119722071
Off-shell effects in the (p, 2p) reaction
Abstract It is shown that, within the limitations of the distorted-wave off-shell impulse approximation, future accurate (p, 2p) experiments near 150 MeV incident energy can be helpful in distinguishing between some of six phenomenological nuclear potentials that are examined. On-shell differences between the Reid soft-core and five different one-boson-exchange potentials are amplified in the (p, 2p) context in such a way that an accurately measurable feature, the peak height ratio for 1p knockout, is sensitive to them.
209831780
Electron-atom collisions
Preface 1. Introduction 2. Experimental techniques for cross-section measurements 3. Background quantum mechanics in the atomic context 4. One-electron problems 5. Theory of atomic bound states 6. Formal scattering theory 7. Calculation of scattering amplitudes 8. Spin-independent scattering observables 9. Spin-dependent scattering observables 10. Ionisation 11. Electron momentum spectroscopy References Index.
123046682
Optical Model in the Interior of the Nucleus
It is shown how phase relationships between partial waves in the optical model result in focusing effects which are largest for surface partial waves. In the case of very low incident energies where the wavelength of a partial wave is large compared to the nuclear surface thickness, it is shown how the phase relationships may enable surfaceand volume-reaction mechanisms for a direct reaction to be distinguished by looking at gross features of the argular distribution. Thus, low-energy direct interactions may be used as a probe for the nuclear interior. (auth)
121500029
Time-Dependent Scattering Experiment
Theory is developed for a time-dependenit scatternng experiment that consists in following the time development of a photon wave packet scattered resonantly from the metastable state of an appropriate nucleus. (C.E.S.)
120920307
Core Excitation in a Stripping Reaction
120056204
Continuum in the atomic optical model
The optical potential in principle gives a complete description of the effect of the target continuum on an electron-scattering problem truncated to a discrete set of channels. A momentum-space form for the optical potential has been derived and computed for the continuum in various approximations for the case of hydrogen. The total ionization cross section is excellently reproduced in the weak-coupling approximation above 100 eV by the most detailed model, and is tolerable as low as 50 eV.
14677179
On the origin of cores in simulated galaxy clusters
The diffuse plasma that fills galaxy groups and clusters (the intracluster medium) is a by-product of galaxy formation. The present thermal state of this gas results from a competition between gas cooling and heating. The heating comes from two distinct sources: gravitational heating associated with the collapse of the dark matter halo and additional thermal input from the formation of galaxies and their black holes. A long-term goal of this research is to decode the observed temperature, density and entropy profiles of clusters and to understand the relative roles of these processes. However, a long-standing problem has been that cosmological simulations based on smoothed particle hydrodynamics (SPH) and Eulerian mesh-based codes predict different results even when cooling and galaxy/black hole heating are switched off. Clusters formed in SPH simulations show near power-law entropy profiles, while those formed in Eulerian simulations develop a core and do not allow gas to reach such low entropies. Since the cooling rate is closely connected to the minimum entropy of the gas distribution, the differences are of potentially key importance. 
 
In this paper, we investigate the origin of this discrepancy. By comparing simulations run using the GADGET-2 SPH code and the FLASH adaptive Eulerian mesh code, we show that the discrepancy arises during the idealized merger of two clusters and that the differences are not the result of the lower effective resolution of Eulerian cosmological simulations. The difference is not sensitive to the minimum mesh size (in Eulerian codes) or the number of particles used (in SPH codes). We investigate whether the difference is the result of the different gravity solvers, the Galilean non-invariance of the mesh code or an effect of unsuitable artificial viscosity in the SPH code. Instead, we find that the difference is inherent to the treatment of vortices in the two codes. Particles in the SPH simulations retain a close connection to their initial entropy, while this connection is much weaker in the mesh simulations. The origin of this difference lies in the treatment of eddies and fluid instabilities. These are suppressed in the SPH simulations, while the cluster mergers generate strong vortices in the Eulerian simulations that very efficiently mix the fluid and erase the low-entropy gas. We discuss the potentially profound implications of these results.
15928892
Photoevaporation of Circumstellar Disks around Young Stars
We examine the ability of photoevaporative disk winds to explain the low-velocity components observed in the forbidden line spectra of low-mass T Tauri stars. Using the analytic model of Shu and coworkers and Hollenbach and coworkers as a basis, we examine the characteristics of photoevaporative outflows with hydrodynamic simulations. General results from the simulations agree well with the analytic predictions, although some small differences are present. Most importantly, the flow of material from the disk surface develops at smaller radii than in the analytic approximations, and the flow velocity from the disk surface is only one-third the sound speed. A detailed presentation of observational consequences of the model is given, including predicted line widths, blueshifts, and integrated luminosities of observable sulfur and nitrogen emission lines. We demonstrate that these predictions are in agreement with current observational data on the low-velocity forbidden line emission of ionized species from T Tauri stars. This is in contrast to magnetic wind models, which systematically underpredict these forbidden line luminosities. However, the present model cannot easily account for the luminosities of neutral oxygen lines in T Tauri stars.
54971852
Cosmological simulations of the formation of the stellar haloes around disc galaxies
We use the Galaxies-Intergalactic Medium Interaction Calculation (GIMIC) suite of cosmological hydrodynamical simulations to study the formation of stellar spheroids of Milky Way mass disc galaxies. The simulations contain accurate treatments of metal-dependent radiative cooling, star formation, supernova feedback and chemodynamics, and the large volumes that have been simulated yield an unprecedentedly large sample of ≈400 simulated ∼L∗ disc galaxies. The simulated galaxies are surrounded by low-mass, low surface brightness stellar haloes that extend out to ∼100 kpc and beyond. The diffuse stellar distributions bear a remarkable resemblance to those observed around the Milky Way, M31 and other nearby galaxies, in terms of mass density, surface brightness and metallicity profiles. We show that in situ star formation typically dominates the stellar spheroids by mass at radii of r 30 kpc, whereas accretion of stars dominates at larger radii and this change in origin induces a change in the slope of the surface brightness and metallicity profiles, which is also present in the observational data. The system-to-system scatter in the in situ mass fractions of the spheroid, however, is large and spans over a factor of 4. Consequently, there is a large degree of scatter in the shape and normalization of the spheroid density profile within r 30 kpc (e.g. when fitted by a spherical power-law profile, the indices range from −2.6 to −3.4). We show that the in situ mass fraction of the spheroid is linked to the formation epoch of the system. Dynamically, older systems have, on average, larger contributions from in situ star formation, although there is significant system-to-system scatter in this relationship. Thus, in situ star formation likely represents the solution to the long-standing failure of pure accretion-based models to reproduce the observed properties of the inner spheroid.
55320407
The case for AGN feedback in galaxy groups
The relatively recent insight that energy input from supermassive black holes (BHs) can have a substantial effect on the star formation rates (SFRs) of galaxies motivates us to examine the effects of BH feedback on the scale of galaxy groups. At present, groups contain most of the galaxies and a significant fraction of the overall baryon content of the universe and, along with massive clusters, they represent the only systems for which it is possible to measure both the stellar and gaseous baryonic components directly. To explore the effects of BH feedback on groups, we analyse two high resolution cosmological hydrodynamic simulations from the OverWhelmingly Large Simulations (OWLS) project. While both include galactic winds driven by supernovae, only one of the models includes feedback from accreting BHs. We compare the properties of the simulated galaxy groups to a wide range of observational data, including the entropy and temperature profiles of the intragroup medium, hot gas mass fractions, the luminosity temperature and mass temperature scaling relations, the K-band luminosity of the group and its central brightest galaxy (CBG), star formation rates and ages of the CBG, and gas- and stellar-phase metallicities. Both runs yield entropy distributions similar to the data, while the run without AGN feedback yields highly peaked temperature profiles, in discord with the observations. Energy input from supermassive BHs significantly reduces the gas mass fractions of galaxy groups with masses less than a few times 10 14 M� , yielding a gas mass fraction and X-ray luminosity scaling with system temperature that is in excellent agreement with the data, although the detailed scatter in the L T relation is not quite correct. The run without AGN feedback suffers from the well known overcooling problem — the resulting stellar mass fractions are several times larger than observed and present-day cooling flows operate uninhibitedly. By contrast, the run that includes BH feedback yields stellar mass fractions, SFRs, and stellar age distributions in excellent agreement with current estimates, thus resolving the long standing ‘cooling crisis’ of simulations on the scale of groups. Both runs yield very similar gas-phase metal abundance profiles that match X-ray measurements, but they predict very different stellar metallicities. Based on the above, galaxy groups provide a compelling case that feedback from supermassive BHs is a crucial ingredient in the formation of massive galaxies.
53396981
Probing the cosmic web: intercluster filament detection using gravitational lensing
The problem of detecting dark matter filaments in the cosmic web is considered. Weak lensing is an ideal probe of dark matter, and therefore forms the basis of particularly promising detection methods. We consider and develop a number of weak lensing techniques that could be used to detect filaments in individual or stacked cluster fields, and apply them to synthetic lensing data sets in the fields of clusters from the Millennium Simulation. These techniques are multipole moments of the shear and convergence, mass reconstruction and parametrized fits to filament mass profiles using a Markov chain Monte Carlo approach. In particular, two new filament detection techniques are explored (multipole shear filters and Markov chain Monte Carlo mass profile fits), and we outline the quality of data required to be able to identify and quantify filament profiles. We also consider the effects of large-scale structure on filament detection. We conclude that using these techniques, there will be realistic prospects of detecting filaments in data from future space-based missions. The methods presented in this paper will be of great use in the identification of dark matter filaments in future surveys.