Tangential Dispersions Research Articles

The redshift-space cluster–galaxy cross-correlation function – I. Modelling galaxy infall on to Millennium simulation clusters and SDSS groups

The large scale infall of galaxies around massive clusters provides a potentially powerful diagnostic of structure growth, dark energy, and cosmological deviations from General Relativity. We develop and test a method to recover galaxy infall kinematics (GIK) from measurements of the redshift-space cluster-galaxy cross-correlation function \xi_{cg}(r_p,r_\pi). Using galaxy and halo samples from the Millennium simulation, we calibrate an analytic model of the galaxy kinematic profiles comprised of a virialized component with an isotropic Gaussian velocity distribution and an infall component described by a skewed 2D t-distribution with a characteristic infall velocity v_r and separate radial and tangential dispersions. We show that convolving the real-space cross-correlation function with this velocity distribution accurately predicts the redshift-space \xi_{cg}, and we show that measurements of \xi_{cg} can be inverted to recover the four distinct elements of the GIK profiles. These in turn provide diagnostics of cluster mass profiles, and we expect the characteristic infall velocity v_r(r) in particular to be insensitive to galaxy formation physics that can affect velocity dispersions within halos. As a proof of concept we measure \xi_{cg} for rich galaxy groups in the Sloan Digital Sky Survey and recover GIK profiles for groups in two bins of central galaxy stellar mass. The higher mass bin has a v_r(r) curve very similar to that of 10^{14} Msun halos in the Millennium simulation, and the recovered kinematics follow the expected trends with mass. GIK modeling of cluster-galaxy cross-correlations can be a valuable complement to stacked weak lensing analyses, allowing novel tests of modified gravity theories that seek to explain cosmic acceleration.

Ultrasonic velocity dispersion in bovine cortical bone

The evaluation of cortical bone quality has become possible in clinical practice, but the interaction between a broadband ultrasonic pulse and this complex multiscale medium remains poorly understood. Specifically, the frequency dependence of phase velocity has been sparsely investigated. This study aims at evaluating the determinants of the frequency dependence of phase velocity in bovine femoral cortical bone samples using an in vitro ultrasonic transmission device. Phase velocity is shown to vary quasi linearly in a 1 MHz restricted bandwidth around 4 MHz, which enables dispersion evaluation. Axial dispersion is significantly higher than radial and tangential dispersions. Significant differences in dispersion are obtained according to the anatomical location. The microstructure of each sample is determined using an optical microscope, which allows assessing the dependence of dispersion on the type of bone microstructure. Mostly positive, but also negative values of dispersion are measured. Negative dispersion is obtained mostly in samples constituted of mixed microstructure, which may be explained by phase cancellation effects due to the presence of different microstructures within the same sample. Dispersion is shown to be related to broadband ultrasonic attenuation values, especially in the radial direction. This dependence is compared with results derived from the local Kramers‐Kronig relationships.

A Critique of Scaling Behavior in Nonlinear Structure Formation Scenarios

Moments of the BBGKY equations for spatial correlation functions of cosmological density perturbations are used to obtain a differential equation for the evolution of the dimensionless function, h = -(v/x), where v is the mean relative pair velocity. The BBGKY equations are closed using a hierarchical scaling Ansatz for the three-point correlation function. Scale-invariant solutions derived earlier by Davis and Peebles are then used in the nonlinear regime, along with the generalized stable-clustering hypothesis (h → const), to obtain an expression for the asymptotic value of h, in terms of the power-law index of clustering, γ, and the tangential and radial velocity dispersions. The Davis-Peebles solution is found to require that tangential dispersions are larger than radial ones, in the nonlinear regime; this can be understood on physical grounds. Finally, stability analysis of the solution demonstrates that the allowed asymptotic values of h, consistent with the stable-clustering hypothesis, lie in the range 0 ≤ h ≤ 1/2. Thus, if the Davis-Peebles scale-invariant solution (and the hierarchical model for the three-point function) is correct, the standard stable-clustering picture (h → 1 as → ∞) is not allowed in the nonlinear regime of structure formation.

Dynamics of the elliptical galaxy NGC 7097 measured from gas and stars

view Abstract Citations (58) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamics of the Elliptical Galaxy NGC 7097 Measured from Gas and Stars Caldwell, N. ; Kirshner, R. P. ; Richstone, D. O. Abstract The E4 galaxy NGC 7097 is normal on broad-band photographs, and has the surface-brightness profile of an ordinary elliptical galaxy. Spectra taken at the CTIO 4 m telescope show that NGC 7097 has extended O II and O III forbidden line emission, which provides an unusual opportunity to use the motion of the gas to probe the radial mass distribution of an elliptical galaxy. These observations are consistent with a centrally concentrated gas disk of 4-kpc radius, inclined at 60 deg to the plane of the sky. If the gas velocities are circular, they can be used to determine M(r) for this galaxy. The inferred mass distribution of the galaxy has a larger scale length than the scale length of the stellar distribution, so that the mass-to-light ratio increases from below 1 at the center to 3.5 at 3 kpc. In the absence of a variation in the stellar composition, this is evidence for dark matter in an elliptical galaxy. Analysis of stellar rotation and velocity dispersion distributions through linear programming methods shows that, to match the observed mass and light distributions, the radial component of the stellar velocity dispersions must be larger than the tangential dispersions. Publication: The Astrophysical Journal Pub Date: June 1986 DOI: 10.1086/164234 Bibcode: 1986ApJ...305..136C Keywords: Elliptical Galaxies; Galactic Rotation; Gas Dynamics; Stellar Motions; Astronomical Spectroscopy; Dark Matter; Emission Spectra; Mass Distribution; Mass To Light Ratios; Oxygen Ions; Star Distribution; Velocity Distribution; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 7097; GALAXIES: INTERNAL MOTIONS full text sources ADS | data products SIMBAD (1) NED (1)