Small Impact Parameters Research Articles

MUSE-ALMA haloes V: physical properties and environment of z ≤ 1.4 H i quasar absorbers

ABSTRACT We present results of the MUSE-ALMA haloes, an ongoing study of the circumgalactic medium (CGM) of low-redshift galaxies (z ≤ 1.4), currently comprising 14 strong H i absorbers in 5 quasar fields. We detect 43 galaxies associated with absorbers down to star formation rate (SFR) limits of 0.01–0.1 M⊙ yr−1, found within impact parameters (b) of 250 kpc from the quasar sightline. Excluding the targeted absorbers, we report a high detection rate of 89 per cent and find that most absorption systems are associated with pairs or groups of galaxies (3–11 members). We note that galaxies with the smallest impact parameters are not necessarily the closest to the absorbing gas in velocity space. Using a multiwavelength data set (UVES/HIRES, HST, MUSE), we combine metal and H i column densities, allowing for derivation of the lower limits of neutral gas metallicity as well as emission-line diagnostics (SFR, metallicities) of the ionized gas in the galaxies. We find that groups of associated galaxies follow the canonical relations of N(H i)–b and Wr(2796)–b, defining a region in parameter space below which no absorbers are detected. The metallicity of the ISM of associated galaxies, when measured, is higher than the metallicity limits of the absorber. In summary, our findings suggest that the physical properties of the CGM of complex group environments would benefit from associating the kinematics of individual absorbing components with each galaxy member.

Mechanism Change in the Dynamics of the O' + O2 → O'O + O Atom Exchange Reaction at High Collision Energies.

The extreme velocity and the large available energy of atoms with hyperthermal kinetic energies can give rise to novel mechanisms and behavior of chemical reactions unseen at thermal conditions. Crossed-molecular-beams experiments combined with isotope labeling on the reaction of hyperthermal O atoms with O2 molecules have provided an example of the arising complexity of such systems. Quasiclassical trajectory (QCT) calculations proved to be instructive in the exploration of the microscopic mechanism of the reactive and inelastic scattering observed, and a new mechanism has been identified: there are reactive collisions in which the potential energy remains repulsive during the entire encounter ("direct" reactions in which, in a sense, no complex is formed). In this work, the effect of the magnitude of the collision energy on this mechanism is explored. At hyperthermal collision energies, the reaction is characterized by a unique impact parameter window favorable for reaction through complex formation, while the direct collisions take place exclusively at small impact parameters. In direct reactive collisions, contributing as much as 12% to the reaction cross section, first the existing bond is broken, and the new bond is formed afterward. This kind of collision is unique to extremely high collision energies. Analysis of various correlations was used to find out the details of the reaction dynamics. The observed phenomena indicate that when the collision energy is extremely high, one can expect deviation from what an extrapolation from the more familiar energy ranges would predict.

Anion and cation emission from water molecules after collisions with 6.6-keV 16O+ ions

Anion and cation emission following water dissociation was studied for 6.6-keV $^{16}$O$^{+}$ + H$_{2}$O collisions. Absolute cross sections for the emission of all positively and negatively charged fragments, differential in both energy and observation angle, were measured. The fragments formed in hard, binary collisions appearing in peaks were distinguishable from those created in soft collisions with many-body dynamics that result in a broad energy spectrum. A striking feature is that anions and cations are emitted with similar energy and angular distributions, with a nearly constant ratio of about 1:100 for H$^{-}$ to H$^{+}$. Model calculations were performed at different levels of complexity. Four-body scattering simulations reproduce the measured fragment distributions if adequate kinetic-energy release of the target is taken into account. Providing even further insight into the underlying processes, predictions of a thermodynamic model indicate that transfer ionization at small impact parameters is the dominant mechanism for H$^{+}$ creation. The present findings confirm our earlier observation that in molecular fragmentation induced by slow, singly charged ions, the charge states of the emitted hydrogen fragments follow a simple statistical distribution independent of the way they are formed.

Transverse expansion of hot magnetized Bjorken flow in heavy ion collisions

We argue that the existence of an inhomogeneous external magnetic field can lead to radial flow in transverse plane. Our aim is to show how the introduction of a magnetic field generalizes the Bjorken flow. We investigate the effect of an inhomogeneous weak external magnetic field on the transverse expansion of in-viscid fluid created in high energy nuclear collisions. In order to simplify our calculation and compare with Gubser model, we consider the fluid under investigation to be produced in central collisions, at small impact parameter; azimuthal symmetry has been considered. In our model, we assume an inhomogeneous external magnetic field following the power-law decay in proper time and having radial inhomogeneity perpendicular to the radial velocity of the in-viscid fluid in the transverse plane; then the space time evolution of the transverse expansion of the fluid is obtained. We also show how the existence of an inhomogeneous external magnetic field modifies the energy density. Finally we use the solutions for the transverse velocity and energy density in the presence of a weak magnetic field, to estimate the transverse momentum spectrum of protons and pions emerging from the Magneto-hydrodynamic solutions.

Soft gravitational radiation from ultra-relativistic collisions at sub- and sub-sub-leading order

Using soft-graviton theorems a well-known zero-frequency limit (ZFL) for the gravitational radiation flux dEGW/dω is re-derived and extended to order mathcal{O}left(omega right) and mathcal{O}left({omega}^2right) for arbitrary massless multi-particle collisions. The (angle-integrated, unpolarized) mathcal{O}left(omega right) correction to the flux turns out to be absent in the case of two-particle elastic collisions. The mathcal{O}left({omega}^2right) correction is instead non-vanishing and takes a simple general expression which is then applied to bremsstrahlung from two-particle elastic collisions. For a tree-level process the outcome is finite and consistent with expectations. Instead, if the tree-level form of the soft theorems is used at sub-sub-leading order even when the elastic amplitude needs an all-loop (eikonal) resummation, an unphysical infrared singularity occurs. Its origin can be traced to the infinite Coulomb phase of gravitational scattering in four dimensions. We briefly discuss how to get rid, in principle, of the unwanted divergences and indicate -without carrying out- a possible procedure to find the proper correction to the naive soft theorems. Nevertheless, if a simple recipe recently proposed for handling these divergences is adopted, we find surprisingly good agreement with results obtained independently via the eikonal approach to transplanckian-energy scattering at large (small) impact parameter (deflection angle), where such Coulomb divergences explicitly cancel out.

Production of exotic neutron-deficient isotopes near N, Z = 50 in multinucleon transfer reactions * * Supported by National Natural Science Foundation of China (11635003, 11025524, 11161130520), the National Basic Research Program of China (2010CB832903) and the European Commissions 7th Framework Programme (FP7-PEOPLE-2010-IRSES) (269131)

The multinucleon transfer reaction in the collisions of 40Ca+ 124Sn at MeV is investigated using the improved quantum molecular dynamics model. The measured angular distributions and isotopic distributions of the products are reproduced reasonably well by the calculations. The multinucleon transfer reactions of 40Ca + 112Sn, 58Ni + 112Sn, 106Cd + 112Sn, and 48Ca + 112Sn are also studied. This demonstrates that the combinations of neutron-deficient projectile and target are advantageous for the production of exotic neutron-deficient nuclei near N, Z = 50. The charged particles' emission plays an important role at small impact parameters in the de-excitation processes of the system. The production cross sections of the exotic neutron-deficient nuclei in multinucleon transfer reactions are much larger than those measured in the fragmentation and fusion-evaporation reactions. Several new neutron-deficient nuclei can be produced in the 106Cd + 112Sn reaction. The corresponding production cross sections for the new neutron-deficient nuclei, 101, 112Sb, 103Te, and 106, 107I, are 2.0 nb, 4.1 nb, 6.5 nb, 0.4 b and 1.0 b, respectively.

Models of multicomponent splash bridges in face-on galaxy disc collisions

We use an inelastic particle code with shocks and cooling calculated on a subgrid level to study the gas in direct collisions between galaxy discs. The interstellar media (ISM) of the discs are modeled with continuous thermal phases. The models produce many unique structures, collectively called splash bridges. They range from central bridge discs to swirled sheets, which resemble those observed in interacting galaxies. These morphologies are sensitive to the rotation, relative mass, disc offsets and the gas structure in the discs. In the case of the Taffy galaxies - NGC 12914/15, extensive observations have revealed radio continuum emitting gas, HI gas, hot X-rays from hot diffuse gas and more $H_2$ than exists in the Milky Way coexisting in the bridge. The origins of the $H_2$ and large asymmetric distribution of ISM are not clear. We show that for small disc impact parameters, multiple phases of ISM with densities over many orders of magnitude can be removed from their host galaxies into a Taffy-like bridge. The orientation of the discs initial overlap can have a great effect on the distributions of each phase of ISM. In some cases, the models also predict the creation of a possible `dark galaxy,' a large flat region of dense ISM far from the stellar disc potential of either galaxy.

Evidence for diffuse molecular gas and dust in the hearts of gamma-ray burst host galaxies

Context. Damped Lyman-α (DLA) absorption-line systems at the redshifts of gamma-ray burst (GRB) afterglows offer a unique way to probe the physical conditions within star-forming galaxies in the early Universe. Aims. Here we built up a large sample of 22 GRBs at redshifts z > 2 observed with VLT/X-shooter in order to determine the abundances of hydrogen, metals, dust, and molecular species. This allows us to study the metallicity and dust depletion effects in the neutral interstellar medium at high redshift and to answer the question of whether (and why) there might be a lack of H2 in GRB-DLAs. Methods. We developed new methods based on the Bayesian inference package, PyMC, to FIT absorption lines and measure the column densities of different metal species as well as atomic and molecular hydrogen. The derived relative abundances are used to FIT dust depletion sequences and determine the dust-to-metals ratio and the host-galaxy intrinsic visual extinction. Additionally, we searched for the absorption signatures of vibrationally-excited H2 and carbon monoxide. Results. We find that there is no lack of H2-bearing GRB-DLAs. We detect absorption lines from molecular hydrogen in 6 out of 22 GRB afterglow spectra, with molecular fractions ranging between f ≃ 5 × 10−5 and f ≃ 0.04, and claim tentative detections in three additional cases. For the remainder of the sample, we measure, depending on S/N, spectral coverage and instrumental resolution, more or less stringent upper limits. The GRB-DLAs in our sample have on average low metallicities, [X/H]¯ ≈ −1.3, comparable to the population of extremely-strong QSO-DLAs (log N(H I) > 21.5). Furthermore, H2-bearing GRB-DLAs are found to be associated with significant dust extinction, AV > 0.1 mag, and dust-to-metals ratios DTM > 0.4, confirming the importance of dust grains for the production of molecules. All these systems exhibit neutral hydrogen column densities log N(H I) > 21.7. The overall fraction of H2 detections in GRB-DLAs is ≥ 27% (41% including tentative detections), which is three to four times larger than in the general QSO-DLA population. For 2 < z < 4, and considering column densities log N(H I) > 21.7, the H2 detection fraction is 60–80% in GRB-DLAs and in extremely strong QSO-DLAs. This is likely due to the fact that both GRB- and QSO-DLAs with high neutral hydrogen column densities are probed by sight-lines with small impact parameters, indicating that the absorbing gas is associated with the inner regions of the absorbing galaxy, where the gas pressure is higher and the conversion of H I to H2 takes place. In the case of GRB hosts, this diffuse molecular gas is located at distances ≳ 500 pc from the GRB and hence is unrelated to the star-forming region where the event occurred.

Roles of NΔ → NN and πN → Δ Reactions in Heavy-Ion Collisions at Intermediate Energies**Supported by the National Natural Science Foundation of China under Grant No. 11505150; the Yuncheng University Research Project under Grant No. YQ-2014014; the National Natural Science Foundation of China under Grant No. 11405278

Within the framework of the isospin-dependent transport model, the roles of the reactions NΔ → NN and πN → Δ are investigated through simulating heavy-ion collisions at 1000 MeV/nucleon. The absorption process NΔ → NN plays an important role for heavy impact systems and small impact parameters than for light impact systems and large impact parameters. The resorption process πN → Δ is of importance for heavy impact systems and large impact parameters than for light impact systems and small impact parameters. Thus the influences of the reaction NΔ → NN (πN → Δ) on pion production dynamics can be neglected in heavy-ion collisions for smaller (larger) impact parameters and light systems. It is the reaction πN → Δ that causes the anti-correlation of pions and nucleons in the rapidity dependence of the directed flow.

High-energy direct photoelectron spectroscopy in strong-field ionization

Recently, in a strong Coulomb field regime of tunneling ionization an unexpected large enhancement of photoelectron spectra due to the Coulomb field of the atomic core has been identified by numerical solution of time-dependent Schr\"odinger equation [Phys. Rev. Lett. \textbf{117}, 243003 (2016)] in the upper energy range of the tunnel-ionized direct electrons. We investigate the origin of the enhancement employing a classical theory with Monte Carlo simulations of trajectories, and a quantum theory of Coulomb-corrected strong field approximation based on the generalized eikonal approximation for the continuum electron. Although the quantum effects at recollisions with a small impact parameter yield an overall enhancement of the spectrum relative to the classical prediction, the high energy enhancement itself is shown to have a classical nature and is due to momentum space bunching of photoelectrons released not far from the peak of the laser field. The bunching is caused by a large and nonuniform, with respect to the ionization time, Coulomb momentum transfer at the ionization tunnel exit.

Inelastic profiles of protons at 7 and 13 TeV

The shapes of the inelastic interaction region of protons with energies sqrt{s}=7 and 13 GeV are considered. The unitarity condition with different assumptions about the elastic scattering amplitude is used. It is shown that the sign of the imaginary part of the amplitude at large transferred momenta is especially important for the conclusion about this shape at small impact parameters (central collisions).

Option pricing with linear market impact and nonlinear Black–Scholes equations

We consider a model of linear market impact, and address the problem of replicating a contingent claim in this framework. We derive a nonlinear Black–Scholes equation that provides an exact replication strategy. This equation is fully nonlinear and singular, but we show that it is well posed, and we prove existence of smooth solutions for a large class of final payoffs, both for constant and local volatility. To obtain regularity of the solutions, we develop an original method based on Legendre transforms. The close connections with the problem of hedging with gamma constraints [SIAM J. Control Optim. 39 (2000) 73–96, Math. Finance 17 (2007) 59–80, Ann. Inst. H. Poincare Anal. Non Lineaire 22 (2005) 633–666], with the problem of hedging under liquidity costs [Finance Stoch. 14 (2010) 317–341] are discussed. The optimal strategy and associated diffusion are related with the second-order target problems of [Ann. Appl. Probab. 23 (2013) 308–347], and with the solutions of optimal transport problems by diffusions of [Ann. Probab. 41 (2013) 3201–3240]. We also derive a modified Black–Scholes formula valid for asymptotically small impact parameter, and finally provide numerical simulations as an illustration.

Bending of light in a Coulomb gas

Photons traveling in a background electromagnetic field may bend via the vacuum polarization effect with the background field. The bending in a Coulomb field by a heavy nucleus is small even at a large atomic number, rendering it difficult to detect experimentally. As an amplifying mechanism of the effect we consider the bending of light traveling in a chamber of Coulomb gas. The Gaussian nature of the bending in the gas increases the total bending angle in proportion to the square root of the photon travel distance. The enhancement can be orders of magnitude over the bending by a single nucleus at a small impact parameter, which may help experimental observation of the Coulombic bending.

Formation of Moon-induced Gaps in Dense Planetary Rings: Application to the Rings of Saturn

Abstract We develop an axisymmetric diffusion model to describe radial density profiles in the vicinity of tiny moons embedded in planetary rings. Our diffusion model accounts for the gravitational scattering of the ring particles by an embedded moon and for the viscous diffusion of the ring matter back into the gap. With test particle simulations, we show that the scattering of the ring particles passing the moon is larger for small impact parameters than estimated by Goldreich & Tremaine and Namouni. This is significant for modeling the Keeler gap. We apply our model to the gaps of the moons Pan and Daphnis embedded in the outer A ring of Saturn with the aim to estimate the shear viscosity of the ring in the vicinity of the Encke and Keeler gap. In addition, we analyze whether tiny icy moons whose dimensions lie below Cassini’s resolution capabilities would be able to explain the gap structure of the C ring and the Cassini division.

State-to-state quantum dynamics of the H + LiF → Li + HF reaction on an accurate ab initio potential energy surface

Quantum state-to-state dynamics calculations are reported for the H + LiF(vi, ji) → HF(vf, jf) + Li reaction on a recently reported ground state X̃2A′ potential energy surface. It was found that at low collision energies the title reaction is dominated by long-lived resonances, while the lifetimes of resonances become shorter at higher energies. The existence of resonances is also supported by the differential cross section, which has mostly forward and sideways bias. The integral cross section rises with collision energy slowly, and the opacity function suggests that the reaction is dominated by small impact parameter collisions. The HF product has cold vibrational state distributions in range of the collision energy studied here, although its rotational state distributions peak near the highest accessible rotational states. This early-barrier reaction shows some enhancement by translational energy, but internal excitations of the LiF reactant have more complex behaviors.

Revealing the Host Galaxy of a Quasar 2175 Å Dust Absorber at z = 2.12

Abstract We report the first detection of the host galaxy of a strong 2175 Å dust absorber at z = 2.12 toward the background quasar SDSS J121143.42+083349.7 using Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) IR F140W direct imaging and G141 grism spectroscopy. The spectroscopically confirmed host galaxy is located at a small impact parameter of ∼5.5 kpc (∼0.″65). The F140W image reveals a disk-like morphology with an effective radius of 2.24 ± 0.08 kpc. The extracted 1D spectrum is dominated by a continuum with weak emission lines ([O iii] and [O ii]). The [O iii]-based unobscured star formation rate (SFR) is 9.4 ± 2.6 M ⊙ yr−1, assuming an [O iii]/Hα ratio of 1. The moderate 4000 Å break (Dn(4000) index ∼1.3) and Balmer absorption lines indicate that the host galaxy contains an evolved stellar population with an estimated stellar mass M * of (3–7) × 1010 M ⊙. The SFR and M * of the host galaxy are comparable to, though slightly lower than, those of typical emission-selected galaxies at z ∼ 2. As inferred from our absorption analysis in Ma et al., the host galaxy is confirmed to be a chemically enriched, evolved, massive, and star-forming disk-like galaxy that is likely in the transition from a blue star-forming galaxy to a red quiescent galaxy.

Effect of in-medium nucleon-nucleon cross section on proton-proton momentum correlation in intermediate-energy heavy-ion collisions

The proton-proton momentum correlation function from different rapidity regions is systematically investigated for the Au + Au collisions at different impact parameters and different energies from $400A$ MeV to $1500A$ MeV in the framework of the isospin-dependent quantum molecular dynamics model complemented by the Lednick\'y-Lyuboshitz analytical method. In particular, the in-medium nucleon-nucleon cross-section dependence of the correlation function is brought into focus, while the impact parameter and energy dependence of the momentum correlation function are also explored. The sizes of the emission source are extracted by fitting the momentum correlation functions using the Gaussian source method. We find that the in-medium nucleon-nucleon cross section obviously influences the proton-proton momentum correlation function, which is from the whole-rapidity or projectile or target rapidity region at smaller impact parameters, but there is no effect on the mid-rapidity proton-proton momentum correlation function, which indicates that the emission mechanism differs between projectile or target rapidity and mid-rapidity protons.

A study of small impact parameter ion channeling effects in thin crystals

We have recorded channeling patterns produced by 1 to 2 MeV protons aligned with <111> axes in 55 nm thick silicon crystals which exhibit characteristic angular structure for deflection angles up to and beyond the axial critical angle, psi(a). Such large angular deflections are produced by ions incident on atomic strings with small impact parameters, resulting in trajectories which pass through several radial rings of atomic strings before exiting the thin crystal. Each ring may focus, steer or scatter the channeled ions in the transverse direction and the resulting characteristic angular structure beyond 0.6psi(a) at different depths can be related to peaks and troughs in the nuclear encounter probability. Such radial focusing underlies other axial channeling phenomena in thin crystals including planar channeling of small impact parameter trajectories, peaks around the azimuthal distribution at small tilts and large shoulders in the nuclear encounter probability at tilts beyond psi(a).

Unitarity restoring graviton radiation in the collapse regime of gravitational scattering

We investigate graviton radiation in gravitational scattering at small impact parameters $b<R\equiv 2G\sqrt{s}$ and extreme energies $s\gg M_P^2$, a regime in which classical collapse is thought to occur, and thus radiation may be suppressed also. Here however, by analyzing the soft-based representation of radiation recently proposed in the semiclassical ACV framework, we argue that gravitons can be efficiently produced in the untrapped region $|\boldsymbol{x}|\gtrsim R>b$, so as to suggest a possible completion of the unitarity sum. In fact, such energy radiation at large distances turns out to compensate and to gradually reduce to nothing the amount of energy $E'$ being trapped at small-$b$'s, by thus avoiding the quantum tunneling suppression of the elastic scattering and suggesting a unitary evolution. We finally look at the coherent radiation sample so obtained and we find that, by energy conservation, it develops an exponential frequency damping corresponding to a "quasi-temperature" of order $\hbar/R$, which is naturally related to a Hawking radiation and is suggestive of a black-hole signal at quantum level.

Theoretical study on collision dynamics of H+ + H2O at low energies

ABSTRACTScattering dynamics of a water molecule in collision with proton is studied based on a time-dependent density functional theory and coupled with the molecular dynamics method, in which the electrons are described by quantum mechanics and the nuclei are described by classical mechanics. Four different incident directions at 46 eV are chosen in order to investigate the orientations effect, and the energy-dependent effect in low energy region is explored under impact energies 27, 36 and 46 eV. Reaction channels, scattering angles and energy loss of protons are calculated. The differences between those characteristics are unobvious in large impact parameters, which are irrespective of the incident orientations due to weak projectile-target interaction. In small impact parameters, the results strongly depend on the collision energy and orientation.