Recently, some studies showed that the GeV gamma-ray excess signal from the central Milky Way can be explained by the annihilation of -40 GeV dark matter through the bb channel. Based on the morphology of the gamma-r...Recently, some studies showed that the GeV gamma-ray excess signal from the central Milky Way can be explained by the annihilation of -40 GeV dark matter through the bb channel. Based on the morphology of the gamma-ray flux, the best-fit inner slope of the dark matter density profile is γ = 1.26. However, recent analyses of the Milky Way dark matter profile favor γ = 0.6 - 0.8. In this article, we show that the GeV gamma-ray excess can also be explained by the Sommerfeld-enhanced dark matter annihilation through the bb channel with γ = 0.85 - 1.05. We constrain the parameters of the Sommerfeld-enhanced annihilation by using data from Fermi-LAT. We also show that the predicted gamma-ray fluxes emitted from dwarf galaxies generally satisfy recent upper limits on gamma-ray fluxes detected by Fermi-LAT.展开更多
Recently, a very strong correlation between the central surface density of stars and dynamical mass in 135 disk galaxies has been obtained. It has been shown that this central-surface-densities relation agrees very we...Recently, a very strong correlation between the central surface density of stars and dynamical mass in 135 disk galaxies has been obtained. It has been shown that this central-surface-densities relation agrees very well with Modified Newtonian Dynamics（MOND）. In this article, we show that if we assume the baryons have an isothermal distribution and dark matter exists, then it is possible to derive by means of the Jeans equation an analytic central-surface-densities relation connecting dark matter and baryons that agrees with the observed relation. We find that the observed central-surface-densities relation can also be accommodated in the context of dark matter provided the latter is described by an isothermal profile. Therefore, the observed relation is consistent with not only MOND.展开更多
We use the redshift Hubble parameter H（z） data derived from relative galaxy ages, distant type Ia supernovae （SNe Ia）, the Baryonic Acoustic Oscillation （BAO） peak, and the Cosmic Microwave Background （CMB） sh...We use the redshift Hubble parameter H（z） data derived from relative galaxy ages, distant type Ia supernovae （SNe Ia）, the Baryonic Acoustic Oscillation （BAO） peak, and the Cosmic Microwave Background （CMB） shift parameter data, to constrain cosmological parameters in the Undulant Universe. We marginalize the like- lihood functions over h by integrating the probability density 19 ∝ e-x^2/2. By using a Markov Chain Monte Carlo （MCMC） technique, we obtain the best fitting results and give the confidence regions in the b - Ωm0 plane. Then we compare their constraints. Our results show that the H（z） data play a similar role with the SNe Ia data in cosmological study. By presenting the independent and joint constraints, we find that the BAO and CMB data play very important roles in breaking the degeneracy compared with the H（z） and SNe Ia data alone. Combined with the BAO or CMB data, one can remarkably improve the constraints. The SNe Ia data sets constrain Ωm0 much tighter than the H（z） data sets, but the H（z） data sets constrain b much tighter than the SNe Ia data sets. All these results show that the Undulant Universe approaches the ACDM model. We expect more H（z） data to constrain cosmological parameters in the future.展开更多
In the warm dark matter scenario, the Press-Schechter formalism is valid only for galaxy masses greater than the “velocity dispersion cut-off”. In this work we extend the predictions to masses below the velocity dis...In the warm dark matter scenario, the Press-Schechter formalism is valid only for galaxy masses greater than the “velocity dispersion cut-off”. In this work we extend the predictions to masses below the velocity dispersion cut-off, and thereby address the “Missing Satellites Problem” of the cold dark matter ΛCDM scenario, and the rest-frame ultra-violet luminosity cut-off required to not exceed the measured reionization optical depth. For warm dark matter we find agreement between predictions and observations of these two phenomena. As a by-product, we obtain the empirical Tully-Fisher relation from first principles.展开更多
We use controlled N-body simulation to investigate the dynamical processes(dynamical friction, tidal truncation, etc.) involved in the merging of small satellites into biggerhalos. We confirm the validity of some anal...We use controlled N-body simulation to investigate the dynamical processes(dynamical friction, tidal truncation, etc.) involved in the merging of small satellites into biggerhalos. We confirm the validity of some analytic formulae proposed earlier based on simplearguments. For rigid satellites represented by softened point masses, the merging time scale dependson both the orbital shape and concentration of the satellite. The dependence on orbital ellipticityis roughly a power law, as suggested by Lacey & Cole, and the dependence on satellite concentrationis similar to that proposed by White. When merging satellites are represented by non-rigid objects,Tidal effects must be considered. We found that material beyond the tidal radius are stripped off.The decrease in the satellite mass might mean an increase in the merging time scale, but in fact,the merging time is decreased, because the stripped-off material carries away a proportionatelylarger amount of of orbital energy and angular momentum.展开更多
Axion-like particles(ALPs) are a promising kind of dark matter candidate particle that are predicted to couple with photons in the presence of magnetic fields. The oscillations between photons and ALPs traveling in th...Axion-like particles(ALPs) are a promising kind of dark matter candidate particle that are predicted to couple with photons in the presence of magnetic fields. The oscillations between photons and ALPs traveling in the magnetic fields have been used to constrain ALP properties. In this work, we obtain some new constraints on the ALP mass ma and the photon-ALP coupling constant g with two different magnetic field models through TeV photons from PKS 2155–304. The first is the discrete-Φ model in which the magnetic field has the orientation angle Φ that changes discretely and randomly from one coherent domain to the next, and the second is the linearly-continuous-Φ model in which the magnetic field orientation angle Φ varies continuously across neighboring coherent domains. For the discrete-Φ model, we can obtain the best constraints on the ALP mass m1 = ma/(1 neV)= 0.1 and on the photon-ALP coupling constant g11= g/(10^-11 GeV^-1)= 5. The reasonable range of the ALP mass m1 is 0.08 ~ 0.2 when g11 = 5,and the only reasonable value of the photon-ALP coupling constant is g11 = 5 when m1 = 0.1. For the linearly-continuous-Φ model, we can obtain the best constraints on the ALP mass m1 = 0.1 and on the photon-ALP coupling constant g11 = 0.7. The reasonable range of the ALP mass m1 is 0.05 ~ 0.4 when g11= 0.7, and the reasonable range of the photon-ALP coupling constant g11 is 0.5 ~ 1 when m1 = 0.1.All of the results are consistent with the upper bound(g < 6.6 × 10^-11 GeV^-1, i.e., g11 < 6.6) set by the CAST experiment.展开更多
We calculate the gravitational lensing probabilities by cold dark matter (CDM) halos with different density profiles, and compare them with current observations from the Cosmic Lens All-Sky Survey (CLASS) and the Jodr...We calculate the gravitational lensing probabilities by cold dark matter (CDM) halos with different density profiles, and compare them with current observations from the Cosmic Lens All-Sky Survey (CLASS) and the Jodrell-Bank VLA Astrometric Survey (JVAS). We find that the lensing probability is dramatically sensitive to the clumping of the dark matter, or quantitatively, the concentration parameter. We also find that our predicted lensing probabilities in most cases show inconsistency with the observations. It is argued that high lensing probability may not be an effective tool for probing the statistical properties of inner structures of dark matter halos.展开更多
Clusters of galaxies are the most massive objects in the Universe and precise knowledge of their mass structure is important to understand the history of structure formation and constrain still unknown types of dark c...Clusters of galaxies are the most massive objects in the Universe and precise knowledge of their mass structure is important to understand the history of structure formation and constrain still unknown types of dark contents of the Universe. X-ray spectroscopy of galaxy clusters provides rich information about the physical state of hot intracluster gas and the underlying potential structure. In this paper, starting from the basic description of clusters under equilibrium conditions, we review properties of clusters revealed primarily through X-ray observations considering their thermal and dynamical evolutions. The future prospects of cluster studies using upcoming X-ray missions are also mentioned.展开更多
Arising from gravitational deflections of light rays by large-scale struc- tures in the Universe, weak-lensing effects have been recognized as one of the most important probes in cosmological studies. In this paper, w...Arising from gravitational deflections of light rays by large-scale struc- tures in the Universe, weak-lensing effects have been recognized as one of the most important probes in cosmological studies. In this paper, we review the main progress in weak-lensing analyses, and discuss the challenges in future investigations aiming to understand the dark side of the Universe with unprecedented precisions.展开更多
When a satellite galaxy falls into a massive dark matter halo, it suffers from the dynamical friction force which drags it into the halo＇s center, where it finally merges with the central galaxy. The time interval be...When a satellite galaxy falls into a massive dark matter halo, it suffers from the dynamical friction force which drags it into the halo＇s center, where it finally merges with the central galaxy. The time interval between entry and merger is called the dynamical friction timescale （Tdf）. Many studies have been dedicated to deriving Tdf using analytical models or N-body simulations. These studies have obtained qualitative agreements on how Zdf depends on the orbital parameters, and the mass ratio between the satellite and the host＇s halo. However, there are still disagreements on deriving an accurate form for Tdf. We present a semi-analytical model to predict Tdf and we focus on interpreting the discrepancies among different studies. We find that the treatment of mass loss from the satellite by tidal stripping dominates the behavior of Tdf. We also identify other model parameters which affect the predicted Tdf.展开更多
Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curv...Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter domi- nated low surface brightness （LSB） galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simula- tions predict a divergent （cuspy） density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey （CLASS） and Jodrell Bank/Very Large Array Astrometric Survey （JVAS）. The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statis- tics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. This places contradictory demands upon the effects of baryons on the central mass profiles of galaxies.展开更多
We investigate the potential of constraining the mass to light ratio of field galaxies using weak lensing shear and flexions. A suite of Monte Carlo simulations are used to generate weak lensing observations with diff...We investigate the potential of constraining the mass to light ratio of field galaxies using weak lensing shear and flexions. A suite of Monte Carlo simulations are used to generate weak lensing observations with different noise models. Using mock data, we find that the inclusion of flexions can improve the estimate of foreground halo parameters, but the details are strongly dependent on noise in the model. In the intrinsic noise limit, both shear and flexions are promising tools to study the mass to light ratio of galaxies. However, if the noise model of flexions follows the form described by Rowe et al., there is only - 5% improvement in the constraints even with next generation lensing observations.展开更多
N-body simulations predict that dark matter halos with different mass scales are described by a universal model, the Navarro-Frenk-White （NFW） den- sity profiles. As a consequence of baryonic cooling effects, these ...N-body simulations predict that dark matter halos with different mass scales are described by a universal model, the Navarro-Frenk-White （NFW） den- sity profiles. As a consequence of baryonic cooling effects, these halos will become more concentrated, and similar to an isothermal sphere over a large range in radii （～ 300 h-1 kpc）. The singular isothermal sphere （SIS） model however has to be trun- cated artificially at large radii since it extends to infinity. We model a massive galaxy halo as a combination of an isothermal sphere and an NFW density profile. We give an approximation for the mass concentration at different baryon fractions and present exact expressions for the weak lensing shear and flexion for such a halo. We compare the lensing properties with the SIS and NFW profiles. We find that the combined pro- file can generate higher order lensing signals at small radii and is more efficient in generating strong lensing events. In order to distinguish such a halo profile from the SIS or NFW profiles, one needs to combine strong and weak lensing constraints for small and large radii.展开更多
基金partially supported by a grant from The Education University of Hong Kong(Project No.RG57/2015-2016R)
文摘Recently, some studies showed that the GeV gamma-ray excess signal from the central Milky Way can be explained by the annihilation of -40 GeV dark matter through the bb channel. Based on the morphology of the gamma-ray flux, the best-fit inner slope of the dark matter density profile is γ = 1.26. However, recent analyses of the Milky Way dark matter profile favor γ = 0.6 - 0.8. In this article, we show that the GeV gamma-ray excess can also be explained by the Sommerfeld-enhanced dark matter annihilation through the bb channel with γ = 0.85 - 1.05. We constrain the parameters of the Sommerfeld-enhanced annihilation by using data from Fermi-LAT. We also show that the predicted gamma-ray fluxes emitted from dwarf galaxies generally satisfy recent upper limits on gamma-ray fluxes detected by Fermi-LAT.
基金supported by a grant from The Education University of Hong Kong (Project No.:RG4/2016-2017R)
文摘Recently, a very strong correlation between the central surface density of stars and dynamical mass in 135 disk galaxies has been obtained. It has been shown that this central-surface-densities relation agrees very well with Modified Newtonian Dynamics（MOND）. In this article, we show that if we assume the baryons have an isothermal distribution and dark matter exists, then it is possible to derive by means of the Jeans equation an analytic central-surface-densities relation connecting dark matter and baryons that agrees with the observed relation. We find that the observed central-surface-densities relation can also be accommodated in the context of dark matter provided the latter is described by an isothermal profile. Therefore, the observed relation is consistent with not only MOND.
基金supported by the National Natural Science Foundation of China (Grant No.10473002)the Ministry of Science and Technology National Basic Science program (project 973,Grant No.2009CB24901)the Fundamental Research Funds for the Central Universities
文摘We use the redshift Hubble parameter H（z） data derived from relative galaxy ages, distant type Ia supernovae （SNe Ia）, the Baryonic Acoustic Oscillation （BAO） peak, and the Cosmic Microwave Background （CMB） shift parameter data, to constrain cosmological parameters in the Undulant Universe. We marginalize the like- lihood functions over h by integrating the probability density 19 ∝ e-x^2/2. By using a Markov Chain Monte Carlo （MCMC） technique, we obtain the best fitting results and give the confidence regions in the b - Ωm0 plane. Then we compare their constraints. Our results show that the H（z） data play a similar role with the SNe Ia data in cosmological study. By presenting the independent and joint constraints, we find that the BAO and CMB data play very important roles in breaking the degeneracy compared with the H（z） and SNe Ia data alone. Combined with the BAO or CMB data, one can remarkably improve the constraints. The SNe Ia data sets constrain Ωm0 much tighter than the H（z） data sets, but the H（z） data sets constrain b much tighter than the SNe Ia data sets. All these results show that the Undulant Universe approaches the ACDM model. We expect more H（z） data to constrain cosmological parameters in the future.
文摘In the warm dark matter scenario, the Press-Schechter formalism is valid only for galaxy masses greater than the “velocity dispersion cut-off”. In this work we extend the predictions to masses below the velocity dispersion cut-off, and thereby address the “Missing Satellites Problem” of the cold dark matter ΛCDM scenario, and the rest-frame ultra-violet luminosity cut-off required to not exceed the measured reionization optical depth. For warm dark matter we find agreement between predictions and observations of these two phenomena. As a by-product, we obtain the empirical Tully-Fisher relation from first principles.
基金Supported by the National Natural Science Foundation of China
文摘We use controlled N-body simulation to investigate the dynamical processes(dynamical friction, tidal truncation, etc.) involved in the merging of small satellites into biggerhalos. We confirm the validity of some analytic formulae proposed earlier based on simplearguments. For rigid satellites represented by softened point masses, the merging time scale dependson both the orbital shape and concentration of the satellite. The dependence on orbital ellipticityis roughly a power law, as suggested by Lacey & Cole, and the dependence on satellite concentrationis similar to that proposed by White. When merging satellites are represented by non-rigid objects,Tidal effects must be considered. We found that material beyond the tidal radius are stripped off.The decrease in the satellite mass might mean an increase in the merging time scale, but in fact,the merging time is decreased, because the stripped-off material carries away a proportionatelylarger amount of of orbital energy and angular momentum.
文摘Axion-like particles(ALPs) are a promising kind of dark matter candidate particle that are predicted to couple with photons in the presence of magnetic fields. The oscillations between photons and ALPs traveling in the magnetic fields have been used to constrain ALP properties. In this work, we obtain some new constraints on the ALP mass ma and the photon-ALP coupling constant g with two different magnetic field models through TeV photons from PKS 2155–304. The first is the discrete-Φ model in which the magnetic field has the orientation angle Φ that changes discretely and randomly from one coherent domain to the next, and the second is the linearly-continuous-Φ model in which the magnetic field orientation angle Φ varies continuously across neighboring coherent domains. For the discrete-Φ model, we can obtain the best constraints on the ALP mass m1 = ma/(1 neV)= 0.1 and on the photon-ALP coupling constant g11= g/(10^-11 GeV^-1)= 5. The reasonable range of the ALP mass m1 is 0.08 ~ 0.2 when g11 = 5,and the only reasonable value of the photon-ALP coupling constant is g11 = 5 when m1 = 0.1. For the linearly-continuous-Φ model, we can obtain the best constraints on the ALP mass m1 = 0.1 and on the photon-ALP coupling constant g11 = 0.7. The reasonable range of the ALP mass m1 is 0.05 ~ 0.4 when g11= 0.7, and the reasonable range of the photon-ALP coupling constant g11 is 0.5 ~ 1 when m1 = 0.1.All of the results are consistent with the upper bound(g < 6.6 × 10^-11 GeV^-1, i.e., g11 < 6.6) set by the CAST experiment.
基金This work was partially supported by the National Natural Science Foundation of China under Grant No. 10003002.
文摘We calculate the gravitational lensing probabilities by cold dark matter (CDM) halos with different density profiles, and compare them with current observations from the Cosmic Lens All-Sky Survey (CLASS) and the Jodrell-Bank VLA Astrometric Survey (JVAS). We find that the lensing probability is dramatically sensitive to the clumping of the dark matter, or quantitatively, the concentration parameter. We also find that our predicted lensing probabilities in most cases show inconsistency with the observations. It is argued that high lensing probability may not be an effective tool for probing the statistical properties of inner structures of dark matter halos.
文摘Clusters of galaxies are the most massive objects in the Universe and precise knowledge of their mass structure is important to understand the history of structure formation and constrain still unknown types of dark contents of the Universe. X-ray spectroscopy of galaxy clusters provides rich information about the physical state of hot intracluster gas and the underlying potential structure. In this paper, starting from the basic description of clusters under equilibrium conditions, we review properties of clusters revealed primarily through X-ray observations considering their thermal and dynamical evolutions. The future prospects of cluster studies using upcoming X-ray missions are also mentioned.
基金Supported by the National Natural Science Foundation of China
文摘Arising from gravitational deflections of light rays by large-scale struc- tures in the Universe, weak-lensing effects have been recognized as one of the most important probes in cosmological studies. In this paper, we review the main progress in weak-lensing analyses, and discuss the challenges in future investigations aiming to understand the dark side of the Universe with unprecedented precisions.
基金funded by the National Natural Science Foundation of China (Grant No. 10573028)the Key Project (Grant No. 10833005)+4 种基金the Group Innovation Project (Grant No. 10821302)the National Basic Research Program of China (973 ProgramNo. 2007CB815402)supported by the One Hundred Talents Project of the Chinese Academy of Sciencesthe foundation for the authors of CAS excellent doctoral dissertations
文摘When a satellite galaxy falls into a massive dark matter halo, it suffers from the dynamical friction force which drags it into the halo＇s center, where it finally merges with the central galaxy. The time interval between entry and merger is called the dynamical friction timescale （Tdf）. Many studies have been dedicated to deriving Tdf using analytical models or N-body simulations. These studies have obtained qualitative agreements on how Zdf depends on the orbital parameters, and the mass ratio between the satellite and the host＇s halo. However, there are still disagreements on deriving an accurate form for Tdf. We present a semi-analytical model to predict Tdf and we focus on interpreting the discrepancies among different studies. We find that the treatment of mass loss from the satellite by tidal stripping dominates the behavior of Tdf. We also identify other model parameters which affect the predicted Tdf.
基金supported by the National Natural Science Foundation of China (Grant 11073023)the National Basic Research Program of China (973 Program+1 种基金Grant 2009CB24901)the National Science Foundation of the United States (Grant AST0908370)
文摘Standard cosmology has many successes on large scales, but faces some fundamental difficulties on small, galactic scales. One such difficulty is the cusp/core problem. High resolution observations of the rotation curves for dark matter domi- nated low surface brightness （LSB） galaxies imply that galactic dark matter halos have a density profile with a flat central core, whereas N-body structure formation simula- tions predict a divergent （cuspy） density profile at the center. It has been proposed that this problem can be resolved by stellar feedback driving turbulent gas motion that erases the initial cusp. However, strong gravitational lensing prefers a cuspy density profile for galactic halos. In this paper, we use the most recent high resolution observations of the rotation curves of LSB galaxies to fit the core size as a function of halo mass, and compare the resultant lensing probability to the observational results for the well defined combined sample of the Cosmic Lens All-Sky Survey （CLASS） and Jodrell Bank/Very Large Array Astrometric Survey （JVAS）. The lensing probabilities based on such density profiles are too low to match the observed lensing in CLASS/JVAS. High baryon densities in the galaxies that dominate the lensing statis- tics can reconcile this discrepancy, but only if they steepen the mass profile rather than making it more shallow. This places contradictory demands upon the effects of baryons on the central mass profiles of galaxies.
基金support from the National Basic Research Program of China (973 program Grant No. 2009CB24901)+2 种基金the National Natural Science Foundation of China (Grant No. 10973018)the Partner Group program of the Max Planck Society and an STFC Advanced Fellowshipsupported by the China Postdoctoral Science Foundation (Grant No. 2011M500395)
文摘We investigate the potential of constraining the mass to light ratio of field galaxies using weak lensing shear and flexions. A suite of Monte Carlo simulations are used to generate weak lensing observations with different noise models. Using mock data, we find that the inclusion of flexions can improve the estimate of foreground halo parameters, but the details are strongly dependent on noise in the model. In the intrinsic noise limit, both shear and flexions are promising tools to study the mass to light ratio of galaxies. However, if the noise model of flexions follows the form described by Rowe et al., there is only - 5% improvement in the constraints even with next generation lensing observations.
基金supported by the National Natural Science Foundation of China(Grant No.11203029)
文摘N-body simulations predict that dark matter halos with different mass scales are described by a universal model, the Navarro-Frenk-White （NFW） den- sity profiles. As a consequence of baryonic cooling effects, these halos will become more concentrated, and similar to an isothermal sphere over a large range in radii （～ 300 h-1 kpc）. The singular isothermal sphere （SIS） model however has to be trun- cated artificially at large radii since it extends to infinity. We model a massive galaxy halo as a combination of an isothermal sphere and an NFW density profile. We give an approximation for the mass concentration at different baryon fractions and present exact expressions for the weak lensing shear and flexion for such a halo. We compare the lensing properties with the SIS and NFW profiles. We find that the combined pro- file can generate higher order lensing signals at small radii and is more efficient in generating strong lensing events. In order to distinguish such a halo profile from the SIS or NFW profiles, one needs to combine strong and weak lensing constraints for small and large radii.
