for which $w=-1$) and the case $w=-1/3$, while the Hubble constant increases going from $w=-1$ to $w=-1/3$. Finally, the obtained values of the mass, $M$, and $R_0$ of the studied objects can put constraints to the dark energy equation of state parameter, $w$.

astro-ph.CO↗

arXiv2022

Non self-similar Luminosity-temperature relation and dynamical friction

Extending the results of a previous paper \citep{DelPopolo2005}, by taking into account the role of dynamical friction, we recovered the luminosity-temperature relation (LTR). While by assuming self-similarity, a scaling law in which $L\propto T^2$ is obtained, observations show that the relation between luminosity and temperature is steeper, ${L \propto T^ {\simeq 3}}$. This difference can be explained in terms of energy input by non-gravitational processes, like pre-heating, supernovae feedback, and heating from AGN. In this paper, we studied the LTR by means of a modified version of the punctuated equilibria model \citep{Cavaliere1999}, taking into account in addition dynamical friction, thus extending the approach found in \citep{DelPopolo2005}. The result is a non-self-similar LTR with a bend at $\simeq 2$ keV, with a slope $2.76 \pm 0.18$ at larger energies and $3.4 \pm 0.18$ at energies smaller than 2 keV. This result is in agreement with the XXL survey \citep{Giles2016}. Moreover the steeper slopes at smaller energies is in agreement with some studies claiming a further steepening of the LTR at the low mass end. We also compared the results of our model with the 400d groups sample, finding that in groups the slope is slightly steeper than in clusters, %and namely $3.35 \pm 0.3$, in agreement with the \citep{Zou2016} study for the 400d groups sample, that gives a slope $3.29 \pm 0.33$.

astro-ph.CO↗

arXiv2022

Investigating the nature of mass distribution surrounding the Galactic supermassive black hole

In the past three decades, many stars orbiting about the supermassive black hole (SMBH) at the Galactic Centre (Sgr A*) were identified. Their orbital nature can give stringent constraints for the mass of the SMBH. In particular, the star S2 has completed at least one period since our first detection of its position, which can provide rich information to examine the properties of the SMBH, and the astrophysical environment surrounding the SMBH. Here, we report an interesting phenomenon that if a significant amount of dark matter or stellar mass is distributed around the SMBH, the precession speed of the S2 stellar orbit could be `slow down' by at most 27\% compared with that without dark matter surrounding the SMBH, assuming the optimal dark matter scenario. We anticipate that future high quality observational data of the S2 stellar orbit or other stellar orbits can help reveal the actual mass distribution near the SMBH and the nature of dark matter.

astro-ph.GA↗

arXiv2022

The effect of magnetic field on the inner Galactic rotation curve

In the past few decades, some studies pointed out that magnetic field might affect the rotation curves in galaxies. However, the impact is relatively small compared with the effects of dark matter and the baryonic components. In this letter, we revisit the impact of magnetic field on the rotation curve of our Galaxy. We show that the inner Galactic rotation curve could be affected significantly by the magnetic field. The addition of the inner bulge component, which has been proposed previously to account for the inner rotation curve data, is not necessary. The magnetic field contribution can fully account for the excess of the inner rotation velocity between 5 pc to 50 pc from the Galactic Centre. Our analysis can also constrain the azimuthal component of the central regular magnetic field strength to $B_0 \sim 50-60$ $μ$G, which is consistent with the observed range.

astro-ph.GA↗

arXiv2022

Constraining annihilating dark matter by radio continuum spectrum of the Large Magellanic Cloud

Recent radio observations have obtained stringent constraints for annihilating dark matter. In this article, we use the radio continuum spectral data of the Large Magellanic Cloud (LMC) to analyze the dark matter annihilation signals. We have discovered a slightly positive signal of dark matter annihilation with a $1.5σ$ statistical significance. The overall best-fit dark matter mass is $m_{\rm DM} \approx 90$ GeV, annihilating via $b\bar{b}$ channel. We have also constrained the $3σ$ lower limits of dark matter mass with the standard thermal dark matter annihilation cross section for the $e^+e^-$, $μ^+μ^-$, $τ^+τ^-$ and $b\bar{b}$ channels.

astro-ph.GA↗

arXiv2022

Constraints on annihilating dark matter in the Omega Centauri cluster

Recent gamma-ray and radio studies have obtained some stringent constraints on annihilating dark matter properties. However, only a few studies have focussed on using X-ray data to constrain annihilating dark matter. In this article, we perform the X-ray analysis of annihilating dark matter using the data of the Omega Centauri cluster. If dark matter is the correct interpretation of the non-luminous mass component derived in the Omega Centauri cluster, the conservative lower limits of thermal dark matter mass annihilating via the $τ^+τ^-$, $b\bar{b}$ and $W^+W^-$ channels can be significantly improved to 104(43) GeV, 650(167) GeV and 480(137) GeV respectively, assuming the diffusion coefficient $D_0 \le 10^{26}(10^{27})$ cm$^2$/s. These constraints can safely rule out the recent claims of dark matter interpretation of the gamma-ray excess and anti-proton excess seen in our Galaxy. Generally speaking, the conservative lower limits obtained for non-leptophilic annihilation channels are much more stringent than that obtained by gamma-ray analysis of nearby dwarf galaxies. We anticipate that this would open a new window for constraining annihilating dark matter.

astro-ph.GA↗

arXiv2022

There is no universal acceleration scale in galaxies

Recently, many studies seem to reveal the existence of some correlations between dark matter and baryonic matter. In particular, the unexpected tight Radial Acceleration Relation (RAR) discovered in rotating galaxies has caught much attention. The RAR suggests the existence of a universal and fundamental acceleration scale in galaxies, which seems to challenge the $Λ$CDM model and favour some modified gravity theories. A large debate about whether RAR is compatible with the $Λ$CDM model has arisen. Here, by analysing the high quality velocity dispersion profiles of 13 E0-type elliptical galaxies in the SDSS-IV MaNGA sample and assuming a power-law function of radius $r$ for the 3-dimensional velocity dispersion in each galaxy, we report the RAR for E0-type elliptical galaxies and we show that the resultant RAR has more than $5σ$ deviations from the RAR in late-type galaxies. This new RAR provides an independent probe to falsify the existence of any universal acceleration scale in galaxies. Our result significantly challenges those modified gravity theories which suggest the existence of any universal acceleration scale.

astro-ph.GA↗

arXiv2022

The analytic radial acceleration relation for galaxy clusters

Recently, a tight correlation between the dynamical radial acceleration and the baryonic radial acceleration in galaxies - the radial acceleration relation - has been discovered. This has been claimed as an indirect support of the modified gravity theories. However, whether the radial acceleration relation could also be found in galaxy clusters is controversial. In this article, we derive and present an analytic radial acceleration relation for the central region of galaxy clusters. We examine the data of some large galaxy clusters and we find that the resulting radial acceleration relation has a very large scatter. Moreover, although the radial acceleration relation for galaxy clusters shows some agreement with the one discovered in galaxies for a certain range of baryonic radial acceleration, their functional forms are somewhat different from each other. This suggests that the radial acceleration relation may not be a universal relation in general.

astro-ph.CO↗

arXiv2022

Constraining dark matter-nucleon scattering cross section by the background electron anti-neutrino flux data

Celestial objects such as stars and planets might be able to capture a large amount of dark matter particles through dark matter-nucleon scattering. Many previous studies have considered different celestial objects such as the Sun and the Earth as natural dark matter detectors and obtained some stringent bounds of the dark matter-nucleon scattering cross section. In this study, we use the $\sim 10$ MeV electron neutrino flux limits obtained by the Super-Kamiokande experiment and consider the Earth as a large natural dark matter detector to constrain the dark matter-nucleon scattering cross section. We show that this method can generally get more stringent limits. For certain ranges of dark matter mass annihilating via the $b\bar{b}$ channel, the limits of cross section for the isospin-independent scattering and proton-only scattering could be more stringent than that obtained in the PICO direct-detection experiment.

hep-ph↗

arXiv2021

Two analytic relations connecting the hot gas astrophysics with the cold dark matter model for galaxy clusters

Galaxy clusters are good targets for examining our understanding of cosmology. Apart from numerical simulations and gravitational lensing, X-ray observation is the most common and conventional way to analyze the gravitational structures of galaxy clusters. Therefore, it is valuable to have simple analytical relations that can connect the observed distribution of the hot, X-ray emitting gas to the structure of the dark matter in the clusters as derived from simulations. In this article, we apply a simple framework that can analytically connect the hot gas empirical parameters with the standard parameters in the cosmological cold dark matter model. We have theoretically derived two important analytic relations, $r_s \approx \sqrt{3}r_c$ and $ρ_s \approx 9βkT/8 πGm_gr_c^2$, which can easily relate the dark matter properties in galaxy clusters with the hot gas properties. This can give a consistent picture describing gravitational astrophysics for galaxy clusters by the hot gas and cold dark matter models.

astro-ph.CO↗

arXiv2021

Constraining the axion-photon coupling using radio data of the Bullet Cluster

Axion is one of the most popular candidates of the cosmological dark matter. Recent studies considering the misalignment production of axions suggest some benchmark axion mass ranges near $m_a \sim 20$ $μ$eV. For such axion mass, the spontaneous decay of axions can give photons in radio band frequency $ν\sim 1-3$ GHz, which can be detected by radio telescopes. In this article, we show that using radio data of galaxy clusters would be excellent to constrain axion dark matter. Specifically, by using radio data of the Bullet cluster (1E 0657-55.8), we find that the upper limit of the axion-photon coupling constant can be constrained to $g_{a γγ} \sim 10^{-12}-10^{-11}$ GeV$^{-1}$ for $m_a \sim 20$ $μ$eV, which is tighter than the limit obtained by the CERN Axion Solar Telescope (CAST).

astro-ph.CO↗

arXiv2021

Turnaround radius in $Λ$CDM, and dark matter cosmologies with shear and vorticity

We determine the relationship between the turnaround radius, $R_{\rm t}$, and mass, $M_{\rm t}$, in $Λ$CDM, and in dark energy scenarios, using an extended spherical collapse model taking into account the effects of shear and vorticity. We find a more general formula than that usually described in literature, showing a dependence of $R_{\rm t}$ from shear, and vorticity. The $R_{\rm t}-M_{\rm t}$ relation differs from that obtained not taking into account shear, and rotation, especially at galactic scales, differing $\simeq 30\%$ from the result given in literature. This has effects on the constraint of the $w$ parameter of the equation of state. We compare the $R_{\rm t}-M_{\rm t}$ relationship obtained for the $Λ$CDM, and different dark energy models to that obtained in the $f(R)$ modified gravity (MG) scenario. The $R_{\rm t}-M_{\rm t}$ relationship in $Λ$CDM, and dark energy scenarios are tantamount to the prediction of the $f(R)$ theories. Then, the $R_{\rm t}-M_{\rm t}$ relationship is not a good probe to test gravity theories beyond Einstein's general relativity.

astro-ph.CO↗

arXiv2021

Turnaround radius in $Λ$CDM, and dark matter cosmologies II: the role of dynamical friction

This paper is an extension of the paper by Del Popolo, Chan, and Mota (2020) to take account the effect of dynamical friction. We show how dynamical friction changes the threshold of collapse, $δ_c$, and the turn-around radius, $R_t$. We find numerically the relationship between the turnaround radius, $R_{\rm t}$, and mass, $M_{\rm t}$, in $Λ$CDM, in dark energy scenarios, and in a $f(R)$ modified gravity model. Dynamical friction gives rise to a $R_{\rm t}-M_{\rm t}$ relation differing from that of the standard spherical collapse. In particular, dynamical friction amplifies the effect of shear, and vorticity already studied in Del Popolo, Chan, and Mota (2020). A comparison of the $R_{\rm t}-M_{\rm t}$ relationship for the $Λ$CDM, and those for the dark energy, and modified gravity models shows, that the $R_{\rm t}-M_{\rm t}$ relationship of the $Λ$CDM is similar to that of the dark energy models, and small differences are seen when comparing with the $f(R)$ models. The effect of shear, rotation, and dynamical friction is particularly evident at galactic scales, giving rise to a difference between the $R_{\rm t}-M_{\rm t}$ relation of the standard spherical collapse of the order of $\simeq 60\%$. Finally, we show how the new values of the $R_{\rm t}-M_{\rm t}$ influence the constraints to the $w$ parameter of the equation of state.

astro-ph.CO↗

arXiv2021

Model-independent constraints on ultra-light dark matter from the SPARC data

Ultra-light dark matter (ULDM) is currently one of the most popular classes of cosmological dark matter. The most important advantage is that ULDM with mass $m \sim 10^{-22}$ eV can account for the small-scale problems encountered in the standard cold dark matter (CDM) model like the core-cusp problem, missing satellite problem and the too-big-to-fail problem in galaxies. In this article, we formulate a new simple model-independent analysis using the SPARC data to constrain the range of ULDM mass. In particular, the most stringent constraint comes from the data of a galaxy ESO563-G021, which can conservatively exclude a ULDM mass range $m=(0.14-3.11)\times 10^{-22}$ eV. This model-independent excluded range is consistent with many bounds obtained by recent studies and it suggests that the ULDM proposal may not be able to alleviate the small-scale problems.

astro-ph.GA↗

arXiv2021

Improved Lemaitre-Tolman model and the mass and turn-around radius in group of galaxies

We extended the modified Lemaitre-Tolman model taking into account the effect of angular momentum and dynamical friction. The inclusion of these quantities in the equation of motion modifies the evolution of a perturbation, initially moving with the Hubble flow. Solving the equation of motions we got the relationships between mass, $M$, and the turn-around radius, $R_0$. Knowing $R_0$, the quoted relation allows the determination of the mass of the object studied. The relationships for the case in which also the angular momentum is taken into account gives a mass $\simeq 90$ \% larger than the standard Lemaitre-Tolman model, and two times the value of the standard Lemaitre-Tolman model, in the case also dynamical friction is taken into account. As a second step, we found relationships between the velocity, $v$, and radius, $R$, and fitted them to data of the Local Group, M81, NGC 253, IC342, CenA/M83, and to the Virgo clusters obtained by Ref.[New Astronomy 11(4):325, A&A 488(3):845]. This allowed us to find optimized values of the mass and Hubble constant of the objects studied. The fit gives values of the masses smaller with respect to the $M-R_0$ relationship method, but in any case 30-40\% larger than the $v-R$ relationship obtained from the standard Lemaitre-Tolman model. Differently from mass, the Hubble parameter becomes smaller with respect to the standard Lemaitre-Tolman model, when angular momentum, and dynamical friction are introduced. This is in agreement with Ref.[New Astronomy 11(4):325, A&A 488(3):845], who improved the standard Lemaitre-Tolman model taking into account the cosmological constant. Finally, we used the mass, $M$, and $R_0$ of the studied objects to put constraints to the dark energy equation of state parameter, $w$. Comparison with previous studies show different constraints on $w$.

astro-ph.CO↗