Jounghun Lee

Cosmic Shear from Galaxy Spins.

We discuss the origin of galactic angular momentum and the statistics of the present-day spin distribution. It is expected that the galaxy spin axes are correlated with the intermediate principal axis of the gravitational shear tensor. This allows one to reconstruct the shear field and thereby the full gravitational potential from the observed galaxy spin fields. We use the direction of the angular momentum vector without any information of its magnitude, which requires a measurement of the position angle and inclination on the sky of each disk galaxy. We present the maximum likelihood shear inversion procedure, which involves a constrained linear minimization. The theory is tested against numerical simulations. We find the correlation strength of nonlinear structures with the initial shear field and show that accurate large-scale density reconstructions are possible at the expected noise level.

ARC STATISTICS IN TRIAXIAL DARK MATTER HALOS: TESTING THE COLLISIONLESS COLD DARK MATTER PARADIGM

Statistics of lensed arcs in clusters of galaxies serve as a powerful probe of both the nonsphericity and the inner slope of dark matter halos. We develop a semianalytic method to compute the number of arcs in triaxial dark matter halos. This combines the lensing cross section from the Monte Carlo ray-tracing simulations and the probability distribution function of the axis ratios evaluated from cosmological N-body simulations. This approach enables one to incorporate both asymmetries in the projected mass density and elongations along the line of sight analytically for the first time in cosmological lensed arc statistics. As expected, triaxial dark matter halos significantly increase the number of arcs relative to spherical models; the difference amounts to more than 1 order of magnitude, while the value of enhancement depends on the specific properties of density profiles. Then we compare our theoretical predictions with the observed number of arcs from 38 X-ray-selected clusters. In contrast to previous claims, our triaxial dark matter halos with inner density profile ρ ∝ r-1.5 in a Λ-dominated cold dark matter (CDM) universe reproduce observations well. Since both the central mass concentration and triaxial axis ratios (minor-to-major axis ratio ~ 0.5) required to account for the observed data are consistent with cosmological N-body simulations, our result may be interpreted to lend strong support to the collisionless CDM paradigm at the mass scale of clusters.

BULLET CLUSTER: A CHALLENGE TO ΛCDM COSMOLOGY

To quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard Λ cold dark matter (CDM) cosmology, we use a large-volume (27 h –3 Gpc3) cosmological N-body MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall velocity distribution is given at (1-3)R 200 of the main cluster (where R 200 is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out by Mastropietro & Burkert has shown that an initial velocity of 3000 km s–1 at about 2R 200 is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, X-ray morphology of the main cluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R 200 is incompatible with the prediction of a ΛCDM model: the probability of finding 3000 km s–1 in (2-3)R 200 is between 3.3 × 10–11 and 3.6 × 10–9. A lower velocity, 2000 km s–1 at 2R 200, is also rare, and moreover, Mastropietro & Burkert have shown that such a low initial velocity does not reproduce the X-ray brightness ratio of the main and subcluster or morphology of the main cluster. Therefore, we conclude that the existence of 1E0657-56 is incompatible with the prediction of a ΛCDM model, unless a lower infall velocity solution for 1E0657-56 with 1800 km s–1 at 2R 200 is found.

Galaxy Spin Statistics and Spin-Density Correlation

We present a theoretical study of galaxy spin correlation statistics, with detailed technical derivations. We also find an expression for the spin-density cross correlation and apply that to the Tully galaxy catalog. The observational results appear qualitatively consistent with the theoretical predictions, yet the error bars are still large. However, we expect that currently ongoing large surveys such as the Sloan Digital Sky Survey (SDSS) will enable us to make a precision measurement of these correlation statistics in the near future. These intrinsic galaxy alignments are expected to dominate over the weak lensing signal in SDSS, and we present the detailed algorithms for the density reconstruction for this case. These observables are tracers of the galaxy-gravity interaction, which may provide deeper insights into the galaxy formation and large-scale matter distribution as well.

Tentative Detection of Galaxy Spin Correlations in the Tully Catalog

We report a tentative detection of spin correlations in the Tully catalogue of nearby galaxies. We define a simple but nontrivial spin correlation function, and find an analytic estimate of it in the frame of the linear perturbation theory. Then, we present the observed spin correlation signal from the Tully galaxies with error bars. The three dimensional spin correlation turns out to be significant at the 97% confidence level, detected out to a few

The Alignments of the Galaxy Spins with the Real-Space Tidal Field Reconstructed from the 2MASS Redshift Survey

h^{-1}

Modeling Intracluster Gas in Triaxial Dark Halos: An Analytic Approach

Mpc. This observed correlation is consistent with the theoretical prediction based on the gravitational instability picture of galaxy formation. An analysis of systematic errors is also presented. The observed strength of correlation may be sufficient to significantly affect blank field of weak lensing searches.

A realistic model for spatial and mass distributions of dark halo substructures: An analytic approach

We report direct observational evidence for the existence of the galaxy spin alignments with the real-space tidal field. We calculate the real-space tidal field from the real-space density field reconstructed recently from the 2MASS Redshift Survey (2MRS) by Erdogdu et al. in 2006. Using a total of 12,122 nearby spiral galaxies from the Tully Galaxy Catalog, we calculate the orientations of their spin axes relative to the 2MRS tidal field. We find a clear signal of the intrinsic correlations between the galaxy spins and the intermediate principal axes of the tidal shears. The null hypothesis of no correlation is rejected at 99.99% confidence level. We also investigate the dependence of the intrinsic correlations on the galaxy morphological type and the environment. It is found that (1) the intrinsic correlation depends weakly on the morphological type of the spiral galaxies, but tends to decrease slightly as the type increases; and (2) it is stronger in the high-density regions than in the low-density regions. The observational result is quantitatively consistent with analytic prediction based on the tidal torque theory. It is concluded that the galaxy spin orientations may provide in principle a new complimentary probe of the dark matter distribution.

Detection of Galaxy Spin Alignments in the Point Source Catalog Redshift Survey Shear Field

We present the first physical model for the non-spherical intra-cluster gas distribution in hydrostatic equilibrium under the gravity of triaxial dark matter halos. Adopting the concentric triaxial density profiles of the dark halos with constant axis ratios proposed by Jing & Suto (2002), we derive an analytical expression for the triaxial halo potential on the basis of the perturbation theory, and find the hydrostatic solutions for the gas density and temperature profiles both in isothermal and polytropic equations of state. The resulting iso-potential surfaces are well approximated by triaxial ellipsoids with the eccentricities dependent on the radial distance. We also find a formula for the eccentricity ratio between the intra-cluster gas and the underlying dark halo. Our results allow one to determine the shapes of the underlying dark halos from the observed intra-cluster gas through the X-ray and/or the Sunyaev-Zel’dovich effects clusters. Subject headings: cosmology:theory – dark matter – galaxies: clusters: general – X-ray: galaxiesWe present the first physical model for the nonspherical intracluster gas distribution in hydrostatic equilibrium under the gravity of triaxial dark matter halos. Adopting the concentric triaxial density profiles of dark halos with constant axis ratios proposed by Jing and Suto in 2002, we derive an analytical expression for the triaxial halo potential on the basis of perturbation theory and find the hydrostatic solutions for the gas density and temperature profiles in both isothermal and polytropic equations of state. The resulting isopotential surfaces are well approximated by triaxial ellipsoids with the eccentricities dependent on the radial distance. We also find a formula for the eccentricity ratio between the intracluster gas and the underlying dark halo. Our results allow one to determine the shapes of the underlying dark halos from the observed intracluster gas through X-ray and/or Sunyaev-Zeldovich effect clusters.

Void ellipticity distribution as a probe of cosmology.

We construct a realistic model for dark halo substructures, and derive analytically their spatial, mass and velocity distributions. Basically, our model is a modification of the Press-Schechter theory to account for the dominant dynamical processes that mark the evolution of dark halo substructures such as tidal stripping and dynamical friction. Our analytic model successfully reproduces all the well-known behaviour of the substructure distributions that have been found in recent numerical simulations: the weak dependence of the mass distributions on the host halo mass; the antibias of the spatial distribution relative to the dark matter particle components; and the nearly power-law shapes of the mass and velocity distributions. We compare our analytic results with recent high-resolution N-body simulation data and find that they are in excellent agreement with each other.