Range Of Impact Parameters Research Articles

THE COS-HALOS SURVEY: RATIONALE, DESIGN, AND A CENSUS OF CIRCUMGALACTIC NEUTRAL HYDROGEN

We present the design and methods of the COS-Halos survey, a systematic investigation of the gaseous halos of 44 z = 0.15-0.35 galaxies using background QSOs observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. This survey has yielded 39 spectra of z_em ~ 0.5 QSOs with S/N ~ 10-15 per resolution element. The QSO sightlines pass within 150 physical kpc of the galaxies, which span early and late types over stellar mass log M* / Msun= 9.5 - 11.5. We find that the CGM exhibits strong HI, averaging 1 Ang in Lya equivalent width out to 150 kpc, with 100% covering fraction for star-forming galaxies and 75% covering for passive galaxies. We find good agreement in column densities between this survey and previous studies over similar range of impact parameter. There is weak evidence for a difference between early- and late-type galaxies in the strength and distribution of HI. Kinematics indicate that the detected material is bound to the host galaxy, such that >~90% of the detected column density is confined within +/-200 km s^-1 of the galaxies. This material generally exists well below the halo virial temperatures at T<~ 10^5 K. We evaluate a number of possible origin scenarios for the detected material, and in the end favor a simple model in which the bulk of the detected HI arises in a bound, cool, low-density photoionized diffuse medium that is generic to all L* galaxies and may harbor a total gaseous mass comparable to galactic stellar masses.

Measuring gravitational redshifts in galaxy clusters

Wojtak et al. have stacked 7800 clusters from the Sloan Digital Sky Survey (SDSS) in redshift space. They find a small net blueshift for the cluster galaxies relative to the brightest cluster galaxies, which agrees quite well with the gravitational redshift predicted from general relativity. Zhao et al. have pointed out that, in addition to the gravitational redshift, one would expect to see transverse Doppler (TD) redshifts, so 〈δz〉 = −〈Φ〉 + 〈β2〉/2 with β the 3D source velocity in units of c, and that these two effects are generally of the same order. Here, we show that there are other corrections that are also of the same order of magnitude. The fact that we observe galaxies on our past light cone results in a bias such that more of the galaxies observed are moving away from us in the frame of the cluster than are moving towards us. This causes the observed average redshift to be |$\langle \delta z \rangle = -\langle \Phi \rangle + \langle \beta ^2 \rangle / 2 + \langle \beta _x^2 \rangle$|⁠, with βx the line-of-sight velocity. That is if we average over galaxies with equal weight. If the galaxies in each cluster are weighted by their fluence, or equivalently if we do not resolve the moving sources, and make an average of the mean redshift giving equal weight per photon, the observed redshift is 〈δz〉 = −〈Φ〉 − 〈β2〉/2, so the kinematical effect is then opposite to the usual transverse Doppler effect. In the Wojtak, Hansen & Hjorth experiment, the weighting is a step-function because of the flux limit for inclusion in the spectroscopic sample and the result is different again, and depends on the details of the luminosity function and the spectral energy distributions of the galaxies. Including these effects substantially modifies the blueshift profile. We show that in-fall and out-flow have very small effect over the relevant range of impact parameters but out-flow becomes significant and needs to be taken into account for measurements on larger scales.

The Story of Planets: Anchoring Numerics in Reality

AbstractBuilding a complete coherent model of planet formation has proven difficult. There are gaps in the observational record, difficult physical processes that we have yet to fully understand, such as planetesimal formation, and an extensive list of observationally determined constraints that the model must fulfil. For example, the diversity of extrasolar planets detected to date is staggering – from single hot-Jupiters to multiple planet systems with several tightly packed super-Earths. In addition, the characteristics of the host stars are broad from single solar-mass stars to tight binaries and low mass, low metalicity stars. Even more surprising, perhaps, is the frequency of detection and thus, the implied efficiency of the planet formation process. Any theoretical model must not just be able to explain how planets form but must also explain the frequency and diversity of planetary systems. So why is planet formation so prolific? What parameters determine the type of planetary system that will result? How important are the initial parameters of the protoplanetary disk, such as composition, versus stochastic effects, such as gravitational scattering events, that occur during the evolution of the planetary system?Current observations of extrasolar planets provide snapshots in time of the earliest and latest stages of planet formation but do not show the evolution between the two. It is at this point that we must rely on numerical models to evolve proto-planetary disks into planets. But how can we validate the results of our numerical simulations if the middle stages of planet formation are effectively invisible? Collisions are a core component of planet formation. Planetesimals, the building blocks of planets, collide with one another as they grow and evolve into planets or planetary cores and are viscously stirred by larger protoplanets and fully-formed planets. The range of impact parameters encountered during growth from planetesimals to planets span multiple collision outcome regimes: cratering, merging, disruption, and hit-and-run events. Most of these collisions produce significant debris and dust. If we have a good understanding of the production of collisional debris we can use it as an indirect tracer of on-going planetary evolution even if the planets themselves are not directly detectable.In this paper I will show how numerical simulations of planet formation including realistic collision modelling can be used to predict, and be constrained by, observations.

Fully differential cross section for O8+-impact ionization of Li

We present various differential cross sections for the single ionization of Li by O${}^{8+}$ ions. We use a time-dependent, close-coupling approach to model the evolution of a one-active-electron wave function in the field of the incoming projectile for a range of impact parameters. In addition, a Fourier transform approach is used to extract differential cross sections for a specific projectile momentum transfer value. This scheme allows us to incorporate information about the interaction of the two heavy nuclei [the so-called nuclear-nuclear (NN) interaction] and to assess its influence in the differential cross sections. We find noticeable differences in the shape of the differential cross sections when we include (neglect) the NN interaction. Our single-differential cross-section calculation shows excellent agreement with experimental data. In addition, recent measured double-differential cross sections as a function of electron energy and transverse momentum transfer are reasonably well reproduced by our theoretical calculations.

Strangeness production in heavy ion collisions -Constraining the KN – potential in medium

We review the strangeness production in heavy ion collisions at energies around the NN production threshold and discuss recent measurements of the FOPI collaboration on charged kaon flow over a wide impact parameter range. The data are compared to comprehensive state-of-the-art transport models. The dense nuclear matter environment produced in those collisions may provide unique opportunities to form strange few body systems. The FOPI detector is especially suited to reconstruct such states by their charged particle decays. Apart from strongly decaying states special emphasis will be put on the search for long living weakly decaying states, i.e. Hyper-Nuclei. Light hyper nuclei are reconstructed by their two body decay channels and the production of Hyper-Tritons is studied with respect to Λ and t(3He).

Sputtering of Al nanoclusters by 1–13 keV monatomic or polyatomic ions studied by Molecular Dynamics simulations

Molecular Dynamics (MD) simulations were employed to study sputtering of freestanding and supported spherical Al nanoclusters with 2–10nm diameters under bombardment by Al1 and Al13 projectiles with energies of 1–13keV at normal and oblique incidence. Both monatomic and clustered yields of secondary emission are found to be larger than those for (111) flat surface of the bulk Al (at equal irradiation conditions). In some events, target nanocluster receives a backward momentum and therefore its major part (more than 1/2 of the mass) is ejected due to produced secondary emission mainly towards the substrate direction. This “recoil effect” is found more pronounced under the impact of cluster projectiles and its probability decreases with increase of the target cluster size. A restricted number of MD simulations were performed to verify whether this “recoil effect” is strong enough to desorb a 4nm Al nanocluster off an Al (111) substrate. Desorption was observed under oblique Al13 impact within the impact parameter range of 0.6–0.9.

Stabilization of diatomic products in recombination of heavy ions

We have studied the kinematic conditions for the formation of the most stable products of direct three-body recombination of ions Cs+ and Br− in the presence of a third-body Xe atom. As the energy of the ions’ central encounters and the energy of the third body range from 1 to 10 eV, the minimal residual energy of the recombination products is found to lie in the interval 0–0.7 eV. The formation of the recombination products with the minimal internal energies is yielded by collisions of the three particles in a triangular configuration in an impact parameter range from 0 to 2.7 a.u. The mutual orientation angles of the vector of the relative ion velocity and the vector of the third body velocity affect the formation of products with the minimal internal energy rather slightly. In most cases, stabilization of the CsBr molecules and high efficiency of the energy transfer to the third body are observed in configurations of the closest approach of the particles with interionic distances close to the equilibrium distance in the CsBr molecule.

WARM GAS IN THE VIRGO CLUSTER. I. DISTRIBUTION OF Lyα ABSORBERS

The first systematic study of the warm gas (T=10^4-5 K) distribution across a galaxy cluster is presented using multiple background QSOs to the Virgo Cluster. We detect 25 Lya absorbers (N_HI = 10^13.1-15.4 cm^-2) in the Virgo velocity range toward 9 of 12 QSO sightlines observed with Cosmic Origin Spectrograph, with a cluster impact parameter range of 0.36-1.65 Mpc (0.23-1.05R_vir). Including 18 Lya absorbers previously detected by STIS or GHRS toward 7 of 11 background QSOs in and around the Virgo Cluster, we establish a sample of 43 absorbers towards a total of 23 background probes for studying the incidence of Lya absorbers in and around the Virgo Cluster. With these absorbers, we find: (1) warm gas is predominantly in the outskirts of the cluster and avoids the X-ray detected hot ICM. Also, Lya absorption strength increases with cluster impact parameter. (2) Lya absorbing warm gas traces cold HI emitting gas in the substructures of the Virgo Cluster. (3) Including the absorbers associated with the surrounding substructures, the warm gas covering fraction (100% for N_HI > 10^13.1 cm^-2) is in agreement with cosmological simulations. We speculate that the observed warm gas is part of large-scale gas flows feeding the cluster both the ICM and galaxies.

Eccentric black hole-neutron star mergers: Effects of black hole spin and equation of state

There is a high level of interest in black hole-neutron star binaries, not only because their mergers may be detected by gravitational wave observatories in the coming years, but also because of the possibility that they could explain a class of short duration gamma-ray bursts. We study black hole-neutron star mergers that occur with high eccentricity as may arise from dynamical capture in dense stellar regions such as nuclear or globular clusters. We perform general relativistic simulations of binaries with a range of impact parameters, three different initial black hole spins (zero, aligned and antialigned with the orbital angular momentum), and neutron stars with three different equations of state. We find a rich diversity across these parameters in the resulting gravitational wave signals and matter dynamics, which should also be reflected in the consequent electromagnetic emission. Before tidal disruption, the gravitational wave emission is significantly larger than perturbative predictions suggest for periapsis distances close to effective innermost stable separations, exhibiting features reflecting the zoom-whirl dynamics of the orbit there. Guided by the simulations, we develop a simple model for the change in orbital parameters of the binary during close-encounters. Depending upon the initial parameters of the system, we find that mass transfer during nonmerging close-encounters can range from essentially zero to a sizable fraction of the initial neutron star mass. The same holds for the amount of material outside the black hole post-merger, and in some cases roughly half of this material is estimated to be unbound. We also see that nonmerging close-encounters generically excite large oscillations in the neutron star that are qualitatively consistent with $f$-modes.

Forward–backward propagation in the mixed quantum–classical theory for the collisional energy transfer

A method of forward–backward propagation is incorporated into the mixed quantum–classical theory for calculations of the collisional energy transfer and ro-vibrational energy flow in a molecule+quencher encounter. This permits to avoid unphysical behavior of the energy transfer function in the range of large impact parameters. A case study is presented on stabilization of the metastable states of ozone in collisions with Ar bath gas.

Systematics of the fusion cross-section in heavy ion reactions

We study the energy dependence of the fusion cross-section with the goal of understanding the evolution of the underlying reaction mechanism as a function of input channel parameters and the causes of its (non-)disappearance. The heart of this work is a systematic and as wide as possible overview of measured cross-sections. Normalized by the reaction cross-section and plotted as a function of the available energy per nucleon corrected for the Coulomb barrier, fusion crosssection show a rather universal behavior leading to a disappearance of the fusion around 10A MeV, except in the case of very asymmetric systems for which incomplete fusion persists and tends towards a constant value that can be explained in the frame of a simple geometrical approach. These trends are confirmed by the semiclassical Landau-Vlasov transport calculations. In heavy ion collisions, one observes complete fusion (CF) and/or incomplete fusion (IF) mechanisms of the two partners in the most central reactions over an impact parameter range whose width depends strongly on the incident energy [1]. From the Coulomb barrier to 6-8A MeV, only CF occurs and it accounts for the largest part of the reaction cross section.

Observation of atomic-size Fraunhofer-type diffraction for single electron capture in He2+ + He collision

Using a reaction microscope in Lanzhou (ReMiLa), clear oscillation structures in an angular distribution of projectile for the He2+ + He (1s2) → He+ (1s) + He+ (1s) reaction are observed. By analogy with Fraunhofer diffraction of light, the oscillation structures are attributed to the diffraction of incident ions on target atoms. On the basis of a molecular Coulombic over-barrier model and a two-state model, we calculate the radius of the circular aperture, with which the angular spacing and fringe position of the diffraction pattern are predicted. By comparing with the measured values, we find that the two-state model could well estimate the range of impact parameters in which an electron is captured with a great probability.

A 2 MV Van de Graaff accelerator as a tool for planetary and impact physics research

Investigating the dynamical and physical properties of cosmic dust can reveal a great deal of information about both the dust and its many sources. Over recent years, several spacecraft (e.g., Cassini, Stardust, Galileo, and Ulysses) have successfully characterised interstellar, interplanetary, and circumplanetary dust using a variety of techniques, including in situ analyses and sample return. Charge, mass, and velocity measurements of the dust are performed either directly (induced charge signals) or indirectly (mass and velocity from impact ionisation signals or crater morphology) and constrain the dynamical parameters of the dust grains. Dust compositional information may be obtained via either time-of-flight mass spectrometry of the impact plasma or direct sample return. The accurate and reliable interpretation of collected spacecraft data requires a comprehensive programme of terrestrial instrument calibration. This process involves accelerating suitable solar system analogue dust particles to hypervelocity speeds in the laboratory, an activity performed at the Max Planck Institut für Kernphysik in Heidelberg, Germany. Here, a 2 MV Van de Graaff accelerator electrostatically accelerates charged micron and submicron-sized dust particles to speeds up to 80 km s(-1). Recent advances in dust production and processing have allowed solar system analogue dust particles (silicates and other minerals) to be coated with a thin conductive shell, enabling them to be charged and accelerated. Refinements and upgrades to the beam line instrumentation and electronics now allow for the reliable selection of particles at velocities of 1-80 km s(-1) and with diameters of between 0.05 μm and 5 μm. This ability to select particles for subsequent impact studies based on their charges, masses, or velocities is provided by a particle selection unit (PSU). The PSU contains a field programmable gate array, capable of monitoring in real time the particles' speeds and charges, and is controlled remotely by a custom, platform independent, software package. The new control instrumentation and electronics, together with the wide range of accelerable particle types, allow the controlled investigation of hypervelocity impact phenomena across a hitherto unobtainable range of impact parameters.

Fully differential cross sections for the single ionization of He by C6 + ions

We present fully differential cross sections for the single ionization of He by C6 + ions. A time-dependent close-coupling approach is used to describe the two-electron wavefunction in the field of the projectile for a range of impact parameters, and a Fourier transform approach is used to extract fully differential cross sections for a specific momentum transfer. Our calculations are compared to the measurements of Schulz et al (2003 Nature 422 48) and we find very good agreement in the scattering plane and good qualitative agreement in the perpendicular plane. In particular, our calculations in the perpendicular plane find a similar ‘double-peak’ structure in the angular distributions to those seen experimentally. We also discuss the various checks made on our calculations by comparing to a one-electron time-dependent calculation.

ECCENTRIC BLACK-HOLE-NEUTRON-STAR MERGERS

Within the next few years gravitational waves (GWs) from merging black holes (BHs) and neutron stars (NSs) may be directly detected, making a thorough theoretical understanding of these systems a high priority. As an additional motivation, these systems may represent a subset of short-duration gamma-ray burst (sGRB) progenitors. BH-NS mergers are expected to result from primordial, quasi-circular inspiral as well as dynamically formed capture binaries. The latter channel allows mergers with high eccentricity, resulting in a richer variety of outcomes. We perform general relativistic simulations of BH-NS interactions with a range of impact parameters, and find significant variation in the properties of these events that have potentially observable consequences, namely the GW signature, remnant accretion disk mass, and amount of unbound material.

Eikonal approximation in the theory of energy loss by fast charged particles

Energy losses in fast charged particles as a result of collisions with atoms are considered in the eikonal approximation. It is shown that the nonperturbative contribution to effective stopping in the range of intermediate impact parameters (comparable with the characteristic sizes of the electron shells of the target atoms) may turn out to be significant as compared to shell corrections to the Bethe-Bloch formula calculated in perturbation theory. The simplifying assumptions are formulated under which the Bethe-Bloch formula can be derived in the eikonal approximation. It is shown that the allowance for nonperturbative effects may lead to considerable (up to 50%) corrections to the Bethe-Bloch formula. The applicability range for the Bethe-Bloch formula is analyzed. It is concluded that calculation of the energy loss in the eikonal approximation (in the range of impact parameters for which the Bethe-Bloch formula is normally used) is much more advantageous than analysis based on the Bethe-Bloch formula and its modifications because not only the Bloch correction is included in the former calculations, the range of intermediate impact parameters is also taken into account nonperturbatively; in addition, direct generalization to the cases of collisions of complex projectiles and targets is possible in this case.

A PARAMETER SPACE EXPLORATION OF GALAXY CLUSTER MERGERS. I. GAS MIXING AND THE GENERATION OF CLUSTER ENTROPY

We present a high-resolution set of adiabatic binary galaxy cluster merger simulations using FLASH. These are the highest-resolution simulations to date of such mergers using an AMR grid-based code with Eulerian hydrodynamics. In this first paper in a series we investigate the effects of merging on the entropy of the hot intracluster gas, specifically with regard to the ability of merging to heat and disrupt cluster "cool-cores." We find, in line with recent works, that the effect of fluid instabilities that are well-resolved in grid-based codes is to significantly mix the gases of the two clusters and to significantly increase the entropy of the gas of the final merger remnant. This result is characteristic of mergers over a range of initial mass ratio and impact parameter. In line with this, we find that the kinetic energy associated with random motions is higher in our merger remnants which have high entropy floors, indicating the motions have efficiently mixed the gas and heated the cluster core with gas of initially high entropy. We examine the implications of this result for the maintenance of high entropy floors in the centers of galaxy clusters and the derivation of the properties of dark matter from the thermal properties of the X-ray emitting gas.

Flower Patterns in Drop Impact on Thin Liquid Films

We describe experimentally the formation of a pattern for drop impacts on thin liquid films for a large range of impact parameters. Using the shallow-water approximation, we are able to explain the main mechanisms leading to these patterns: it consists in the linear instability of the self-similar axisymmetric radial solution of the equations. Agreement between the experiments and the theory is remarkably good, leading, in particular, to the prediction that the most unstable fold number scales like (We/h∞)2/7.

Galaxy group at z=0.3 associated with the damped Lyman α system towards quasar Q1127-145

(Abridged) We performed a spectroscopic galaxy survey, complete to m<20.3 (L_B>0.15L_B* at z=0.3), within 100x100" of the quasar Q1127-145 (z=1.18). The VLT/UVES quasar spectrum contains three z<0.33 MgII absorption systems. We obtained eight new galaxy redshifts, adding to the four previously known, and galaxy star formation rates and metallicities were computed where possible. A strong MgII system [W_r(2796)=1.8A], which is a known DLA, had three previously identified galaxies; we found two additional galaxies associated with this system. These five galaxies form a group with diverse properties, such as a luminosity range of 0.04<L_B<0.63L_B*, an impact parameter range of 17<D<241kpc and velocity dispersion of 115km/s. The DLA group galaxy redshifts span beyond the 350km/s velocity spread of the metallic absorption lines of the DLA itself. The two brightest group galaxies have SFRs of a few Msun/yr and should not have strong winds. We have sufficient spectroscopic information to directly compare three of the five group galaxies' (emission-line) metallicities with the DLA (absorption) metallicity: the DLA metallicity is 1/10th solar, substantially lower than the three galaxies' which range between less than 1/2 solar to solar metallicity. HST/WFPC-2 imaging shows perturbed morphologies for the three brightest group galaxies, with tidal tails extending 25kpc. We favor a scenario where the DLA absorption originates from tidal debris in the group environment. Another absorber exhibits weak MgII absorption [W_r(2796)=0.03A] and had a previously identified galaxy at a similar redshift. We have identified a second galaxy associated with this system. Both galaxies have solar metallicities and unperturbed morphologies. The SFR of one galaxy is much lower than expected for strong outflows. Finally, we have identified five galaxies at large impact parameters with no associated MgII absorption.

High energy bounds on soft $$ \mathcal{N} = 4 $$ SYM amplitudes from AdS/CFT

Using the AdS/CFT correspondence, we study the high-energy behavior of colorless dipole elastic scattering amplitudes in \( \mathcal{N} = 4 \) SYM gauge theory through the Wilson loop correlator formalism and Euclidean to Minkowskian analytic continuation. The purely elastic behavior obtained at large impact-parameter L, through duality from disconnected AdS 5 minimal surfaces beyond the Gross-Ooguri transition point, is combined with unitarity and analyticity constraints in the central region. In this way we obtain an absolute bound on the high-energy behavior of the forward scattering amplitude due to the graviton interaction between minimal surfaces in the bulk. The dominant “Pomeron” intercept is bounded by α ≤ 11=7 using the AdS/CFT constraint of a weak gravitational field in the bulk. Assuming the elastic eikonal approximation in a larger impact-parameter range gives 4/3 ≤ α ≤ 11/7: The actual intercept becomes 4/3 if one assumes the elastic eikonal approximation within its maximally allowed range L ≳ exp Y/3; where Y is the total rapidity. Subleading AdS/CFT contributions at large impact-parameter due to the other d = 10 supergravity fields are obtained. A divergence in the real part of the tachyonic KK scalar is cured by analyticity but signals the need for a theoretical completion of the AdS/CFT scheme.