Zuhui Fan

Probing the intrinsic shape and alignment of dark matter haloes using SDSS galaxy groups

We study the three-dimensional and projected shapes of galaxy groups in the Sloan Digital Sky Survey Data Release 4, and examine the alignment between the orientation of the central galaxy and the spatial distribution of satellite galaxies. The projected ellipticity of a group is measured using the moments of the discrete distribution of its member galaxies. We infer the three-dimensional and projected axis ratios of their dark matter haloes by comparing the measured ellipticity distributions with those obtained from Monte Carlo simulations of projected, triaxial dark matter haloes with different axis ratios. We find that the halo shape has a strong dependence on the halo mass. While the haloes of low-mass groups are nearly spherical, those of massive groups tend to be prolate. For groups containing at least four members, the statistical distribution of their measured ellipticities does not have a strong dependence on the colours of their central galaxies. Our analysis further shows that the average three-dimensional axis ratio for haloes with 12 < log[M/(h −1 M� )] � 15 is about 1 : 0.46 : 0.46, resulting in a projected axis ratio of ∼0.77. Our results for the alignment between the orientation of the central galaxy of a group and the distribution of their satellite galaxies are in broad agreement with those obtained by Yang et al. The distribution of satellite galaxies preferentially aligns with the major axis of the central galaxy, with a clear dependence on both halo mass and galaxy colours. In particular, the alignment is stronger in more massive groups, and the strongest alignment is seen between red centrals and the distribution of red satellites. For groups with blue centrals, no significant alignment is detected. Finally, we examine how the observed alignment can be reproduced with the information about the halo axis ratios. The observed alignment signal can be reproduced if the angle between the major axis of the central galaxy and the projected major axis of the host halo has a Gaussian distribution with a mean of 0 ◦ and a dispersion of ∼23 ◦ .

Periodicities in Solar Coronal Mass Ejections

ABSTRACT Mid-term quasi-periodicities in solar coronal mass ejections (CMEs) during the mostrecent solar maximum cycle 23 are reported here for the first time using the four-year data (February 5, 1999 to February 10, 2003) of the Large Angle SpectrometricCoronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Inparallel, mid-term quasi-periodicities in solar X-ray flares (class >M5.0) from theGeosynchronous Operational Environment Satellites (GOES) and in daily averages ofAp index for geomagnetic disturbances from the World Data Center (WDC) at theInternationalAssociation for Geomagnetism andAeronomy (IAGA)are alsoexaminedfor the same four-year time span. By Fourier power spectral analyses, the CME dataappears to contain significant power peaks at periods of ∼ 358 ± 38, ∼ 272 ± 26,∼ 196±13 days and so forth, while except for the ∼ 259±24-day period, X-ray solarflares of class >∼ M5.0 show the familiar Rieger-type quasi-periods at ∼ 157 ± 11,∼ 122± 5, ∼ 98±3 days and shorter ones until ∼ 34±0.5 days. In the data of dailyaverages of Ap index, the two significant peaks at periods ∼ 273±26 and ∼ 187±12days (the latter is most prominent) could imply that CMEs (periods at ∼ 272±26and∼ 196 ± 13 days) may be proportionally correlated with quasi-periodic geomagneticstorm disturbances; at the speculative level, the ∼ 138 ± 6-day period might implythat X-ray flares of class >∼ M5.0 (period at ∼ 157±11 days) may drive certain typesof geomagnetic disturbances; and the ∼ 28±0.2-day periodicity is most likely causedby recurrent high-speed solar winds at the Earth’s magnetosphere. For the same threedata sets, we further perform Morlet wavelet analysis to derive period-time contoursand identify wavelet power peaks and timescales at the 99 percent confidence level forcomparisons. Several conceptual aspects of possible equatorially trapped Rossby-typewaves at and beneath the solar photosphere are discussed.Key words: oscillations — space weather — Sun: activities — corona — coronalmass ejections — magnetic fields

Overcoming the Circular Problem for Gamma-Ray Bursts in Cosmological Global-Fitting Analysis

Due to the lack of low-redshift long gamma-ray bursts (GRBs), the circular problem has been a severe obstacle for using GRBs as cosmological candles. In this paper, we present a new method to deal with such a problem in Markov chain Monte Carlo (MCMC) global fitting analysis. Assuming a certain type of correlation, for the parameters involved in the correlation relation, we treat them as free parameters and determine them simultaneously with cosmological parameters through MCMC analysis on GRB data together with other observational data. Then the circular problem is naturally eliminated in this procedure. To demonstrate the feasibility of our method, we take the Ghirlanda relation (E-gamma proportional to CEpeakA) as an example, while keeping in mind the debate about its physical validity. Together with SN Ia, WMAP, and SDSS data, we include 27 GRBs with the reported Ghirlanda relation in our study and perform MCMC global fitting. We consider the Lambda CDM model and dynamical dark energy models, respectively. We also include the curvature of the universe in our analysis. In each case, in addition to the constraints on the relevant cosmological parameters, we obtain the best-fit values as well as the distributions of the correlation parameters A and C. With CMB+LSS+SNe+GRB data included in the analysis, the results on A and C for different cosmological models are in agreement well within a 1 sigma range. It is also noted that the distributions of A and C are generally broader than the priors used in many studies in the literature. Our method can be readily applied to other GRB relations, which might be better physically motivated.

Noisy weak-lensing convergence peak statistics near clusters of galaxies and beyond

Taking into account noise from intrinsic ellipticities of source galaxies, in this paper, we study the peak statistics in weak-lensing convergence maps around clusters of galaxies and beyond. We emphasize how the noise peak statistics is affected by the density distribution of nearby clusters, and also how cluster-peak signals are changed by the existence of noise. These are the important aspects to be thoroughly understood in weak-lensing analyses for individual clusters as well as in cosmological applications of weak-lensing cluster statistics. We adopt Gaussian smoothing with the smoothing scale θ G = 0.5 arcmin in our analyses. It is found that the noise peak distribution near a cluster of galaxies sensitively depends on the density profile of the cluster. For a cored isothermal cluster with the core radius R c , the inner region with R ≤ R c . appears noisy containing on average ~2.4 peaks with ν ≥ 5 for R c = 1.7 arcmin and the true peak height of the cluster ν = 5.6, where ν denotes the convergence signal-to-noise ratio. For a Navarro-Frenk-White (NFW) cluster of the same mass and the same central ν, the average number of peaks with ν ≥ 5 within R ≤ R c is ~1.6. Thus a high peak corresponding to the main cluster can be identified more cleanly in the NFW case. In the outer region with R c < R ≤ 5 R c . the number of high noise peaks is considerably enhanced in comparison with that of the pure noise case without the nearby cluster. For ν ≥ 4, depending on the treatment of the mass-sheet degeneracy in weak-lensing analyses, the enhancement factor f is in the range of ~5 to ~55 for both clusters as their outer density profiles are similar. The properties of the main-cluster-peak identified in convergence maps are also significantly affected by the presence of noise. Scatters as well as a systematic shift for the peak height are present. The height distribution is peaked at ν ~ 6.6, rather than at ν = 5.6, corresponding to a shift of Δν ~ 1, for the isothermal cluster. For the NFW cluster, Δν ~ 0.8. The existence of noise also causes a location offset for the weak-lensing identified main-cluster-peak with respect to the true center of the cluster. The offset distribution is very broad and extends to R ~ R c for the isothermal case. For the NFW cluster, it is relatively narrow and peaked at R ~ 0.2 R c . We also analyze NFW clusters of different concentrations. It is found that the more centrally concentrated the mass distribution of a cluster is, the less its weak-lensing signal is affected by noise. Incorporating these important effects and the mass function of NFW dark matter halos, we further present a model calculating the statistical abundances of total convergence peaks, true and false ones, over a large field beyond individual clusters. The results are in good agreement with those from numerical simulations. The model then allows us to probe cosmologies with the convergence peaks directly without the need of expensive follow-up observations to differentiate true and false peaks.

Cosmological constraints from weak lensing peak statistics with Canada-France-Hawaii Telescope Stripe 82 Survey

We derived constraints on cosmological parameters using weak lensing peak statistics measured on the similar to aEuro parts per thousand 130 deg(2) of the Canada-France-Hawaii Telescope Stripe 82 Survey. This analysis demonstrates the feasibility of using peak statistics in cosmological studies. For our measurements, we considered peaks with signal-to-noise ratio in the range of nu = [3, 6]. For a flat I \textgreater cold dark matter model with only (Omega(m), sigma(8)) as free parameters, we constrained the parameters of the following relation I (8) pound = sigma(8)(Omega(m)/0.27)(alpha) to be I (8) pound = 0.82 +/- 0.03 and alpha = 0.43 +/- 0.02. The alpha value found is considerably smaller than the one measured in two-point and three-point cosmic shear correlation analyses, showing a significant complement of peak statistics to standard weak lensing cosmological studies. The derived constraints on (Omega(m), sigma(8)) are fully consistent with the ones from either WMAP9 or Planck. From the weak lensing peak abundances alone, we obtained marginalized mean values of tex-math id=”TM0002” notation=”LaTeX”\\\textbackslashOmega _\\\textbackslashrm m\=0.38\textless^\textgreater\+0.27\_\-0.24\\/tex-math and sigma(8) = 0.81 +/- 0.26. Finally, we also explored the potential of using weak lensing peak statistics to constrain the mass-concentration relation of dark matter haloes simultaneously with cosmological parameters.

Modelling galaxy-galaxy weak lensing with Sloan Digital Sky Survey groups

We use galaxy groups selected from the Sloan Digital Sky Survey (SDSS) together with mass models for individual groups to study the galaxy-galaxy lensing signals expected from galaxies of different luminosities and morphological types. We compare our model predictions with the observational results obtained from the SDSS by Mandelbaum et al. for the same samples of galaxies. The observational results are well reproduced in a Lambda cold dark matter (Lambda CDM) model based on the Wilkinson Microwave Anisotropy Probe (WMAP) 3-yr data, but a Lambda CDM model with higher sigma(8), such as the one based on the WMAP 1-yr data, significantly overpredicts the galaxy-galaxy lensing signal. We model, separately, the contributions to the galaxy-galaxy lensing signals from different galaxies: central versus satellite, early type versus late type and galaxies in haloes of different masses. We also examine how the predicted galaxy-galaxy lensing signal depends on the shape, density profile and the location of the central galaxy with respect to its host halo.

Constraining the substructure of dark matter haloes with galaxy-galaxy lensing

With galaxy groups constructed from the Sloan Digital Sky Survey (SDSS), we analyse the expected galaxy-galaxy lensing signals around satellite galaxies residing in different host haloes and located at different halo-centric distances. We use a Markov chain Monte Carlo method to explore the potential constraints on the mass and density profile of subhaloes associated with satellite galaxies from SDSS-like surveys and surveys similar to the Large Synoptic Survey Telescope (LSST). Our results show that for SDSS-like surveys, we can only set a loose constraint on the mean mass of subhaloes. With LSST-like surveys, however, both the mean mass and the density profile of subhaloes can be well constrained.

Circumnuclear Spiral Arms and Starburst Rings in Magnetized Barred Spiral Galaxies

The Seyfert galaxy NGC 1097 has a prominent bar and a luminous circumnuclear starburst ring. Magnetic fields as revealed by nonthermal radio continuum emissions correlate well with the optical barred spiral structure on large scales, have a gross enhancement overlapping with the optical/infrared ring, and show a trailing swirl around and within the ring. We propose a scenario of bar-excited long-trailing fast magnetohydrodynamic (MHD) density waves at the modified inner Lindblad resonance (mILR), physically identified with the outer rim of the ring. These sustained outgoing long waves are bounced back by the QM-barrier in the form of incoming short-trailing waves. The damping of these waves deposits a negative angular momentum into the magnetized circumnuclear gas disk. Thus, gas materials spiral inward, bring in frozen-in magnetic flux, and accumulate inside the mILR to create a circular zone of high density and magnetic flux vulnerable to massive star formation. Depending on the wave damping efficiency, this process may simultaneously sustain a net mass inflow across the ring and toward the nucleus. A wavelet analysis on a Hubble Space Telescope image of central NGC 1097 shows a distinct two-arm spiral structure extended down to the nucleus as a strong evidence for circumnuclear MHD density waves. We predict that magnetic field observations with improved sensitivity and resolution would eventually reveal a specific correspondence between circumnuclear optical and magnetic field spirals similar to those known to exist on large scales in nearby spiral galaxies, including NGC 1097.

ON USING THE WMAP DISTANCE INFORMATION IN CONSTRAINING THE TIME-EVOLVING EQUATION OF STATE OF DARK ENERGY

Recently, the WMAP group published their 5 year data and considered the constraints on the time-evolving equation of state of dark energy for the first time from the WMAP distance information. In this Letter, we study the effectiveness of the usage of this distance information and find that the compressed CMB information can give similar constraints on dark energy parameters compared with the full CMB power spectrum if dark energy perturbations are included; however, if the dark energy perturbations are incorrectly neglected, the difference of the results is sizable.

THE 21 cm FOREST AS A PROBE OF THE REIONIZATION AND THE TEMPERATURE OF THE INTERGALACTIC MEDIUM

Using high-redshift radio sources as background, the 21 cm forest observations probe the neutral hydrogen absorption signatures of early structures along the lines of sight. Directly sensitive to the spin temperature of the hydrogen atoms, it complements the 21 cm tomography observations, and provides information on the temperature as well as the ionization state of the intergalactic medium (IGM). We use a radiative transfer simulation to investigate the 21 cm forest signals during the epoch of reionization. We first confirmed that the optical depth and equivalent width (EW) are good representations of the ionization and thermal state of the IGM. The features selected by their relative optical depth are excellent tracers of ionization fields, and the features selected by their absolute optical depth are very sensitive to the IGM temperature, so the IGM temperature information could potentially be extracted from 21 cm forest observation, thus breaking a degeneracy in 21 cm tomographic observation. With the EW statistics, we predict some observational consequences for 21 cm forest. From the distributions of EWs and the number evolution of absorbers and leakers with different EWs, we see clearly the cosmological evolution of ionization state of the IGM. The number density of potentially observable features decreases rapidly with increasing gas temperature. The sensitivity of the proposed EW statistic to the IGM temperature makes it a unique and potentially powerful probe of reionization. Missing small-scale structures, such as small filaments and minihalos that are unresolved in our current simulation, and lack of an accurate calculation of the IGM temperature, however, likely have rendered the presented signals quantitatively inaccurate. Finally, we discuss the requirements of the background radio sources for such observations, and find that signals with equivalent widths larger than 1 kHz are hopeful to be detected.