Velocity Anisotropy Parameter Research Articles

The orbital eccentricity distribution of solar-neighbourhood halo stars

We present theoretical calculations for the differential distribution of stellar orbital eccentricity for a sample of solar-neighbour halo stars. Two types of static, spherical gravitational potentials are adopted to define the eccentricity e for given energy E and angular momentum L, such as an isochrone potential and a Navarro-Frenk-White potential that can serve as two extreme ends covering in-between any realistic potential of the Milky Way halo. The solar-neighbour eccentricity distribution \Delta N(e) is then formulated, based on a static distribution function of the form f(E,L) in which the velocity anisotropy parameter \beta monotonically increases in the radial direction away from the galaxy center, such that beta is below unity (near isotropic velocity dispersion) in the central region and asymptotically approaches \sim 1 (radially anisotropic velocity dispersion) in the far distant region of the halo. We find that \Delta N(e) sensitively depends upon the radial profile of \beta, and this sensitivity is used to constrain such profile in comparison with some observational properties of \Delta N_{obs}(e) recently reported by Carollo et al. (2010). Especially, the linear e-distribution and the fraction of higher-e stars for their sample of solar-neighbour inner-halo stars rule out a constant profile of \beta, contrary to the opposite claim by Bond et al. (2010). Our constraint of \beta \lesssim 0.5 at the galaxy center indicates that the violent relaxation that has acted on the inner halo is effective within a scale radius of \sim 10 kpc from the galaxy center. We discuss that our result would help understand the formation and evolution of the Milky Way halo.

QUANTIFYING THE COLLISIONLESS NATURE OF DARK MATTER AND GALAXIES IN A1689

We use extensive measurements of the cluster A1689 to assess the expected similarity in the dynamics of galaxies and dark matter (DM) in their motion as collisionless `particles' in the cluster gravitational potential. To do so we derive the radial profile of the specific kinetic energy of the cluster galaxies from the Jeans equation and observational data. Assuming that the specific kinetic energies of galaxies and DM are roughly equal, we obtain the mean value of the DM velocity anisotropy parameter, and the DM density profile. Since this deduced profile has a scale radius that is higher than inferred from lensing observations, we tested the validity of the assumption by repeating the analysis using results of simulations for the profile of the DM velocity anisotropy. Results of both analyses indicate a significant difference between the kinematics of galaxies and DM within $r \lesssim 0.3r_{\rm vir}$. This finding is reflected also in the shape of the galaxy number density profile, which flattens markedly with respect to the steadily rising DM profile at small radii. Thus, $r \sim 0.3r_{\rm vir}$ seems to be a transition region interior to which collisional effects significantly modify the dynamical properties of the galaxy population with respect to those of DM in A1689

The impact of environment on the dynamical structure of satellite systems

We examine the effects of environment on the dynamical structure of satellite systems based on the Millennium--II Simulation. Satellite halos are defined as sub--halos within the virial radius of a host halo. The satellite sample is restricted to those sub--halos which showed a maximum circular velocity above 30 km/s at the time of accretion. Host halo masses range from 10^11 to 10^14 Msol/h. We compute the satellites' average accretion redshift, z_acc, velocity dispersion, sigma, and velocity anisotropy parameter, beta, utilising stacked satellite samples of equal mass hosts at similar background densities. The main results are: (1) On average satellites within hosts in high density environments are accreted earlier (Delta z~ 0.1$) compared to their counterparts at low densities. For host masses above 5 times10^13 Msol/h this trend weakens and may reverse for higher host masses; (2) The velocity dispersion of satellites in low density environments follows that of the host, i.e. no velocity bias is observed for host halos at low densities independent of host mass. However, for low mass hosts in high density environments the velocity dispersion of the satellites can be up to ~30% larger than that of the host halo, i.e. the satellites are dynamically hotter than their host halos. (3) The anisotropy parameter depends on host mass and environment. Satellites of massive hosts show more radially biased velocity distributions. Moreover in low density environments satellites have more radially biased velocities (Delta beta > 0.1) compared to their counterparts in high density environments. We believe that our approach allows to predict a similar behaviour for observed satellite galaxy systems.

Entropy principle and complementary second law of thermodynamics for self-gravitating systems

(abbreviated) The statistical mechanics of self-gravitating systems is a long-held puzzle. In this work, we employ a phenomenological entropy form of ideal gas, first proposed by White & Narayan, to revisit this issue. By calculating the first-order variation of the entropy, subject to the mass- and energy-conservation constraints, we obtain an entropy stationary equation. Incorporated with the Jeans equation, and by specifying some functional form for the anisotropy parameter beta, we numerically solve the two equations, and demonstrate that the velocity anisotropy parameter plays an important role to attain a density profile that is finite in mass, energy, and spatial extent. If incorporated again with some empirical density profile from simulations, our theoretical predictions of the anisotropy parameter, and the radial pseudo-phase-space density in the outer non-gravitationally degenerate region of the dark matter halo, agree rather well with the simulation data, and the predictions are also acceptable in the middle weak-degenerate region of the dark halo. The second-order variational calculus reveals the seemingly paradoxical but actually complementary consequence that the equilibrium state of self-gravitating systems is the global minimum entropy state for the whole system under long-range violent relaxation, but simultaneously the local maximum entropy state for every and any small part of the system under short-range two-body relaxation and Landau damping. This minimum-maximum entropy duality means that the standard second law of thermodynamics needs to be re-expressed or generalized for self-gravitating systems. We believe that our findings, especially the complementary second law of thermodynamics, may provide crucial clues to the development of the statistical physics of self-gravitating systems as well as other long-range interaction systems.

A THEOREM ON CENTRAL VELOCITY DISPERSIONS

It is shown that, if the tracer population is supported by a spherical dark halo with a core or a cusp diverging more slowly than that of a singular isothermal sphere, the logarithmic cusp slope 'g' of the tracers must be given exactly by g=2b where b is their velocity anisotropy parameter at the center unless the same tracers are dynamically cold at the center. If the halo cusp diverges faster than that of the singular isothermal sphere, the velocity dispersion of the tracers must diverge at the center too. In particular, if the logarithmic halo cusp slope is larger than two, the diverging velocity dispersion also traces the behavior of the potential. The implication of our theorem on projected quantities is also discussed. We argue that our theorem should be understood as a warning against interpreting results based on simplifying assumptions such as isotropy and spherical symmetry.

Phase-space density profiles in scale-free cosmologies

We use a set of high-resolution simulations of scale-free Einstein-de Sitter cosmologies to investigate the logarithmic slope of the phase-space density profile Q(r) = ρ(r)/σ 3 (r) of dark matter (DM) haloes. The initial conditions for the simulations are determined by a power-law power spectrum of the form P(k) ∝ k n . We compute the Q(r) profiles using the radial, tangential and full velocity dispersion, and the velocity anisotropy parameter, β(r). We express Q(r) as a single power-law Q(r) ∝ r α and derive a median slope a in each simulation and for each definition of Q. Our main findings are: (i) the various Q(r) profiles follow a power law to a good approximation. (ii) The slopes depend on the concentration parameter c of the DM haloes, where for c ≥ 10 the slopes steepen with rising concentration and for c ≤ 10 the trend flattens and even turns around. (iii) The asymptotic value of β as r → R vir increases with the value of c. (iv) In accordance with Zait, Hoffman & Shlosman, α rad becomes more negative as the asymptotic value of β at the virial radius increases. (iv) This introduces a weak dependence of the Q(r) slopes on the slope of the power spectrum.

A laboratory study of seismic velocity and attenuation anisotropy in near‐surface sedimentary rocks

ABSTRACTThe laboratory ultrasonic pulse‐echo method was used to collect accurate P‐ and S‐wave velocity (±0.3%) and attenuation (±10%) data at differential pressures of 5–50 MPa on water‐saturated core samples of sandstone, limestone and siltstone that were cut parallel and perpendicular to the vertical borehole axis. The results, when expressed in terms of the P‐ and S‐wave velocity and attenuation anisotropy parameters for weakly transversely isotropic media (ɛ, γ, ɛQ, γQ) show complex variations with pressure and lithology. In general, attenuation anisotropy is stronger and more sensitive to pressure changes than velocity anisotropy, regardless of lithology. Anisotropy is greatest (over 20% for velocity, over 70% for attenuation) in rocks with visible clay/organic matter laminations in hand specimens. Pressure sensitivities are attributed to the opening of microcracks with decreasing pressure. Changes in magnitude of velocity and attenuation anisotropy with effective pressure show similar trends, although they can show different signs (positive or negative values of ɛ, ɛQ, γ, γQ). We conclude that attenuation anisotropy in particular could prove useful to seismic monitoring of reservoir pressure changes if frequency‐dependent effects can be quantified and modelled.

Mass modelling of dwarf spheroidal galaxies: the effect of unbound stars from tidal tails and the Milky Way

We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal (dSph) galaxies. For this purpose we have run a high-resolution N-body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disc and it has a NFW-like dark matter (DM) halo. After 10 Gyr of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions. We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated DM haloes. We model the cleaned-up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio (M/L) and velocity anisotropy parameter. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and M/L of the dwarf with accuracy typically better than 25 per cent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy. We show that after careful removal of interlopers the velocity dispersion profile of Fornax can be reproduced by a model in which mass traces light with a M/L of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of Fornax probably originates from the Milky Way.

Upper critical field of RbOs2O6 analyzed within Eliashberg theory

The upper critical field Hc2(T) of the β-pyrochlore superconductor RbOs2O6 is analyzed within the framework of s-wave Eliashberg theory including anisotropy and scattering effects, yielding a small electron–phonon coupling anisotropy parameter of about 〈a2〉=0.005, and a Fermi velocity anisotropy parameter 〈b2〉=0.3–0.35. Agreement between theory and experiment is achieved for these parameters and a model spectral function α2F(ω) consisting of two Einstein peaks and a Debye contribution. A renormalization of the Fermi velocity by an enhancement factor due to electron–electron correlations 〈vF〉ee=1 is considered in the analysis of the experimental Hc2(T)-data. Comparison between experiment and theory of thermodynamic properties such as the specific-heat difference between superconducting and normal states Cs–Cn, and the thermodynamic critical field Hc(T) support the results from the upper critical field analysis.

Plane-wave attenuation anisotropy in orthorhombic media

Orthorhombic models are often used in the interpretation of azimuthally varying seismic signatures recorded over fractured reservoirs. Here, we develop an analytic framework for describing the attenuation coefficients in orthorhombic media with orthorhombic attenuation (i.e., the symmetry of both the real and imaginary parts of the stiffness tensor is identical) under the assumption of homogeneous wave propagation. The analogous form of the Christoffel equation in the symmetry planes of orthorhombic and VTI (transversely isotropic with a vertical symmetry axis) media helps to obtain the symmetry-plane attenuation coefficients by adapting the existing VTI equations. To take full advantage of this equivalence with transverse isotropy, we introduce a parameter set similar to the VTI attenuation-anisotropy parameters [Formula: see text], [Formula: see text], and [Formula: see text]. This notation, based on the same principle as Tsvankin’s velocity-anisotropy parameters for orthorhombic media, leads to concise linearized equations for thesymmetry-plane attenuation coefficients of all three modes (P, [Formula: see text], and [Formula: see text]).The attenuation-anisotropy parameters also allow us to simplify the P-wave attenuation coefficient [Formula: see text] outside the symmetry planes under the assumptions of small attenuation and weak velocity and attenuation anisotropy. The approximate coefficient [Formula: see text] has the same form as the linearized P-wave phase-velocity function, with the velocity parameters [Formula: see text] and [Formula: see text] replaced by the attenuation parameters [Formula: see text] and [Formula: see text]. The exact attenuation coefficient, however, also depends on the velocity-anisotropy parameters, while the body-wave velocities are almost unperturbed by the presence of attenuation. The reduction in the number of parameters responsible for the P-wave attenuation and the simple approximation for the coefficient [Formula: see text] provide a basis for inverting P-wave attenuation measurements from orthorhombic media. The attenuation processing must be preceded by anisotropic velocity analysis that can be performed (in the absence of pronounced velocity dispersion) using existing algorithms for nonattenuative media.

Galaxy orbits and the intracluster gas temperature in clusters

In this paper, we examine how well galaxies and intracluster gas trace the gravitational potential of clusters. Utilizing mass profiles derived from gravitational lensing and X-ray observations, coupled with measured galaxy velocities, we solve for the velocity anisotropy parameter β_(orb)(r) using the anisotropic Jeans equation. This is done for five clusters, three at low redshift: A2199, A496 and A576 and two at high redshifts: A2390 and MS1358. We use X-ray temperature profiles obtained from Chandra and ASCA/ROSAT data to estimate β_X(r), the ratio of energy in the galaxies compared to the X-ray gas. We find that none of these clusters is strictly in hydrostatic equilibrium. We compare the properties of our sample with clusters that form in high-resolution cosmological N-body simulations that include baryonic physics. Simulations and data show considerable scatter in their β_(orb)(r) and β_X(r) profiles. We demonstrate the future feasibility and potential for directly comparing the orbital structure of clusters inferred from multiwavelength observations with high-resolution simulated clusters.

Plane-wave propagation in attenuative transversely isotropic media

Directionally dependent attenuation in transversely isotropic (TI) media can influence significantly the body-wave amplitudes and distort the results of the AVO (amplitude variation with offset) analysis. Here, we develop a consistent analytic treatment of plane-wave properties for TI media with attenuation anisotropy. We use the concept of homogeneous wave propagation, assuming that in weakly attenuative media the real and imaginary parts of the wave vector are parallel to one another. The anisotropic quality factor can be described by matrix elements [Formula: see text], defined as the ratios of the real and imaginary parts of the corresponding stiffness coefficients. To characterize TI attenuation, we follow the idea of the Thomsen notation for velocity anisotropy and replace the components [Formula: see text] by two reference isotropic quantities and three dimensionless anisotropy parameters [Formula: see text], and [Formula: see text]. The parameters [Formula: see text] and [Formula: see text] quantify the difference between the horizontal- and vertical-attenuation coefficients of P- and SH-waves, respectively, while [Formula: see text] is defined through the second derivative of the P-wave attenuation coefficient in the symmetry direction. Although the definitions of [Formula: see text], and [Formula: see text] are similar to those for the corresponding Thomsen parameters, the expression for [Formula: see text] reflects the coupling between the attenuation and velocity anisotropy. Assuming weak attenuation as well as weak velocity and attenuation anisotropy allows us to obtain simple attenuation coefficients linearized in the Thomsen-style parameters. The normalized attenuation coefficients for P- and SV-waves have the same form as the corresponding approximate phase-velocity functions, but both [Formula: see text] and the effective SV-wave attenuation-anisotropy parameter [Formula: see text] depend on the velocity-anisotropy parameters in addition to the elements [Formula: see text]. The linearized approximations not only provide valuable analytic insight, but they also remain accurate for the practically important range of small and moderate anisotropy parameters — in particular, for near-vertical and near-horizontal propagation directions.

Dark matter distribution in the Draco dwarf from velocity moments

We study the distribution of dark matter in the Draco dwarf spheroidal galaxy by modelling the moments of the line-of-sight velocity distribution of stars obtained from new velocity data of Wilkinson et al. The luminosity distribution is approximated by a Sersic profile fitted to the data by Odenkirchen et al. We assume that the dark matter density profile is given by a formula with an inner cusp and an outer exponential cut-off, as recently proposed by Kazantzidis et al. as a result of simulations of tidal stripping of dwarfs by the potential of the Milky Way. The dark matter distribution is characterized by the total dark mass and the cut-off radius. The models have arbitrary velocity anisotropy parameter, assumed to be constant with radius. We estimate the three parameters by fitting both the line-of-sight velocity dispersion and kurtosis profiles, which allows us to break the degeneracy between the mass distribution and velocity anisotropy. The results of the fitting procedure turn out to be very different depending on the stellar sample considered, that is, on our choice of stars with discrepant velocities to be discarded as interlopers. For our most reliable sample, the model parameters remain weakly constrained, but the robust result is the preference for weakly tangential stellar orbits and high mass-to-light ratios. The best-fitting total mass is then 7 x 10 7 M ○. , much lower than recent estimates, while the mass-to-light ratio is M/L V = 300 M ○. /L ○. and almost constant with radius. If the binary fraction in the stellar population of Draco turns out to be significant, the kurtosis of the global velocity distribution will be smaller and the orbits inferred will be more tangential, while the resulting mass estimate will be lower.

Radial velocity moments of dark matter haloes

Abstract Using cosmological N-body simulations, we study the radial velocity distribution in dark matter haloes, focusing on the lowest-order even moments, dispersion and kurtosis. We determine the properties of 10 massive haloes in the simulation box, approximating their density distribution by the Navarro, Frenk & White (NFW) formula characterized by the virial mass and concentration. We also calculate the velocity anisotropy parameter of the haloes, and find it mildly radial and increasing with distance from the halo centre. The radial velocity dispersion of the haloes shows a characteristic profile with a maximum, while the radial kurtosis profile decreases with distance starting from a value close to Gaussian near the centre. We therefore confirm that dark matter haloes possess intrinsically non-Gaussian, flat-topped velocity distributions. We find that the radial velocity moments of the simulated haloes are quite well reproduced by the solutions of the Jeans equations obtained for the halo parameters with the anisotropy measured in the simulations. We also study the radial velocity moments for a composite cluster made of 10 haloes out to 10 virial radii. In this region the velocity dispersion decreases systematically to reach the value of the background, while kurtosis increases from below to above the Gaussian value of 3, signifying a transition from a flat-topped to a strongly peaked velocity distribution with respect to the Gaussian, which can be interpreted as the dominance of ordered flow with a small dispersion. We illustrate the transition by showing explicitly the velocity distribution of the composite cluster in a few radial bins.

Density profiles of dark matter halos with anisotropic velocity tensors

We present density profiles, that are solutions of the spherical Jeans equation, derived under the following two assumptions: (i) the coarse grained phase-density follows a power-law of radius, ρ/σ 3 ∝ r −α , and (ii) the velocity anisotropy parameter is given by the relation β a ( r)= β 1+2 β 2 ( r/ r *)/[1+( r/ r *) 2] where β 1, β 2 are parameters and r * equals twice the virial radius, r vir, of the system. These assumptions are well motivated by the results of N-body simulations. Density profiles have increasing logarithmic slopes γ, defined by γ=− d ln ρ d ln r . The values of γ at r=10 −2.5 r vir, a distance where the systems could be resolved by large N-body simulations, lie in the range 1.0–1.6. These inner values of γ increase for increasing β 1 and for increasing concentration of the system. On the other hand, slopes at r= r vir lie in the range 2.42–3.82. A model density profile that fits well the results at radial distances between 10 −3 r vir and r vir and connects kinematic and structural characteristics of spherical systems is described.

Superconducting properties ofYxLu1−xNi2B2CandLa3Ni2B2N3−δ:A comparison between experiment and Eliashberg theory

Specific heat and resistivity measurements were performed on polycrystalline samples of the solid-solution ${\mathrm{Y}}_{x}{\mathrm{Lu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ in order to determine thermodynamic properties such as the specific-heat difference $\ensuremath{\Delta}C,$ the thermodynamic critical field ${H}_{c}(T),$ as well as the upper critical field ${H}_{c2}(T).$ These properties were analyzed within the Eliashberg theory including anisotropy effects, yielding electron-phonon coupling anisotropy parameters $〈{a}_{\mathbf{k}}^{2}〉$ ranging between $0.02$ and $0.03$ for the whole series, and Fermi velocity anisotropy parameters of $〈{b}_{\mathbf{k}}^{2}〉=0.245--0.3.$ Excellent agreement between theory and experiment was achieved for these parameters, the Sommerfeld constant $\ensuremath{\gamma}$ and model phonon spectra determined from specific heat measurements. An analysis of the previously investigated boronitride ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}{\mathrm{N}}_{3\ensuremath{-}\ensuremath{\delta}}$ for comparison revealed the electron-phonon anisotropy to be of great significance in describing its thermodynamic properties and the calculations yielded $〈{a}_{\mathbf{k}}^{2}〉\ensuremath{\simeq}0.08$ and $〈{b}_{\mathbf{k}}^{2}〉\ensuremath{\simeq}0.245.$ The ${T}_{c}$ behavior within the series ${\mathrm{Y}}_{x}{\mathrm{Lu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ is discussed in terms of the density of states at the Fermi level $N(0).$

The Dynamics of Poor Systems of Galaxies

We assemble and observe a sample of poor galaxy systems that is suitable for testing N-body simulations of hierarchical clustering and other dynamical halo models. We (1) determine the parameters of the density profile ?(r) and the velocity dispersion profile ?p(R), (2) separate emission-line galaxies from absorption-line galaxies, examining the model parameters and as a function of spectroscopic type, and (3) for the best-behaved subsample, constrain the velocity anisotropy parameter, ?, which determines the shapes of the galaxy orbits. Our sample consists of 20 systems, 12 of which have extended X-ray emission in the ROSAT All-Sky Survey. We measure the 877 optical spectra of galaxies brighter than mR?15.4 within 1.5 h-1 Mpc of the system centers (we take H0=100 h km s-1 Mpc-1). Thus, we sample the system membership to a radius typically three times larger than other recent optical group surveys. The average system population is 30 galaxies, and the average line-of-sight velocity dispersion is ?300 km s-1. The Navarro, Frenk, & White universal profile and the Hernquist model both provide good descriptions of the spatial data. In most cases an isothermal sphere is ruled out. Systems with declining ?p(R) are well-matched by theoretical profiles in which the star-forming galaxies have predominantly radial orbits (?>0); many of these galaxies are probably falling in for the first time. There is significant evidence for spatial segregation of the spectroscopic classes regardless of ?p(R).

The Average Mass and Light Profiles of Galaxy Clusters

The systematic errors in the virial mass-to-light ratio, Mv/L, of galaxy clusters as an estimator of the field M/L value are assessed. We overlay 14 clusters in redshift space to create an ensemble cluster that averages over substructure and asymmetries. The combined sample, including background, contains about 1150 galaxies, extending to a projected radius of about twice r200. The radius r200, defined as where the mean interior density is 200 times the critical density, is expected to contain the bulk of the virialized cluster mass. The dynamically derived M(r200)/L(r200) of the ensemble is (0.82 ? 0.14)Mv/L. The Mv/L overestimate is attributed to not taking into account the surface pressure term in the virial equation. Under the assumption that the velocity anisotropy parameter is in the range 0 ? ? ? 2/3, the galaxy distribution accurately traces the mass profile beyond about the central 0.3r200. There are no color or luminosity gradients in the galaxy population beyond 2r200, but there is 0.11 ? 0.05 mag fading in the r-band luminosities between the field and cluster galaxies. We correct the cluster virial mass-to-light ratio, Mv/L = 289 ? 50 h M?/L? (calculated assuming q0 = 0.1), for the biases in Mv and mean luminosity to estimate the field M/L = 213 ? 59 h M?/L?. With our self-consistently derived field luminosity density, j/?c = 1136 ? 138 h M?/L? (at z 1/3), the corrected M/L indicates ?0 = 0.19 ? 0.06 ? 0.04 (formal 1 ? random error and estimated potential systematic errors) for those components of the mass field in rich clusters.

Photofragment translational spectroscopy of IBr at 304 nm: Polarization dependence and dissociation dynamics

The photodissociation dynamics of IBr has been studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin–orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of IBr at this wavelength. Two sharp velocity distributions observed for the I channel suggest the two dissociation pathways that correlate with ground-state iodine formation. Based on the expected translational energy release and the energy separation between the peaks, the two distributions have been assigned to dissociation of IBr to form I(2P3/2)+Br(2P3/2) and I(2P3/2)+Br*(2P1/2) with the former channel appearing at higher translational energy. The I* distribution shows one strong peak indicating that there is one dominant channel for formation of I* atoms at this wavelength which has been assigned to dissociation of IBr to form I*(2P1/2)+Br(2P3/2) with a quantum yield of 0.1. The I* signal formed from the I*(2P1/2)+Br*(2P1/2) dissociation channel is observed very weakly. The observed anisotropy parameter indicates that the I+Br* product (β=−0.7) is formed mainly from the perpendicular 1Π1(2341)←X transition while the I*+Br channel (β=1.8) is formed predominantly from the parallel 3Π0+(2341)←X transition followed by curve crossing to the 3Σ0+−(2422) state. The recoil energy dependence of the anisotropy parameter in the I atom produced in the I+Br channel shows a positive β value above maximum of the peak recoil energy and a negative value below the peak maximum of the recoil energy distribution. These results are interpreted in terms of the presence of more than one path for the formation of I+Br photoproduct with opposite polarization for their absorbing transitions, most likely the 3Π0+(2341)←X and the 3Π1(2341)←X transitions. The possible excited state dynamics which give the observed results are discussed in terms of the previously proposed potential energy diagrams for IBr and ICl.