The spatial distribution of dark matter annihilation originating from a gamma-ray line signal

The GeV-TeV -γ-ray line signal is the smoking gun signature of dark matter annihilation or decay. The detection of such a signal is one of the main targets of some space-based telescopes, including Fermi-LAT and the upcoming missions CALET, DAMPE and Gamma-400. An important feature of γ-ray line photons that originate from dark-matter-annihilation is that they are concentrated at the center of the Galaxy. So far, no reliable γ-ray line has been detected by Fermi-LAT, and the upper limits on the cross section of annihilation into ＂y-rays have been reported. We use these upper limits to estimate the ＂maximal＂ number of -y-ray line photons detectable for Fermi- LAT, DAMPE and Gamma-400, and then investigate the spatial distribution of these photons. We show that the center of the distribution will usually be offset from the Galactic center （Sgr A＊） due to the limited statistics. Such a result is almost indepen- dent of models of the dark matter distribution, and will render the reconstruction of the dark matter distribution with the γ-ray line signal very challenging for foreseeable space-based detectors.

My C.G.S.I.S.A.H. Theory of Dark Matter

Theory and observations concerning the cosmic reionization epoch are briefly discussed in the context of recent observations attributed to dark matter. A case is made that cold ground state interstellar atomic hydrogen of average density of about one atom per cubic centimeter (1.67 × 10-21?kg·m-3?or 1.67 × 10-24?g·cm-3) appears to be the most likely candidate to explain these observations.

Concordance of Kinematics and Lensing of Elliptical Galaxies with WMAP Cosmology

We explore degeneracies in strong lensing model so to make time delay data consistent with the WMAP （Wilkinson Microwave Anisotropy Probe） cosmology. Previous models using a singular isothermal lens often yield a time delay between the observed multiple images too small than the observed value if we ＂hardwire＂ the now widely quoted post-WMAP ＂high＂ value of the Hubble constant （Ho ～71 ± 4km s^-1 Mpc^-1）. Alternatively, the lens density profile （star plus dark matter） is required to be locally steeper than r-2 （isothermal） profile near the Einstein radius （of the order 3 kpc） to fit the time delays; a naive extrapolation of a very steep profile to large radius would imply a lens halo with a scale length of the order only 3 kpc, too compact to be consistent with CDM. We explore more sophisticated, mathematically smooth, positive lens mass density profiles which are consistent with a large halo and the post-WMAP H0. Thanks to the spherical monopole degeneracy, the ＂reshuffling＂ of the mass in a lens model does not affect the quality of the fit to the image positions, amplifications, and image time delays. Even better, unlike the better-known mass sheet degeneracy, the stellar mass-to-light and the H0 value are not affected either. We apply this monopole degeneracy to the quadruple imaged time-delay system PG 1115＋080. Finally we discuss the implications of the time delay data on the newly proposed relativistic MOND theory.

Cosmological constraints on the Undulant Universe

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.

X-ray spectroscopy of clusters of galaxies

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.

Pairwise Velocity Statistics of Dark Halos

We have accurately evaluated the halo pairwise velocity dispersion and the halo mean streaming velocity in the LCDM model (the fiat ω0 = 0.3 model) using a set of high-resolution N-body simulations. Based on the simulation results, we have developed a model for the pairwise velocity dispersion of halos. Our model agrees with the simulation results over all scales we studied. We have also tested the model of Sheth et al. for the mean streaming motion of halos derived from the pair-conservation equation. We found that their model reproduces the simulation data very well on large scale, but under-predicts the streaming motion on scales r < 10 h-1 Mpc. We have introduced an empirical relation to improve their model. These improved models are useful for predicting the redshift correlation functions and the redshift power spectrum of galaxies if the halo occupation number model, e.g. the cluster weighted model, is given for the galaxies.

Extracting 21 cm signal by frequency and angular filtering

Extracting the neutral hydrogen （HI） signal is a great challenge for cosmological 21 cm experiments; both the astrophysical foregrounds and receiver noise are typically several orders of magnitude greater than the 21 cm signal. However, the different properties of the 21 cm signal, foreground and noise can be exploited to separate these components. The foregrounds are generally smooth or correlated over the frequency space along a line of sight （LOS）, while both the 21 cm signal and noise vary stochastically along the same LoS. The foreground can be removed by filtering out the smooth component in frequency space. The receiver noise is basically uncorrelated for observations at different times, hence for surveys it is also uncorrelated in different directions, while the 21 cm signal, which traces the large scale structure, is correlated up to certain scales. In this exercise, we apply Wiener filters in frequency and angular space to extract the 21 cm signals. We found that the method works well. Inaccurate knowl- edge about the beam could degrade the reconstruction, but the overall result is still good, showing that the method is fairly robust.

A Remark on Using Gravitational Lensing Probability as a Probe of the Central Regions of CDM Halos

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.

The S_(8)tension in light of updated redshift-space distortion data and PAge approximation

One of the most prominent challenges to the standard Lambda cold dark matter(ΛCDM) cosmology is the tension between the structure growth parameter S;constrained by the cosmic microwave background(CMB) data and the smaller one suggested by the cosmic shear data. Recent studies show that, for ΛCDM cosmology, redshift-space distortion(RSD) data also prefers a smaller S;that is ~ 2-3σ lower than the CMB value, but the result is sensitive to the cosmological model. In the present work we update the RSD constraint on S;with the most up-to-date RSD data set where the correlation between data points is properly taken into account. To reduce the model dependence, we add in our Monte Carlo Markov Chain calculation the most up-to-date data sets of Type Ia supernovae(SN) and baryon acoustic oscillations(BAO), whose correlation with RSD is also taken into account, to constrain the background geometry. For ΛCDM cosmology we find S_(8)= 0.812 ± 0.026, which is ~ 2σ larger than previous studies, and hence is consistent with the CMB constraint. By replacing ΛCDM with the parameterization based on cosmic Age(PAge), an almost model-independent description of the late universe, we find that the RSD + SN + BAO constraint on S;is insensitive to the cosmological model.