Prospects for Observing the low-density Cosmic Web in Lyman-alpha Emission

Mapping the intergalactic medium (IGM) in Lyman-$\alpha$ emission would yield unprecedented tomographic information on the large-scale distribution of baryons and potentially provide new constraints on the UV background and various feedback processes relevant to galaxy formation. Here, we use a cosmological hydrodynamical simulation to examine the Lyman-$\alpha$ emission of the IGM due to collisional excitations and recombinations in the presence of a UV background. We focus on gas in large-scale-structure filaments in which Lyman-$\alpha$ radiative transfer effects are expected to be moderate. At low density the emission is primarily due to fluorescent re-emission of the ionising UV background due to recombinations, while collisional excitations dominate at higher densities. We discuss prospects of current and future observational facilities to detect this emission and find that the emission of filaments of the cosmic web will typically be dominated by the halos and galaxies embedded in them, rather than by the lower density filament gas outside halos. Detecting filament gas directly would require a very long exposure with a MUSE-like instrument on the ELT. Our most robust predictions that act as lower limits indicate this would be slightly less challenging at lower redshifts ($z \lesssim 4$). We also find that there is a large amount of variance between fields in our mock observations. High-redshift protoclusters appear to be the most promising environment to observe the filamentary IGM in Lyman-$\alpha$ emission.Comment: 20 pages, 13 figures. Accepted for publication in Astronomy & Astrophysics. Accepted version contains several revisions following suggestions made in the review proces

Dynamical Diffraction Theory for Wave Packet Propagation in Deformed Crystals

We develop a theory for the trajectory of an x ray in the presence of a crystal deformation. A set of equations of motion for an x-ray wave packet including the dynamical diffraction is derived, taking into account the Berry phase as a correction to geometrical optics. The trajectory of the wave packet has a shift of the center position due to a crystal deformation. Remarkably, in the vicinity of the Bragg condition, the shift is enhanced by a factor $\omega /\Delta \omega$ ($\omega$: frequency of an x ray, $\Delta\omega$: gap frequency induced by the Bragg reflection). Comparison with the conventional dynamical diffraction theory is also made.Comment: 4 pages, 2 figures. Title change

Which role do excited states play in radiation damage to organic solid-state compounds?

Ionizing radiation induces radicals in organic materials. When such species are created in biological macromolecules like DNA, they harm living organisms. This detrimental effect is explicitly exploited for the sterilisation of e.g. foodstuffs, and radiation-induced radicals are quantitatively used for radiation dosimetry purposes. For understanding radiation actions at different levels of molecular and cellular organisation, knowledge of the radical structures and their formation mechanisms is of fundamental importance. In this context, radiation-induced processes in solid sugars are studied, among others, to gain insight into the role of the deoxyribose unit in the radiation chemistry of DNA. X-irradiation typically gives rise to a variety of primary radicals in these systems, which transform into stable radicals or diamagnetic species via one or more radical reactions. By combining electron magnetic resonance experiments and density functional theory (DFT) calculations, we recently identified the major stable [1,2], as well as the major primary [3] radiation-induced radicals in solid sucrose (see figure). We are currently investigating how the primary radicals transform into the stable ones. A general but important observation is that in sucrose and similar carbohydrates, e.g. rhamnose, the primary radical formation (typically by way of net H-abstraction) is selective: it preferentially takes place at particular carbons and oxygens. This selectivity apparently cannot be explained simply on thermodynamical grounds. It may be hypothesised that, after the initial oxidation event (leaving the radical cation in an excited state), the hole ‘migrates’ to a particular carbon or oxygen, after which de-excitation and deprotonation processes yield a neutral radical. It is our goal to examine factors possibly explaining the experimentally observed selectivity. So far we have made some preliminary ground-state calculations on energy profiles of deprotonation reactions in rhamnose single crystals, as well as time-dependent DFT calculations of excited states in this system

Systematic {\it ab initio} study of the magnetic and electronic properties of all 3d transition metal linear and zigzag nanowires

It is found that all the zigzag chains except the nonmagnetic (NM) Ni and antiferromagnetic (AF) Fe chains which form a twisted two-legger ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the 3d transition metals in both linear and zigzag structures have a stable or metastable ferromagnetic (FM) state. The electronic spin-polarization at the Fermi level in the FM Sc, V, Mn, Fe, Co and Ni linear chains is close to 90% or above. In the zigzag structure, the AF state is more stable than the FM state only in the Cr chain. It is found that the shape anisotropy energy may be comparable to the electronic one and always prefers the axial magnetization in both the linear and zigzag structures. In the zigzag chains, there is also a pronounced shape anisotropy in the plane perpendicular to the chain axis. Remarkably, the axial magnetic anisotropy in the FM Ni linear chain is gigantic, being ~12 meV/atom. Interestingly, there is a spin-reorientation transition in the FM Fe and Co linear chains when the chains are compressed or elongated. Large orbital magnetic moment is found in the FM Fe, Co and Ni linear chains