HyRec: A fast and highly accurate primordial hydrogen and helium recombination code

We present a state-of-the-art primordial recombination code, HyRec, including all the physical effects that have been shown to significantly affect recombination. The computation of helium recombination includes simple analytic treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He I] 2 3P - 1 1S line, and treats feedback between these lines within the on-the-spot approximation. Hydrogen recombination is computed using the effective multilevel atom method, virtually accounting for an infinite number of excited states. We account for two-photon transitions from 2s and higher levels as well as frequency diffusion in Lyman-alpha with a full radiative transfer calculation. We present a new method to evolve the radiation field simultaneously with the level populations and the free electron fraction. These computations are sped up by taking advantage of the particular sparseness pattern of the equations describing the radiative transfer. The computation time for a full recombination history is ~2 seconds. This makes our code well suited for inclusion in Monte Carlo Markov chains for cosmological parameter estimation from upcoming high-precision cosmic microwave background anisotropy measurements.Comment: Version accepted by PRD. Numerical integration switches adapted to be well behaved for a wide range of cosmologies (Sec. V E). HyRec is available at http://www.tapir.caltech.edu/~yacine/hyrec/hyrec.htm

Ultrafast effective multi-level atom method for primordial hydrogen recombination

Cosmological hydrogen recombination has recently been the subject of renewed attention because of its importance for predicting the power spectrum of cosmic microwave background anisotropies. It has become clear that it is necessary to account for a large number n >~ 100 of energy shells of the hydrogen atom, separately following the angular momentum substates in order to obtain sufficiently accurate recombination histories. However, the multi-level atom codes that follow the populations of all these levels are computationally expensive, limiting recent analyses to only a few points in parameter space. In this paper, we present a new method for solving the multi-level atom recombination problem, which splits the problem into a computationally expensive atomic physics component that is independent of the cosmology, and an ultrafast cosmological evolution component. The atomic physics component follows the network of bound-bound and bound-free transitions among excited states and computes the resulting effective transition rates for the small set of "interface" states radiatively connected to the ground state. The cosmological evolution component only follows the populations of the interface states. By pre-tabulating the effective rates, we can reduce the recurring cost of multi-level atom calculations by more than 5 orders of magnitude. The resulting code is fast enough for inclusion in Markov Chain Monte Carlo parameter estimation algorithms. It does not yet include the radiative transfer or high-n two-photon processes considered in some recent papers. Further work on analytic treatments for these effects will be required in order to produce a recombination code usable for Planck data analysis.Comment: Version accepted by Phys. Rev. D. Proof of equivalence of effective and standard MLA methods moved to the main text. Some rewording

Relative velocity of dark matter and baryonic fluids and the formation of the first structures

At the time of recombination, baryons and photons decoupled and the sound speed in the baryonic fluid dropped from relativistic to the thermal velocities of the hydrogen atoms. This is less than the relative velocities of baryons and dark matter computed via linear perturbation theory, so we infer that there are supersonic coherent flows of the baryons relative to the underlying potential wells created by the dark matter. As a result, the advection of small-scale perturbations (near the baryonic Jeans scale) by large-scale velocity flows is important for the formation of the first baryonic structures. This effect involves a quadratic term in the cosmological perturbation theory equations and hence has not been included in studies based on linear perturbation theory. We show that the relative motion suppresses the abundance of the first bound objects, even if one only investigates dark matter haloes, and leads to qualitative changes in their spatial distribution, such as introducing scale-dependent bias and stochasticity. We discuss the possible observable implications for high-redshift galaxy clustering and reionization

What determines the shape of a Pine-Island-like ice shelf?

Ice shelf shape directly controls ocean heat intrusions, melting near the grounding line, and buttressing. Little is known about what determines ice-shelf shape because ice-ocean coupled simulations typically aim at projecting Antarctica's contribution to sea-level rise and they do not resolve small-scale ice-ocean interactive processes. We conduct ice-ocean coupled simulations for an idealized high-resolution, Pine-Island-like model configuration. We show that ocean melting and ice stretching caused by acceleration thin the ice shelf from the grounding line toward the ice shelf front, consistent with previous studies. In the across-flow direction, ocean melting and ice advection cancel each other out and flatten the ice shelf. More than one-third of the ice thinning from grounding line to ice front can be attributed to ocean melting at depths shallower than 500 m. Our results emphasize the importance of interactive processes between the entire ice shelf and the ocean for determining the ice shelf shape. Plain Language Summary Antarctic ice flows into the ocean and forms a floating extension of land ice called an ice shelf. The ice shelf shape directly controls the amount of ocean heat intrusions, melting near the grounding line, and buttressing. However, little is understood about ice-ocean interactive processes determining ice shelf shape because (a) ocean modelers apply a constant cavity geometry, (b) ice modelers mostly assume simplified melting parameterization, and (c) ice-ocean coupled simulations typically aim at projections of Antarctica's sea-level contributions and they require long model integration. We conduct ice-ocean coupled simulations for an idealized high-resolution Pine-Island-like model configuration. Basal melting and ice stretching create a typical ice shelf shape with steep thinning near the grounding line followed by gradual thinning toward the ice shelf front. In the across-flow direction, ice divergence from the center advects ice toward edges, compensating for melt-driven thinning and flattening ice shelf shape. We also show that ice melting at shallow depths contributes to about one-third of ice-shelf thinning. Although it is thought that ice shelf melting at the grounding line dominantly controls ice shelf behavior, our results suggest the importance of ice-ocean interactive processes for the entire ice shelf cavity for determining the ice shelf shape

Nuclear Photoabsorption at Photon Energies between 300 and 850 Mev

We construct the formula for the photonuclear total absorption cross section using the projection method and the unitarity relation. Our treatment is very effective when interference effects in the absorption processes on a nucleon are strong. The disappearance of the peak around the position of the $D_{13}$ resonance in the nuclear photoabsorption can be explained with the cooperative effect of the interference in two-pion production processes,the Fermi motion, the collision broadenings of $\Delta$ and $N^*$, and the pion distortion in the nuclear medium. The change of the interference effect by the medium plays an important role.Comment: 22pages,7figures,revtex