The Lyman-alpha Forest as a Cosmological Tool

We review recent developments in the theory of the Lyman-alpha forest and their implications for the role of the forest as a test of cosmological models. Simulations predict a relatively tight correlation between the local Lya optical depth and the local gas or dark matter density. Statistical properties of the transmitted flux can constrain the amplitude and shape of the matter power spectrum at high redshift, test the assumption of Gaussian initial conditions, and probe the evolution of dark energy by measuring the Hubble parameter H(z). Simulations predict increased Lya absorption in the vicinity of galaxies, but observations show a Lya deficit within Delta_r ~ 0.5 Mpc/h (comoving). We investigate idealized models of "winds" and find that they must eliminate neutral hydrogen out to comoving radii ~1.5 Mpc/h to marginally explain the data. Winds of this magnitude suppress the flux power spectrum by \~0.1 dex but have little effect on the distribution function or threshold crossing frequency. In light of the stringent demands on winds, we consider the alternative possibility that extended Lya emission from target galaxies replaces absorbed flux, but we conclude that this explanation is unlikely. Taking full advantage of the data coming from large telescopes and from the Sloan Digital Sky Survey will require more complete understanding of the galaxy proximity effect, careful attention to continuum determination, and more accurate numerical predictions, with the goal of reaching 5-10% precision on key cosmological quantities.Comment: 13 pages, to appear in "The Emergence of Cosmic Structure," Proceedings of the 13th Annual Astrophysics Conference in Maryland, eds. S. Holt and C. Reynolds, AIP Press, 200

The Galaxy Proximity Effect in the Lyman-alpha Forest

Hydrodynamic cosmological simulations predict that the average opacity of the Ly-alpha forest should increase in the neighborhood of galaxies because galaxies form in dense environments. Recent observations (Adelberger et al. 2002) confirm this expectation at large scales, but they show a decrease of absorption at comoving separations Delta_r <~ 1 Mpc/h. We show that this discrepancy is statistically significant, especially for the innermost data point at Delta_r <= 0.5 Mpc/h, even though this data point rests on three galaxy-quasar pairs. Galaxy redshift errors of the expected magnitude are insufficient to resolve the conflict. Peculiar velocities allow gas at comoving distances >~ 1 Mpc/h to produce saturated absorption at the galaxy redshift, putting stringent requirements on any ``feedback'' solution. Local photoionization is insufficient, even if we allow for recurrent AGN activity that keeps the neutral hydrogen fraction below its equilibrium value. A simple ``wind'' model that eliminates all neutral hydrogen in spheres around the observed galaxies can marginally explain the data, but only if the winds extend to comoving radii ~1.5 Mpc/h.Comment: 4 pages, 1 figure; To appear in proceedings of the 13th Annual Astrophysics Conference in College Park, Maryland, The Emergence of Cosmic Structure, eds. S.Holt and C. Reynolds, (AIP

Intergalactic Helium Absorption in Cold Dark Matter Models

Observations from the HUT and the HST have recently detected HeII absorption along the lines of sight to two high redshift quasars. We use cosmological simulations with gas dynamics to investigate HeII absorption in the cold dark matter (CDM) theory of structure formation. We consider two Omega=1 CDM models with different normalizations and one Omega_0=0.4 CDM model, all incorporating the photoionizing UV background spectrum computed by Haardt & Madau (1996). The simulated gas distribution, combined with the H&M spectral shape, accounts for the relative observed values of taubar_HI and taubar_HeII, the effective mean optical depths for HI and HeII absorption. If the background intensity is as high as H&M predict, then matching the absolute values of taubar_HI and taubar_HeII requires a baryon abundance larger (by factors between 1.5 and 3 for the various CDM models) than our assumed value of Omega_b h^2=0.0125. The simulations reproduce the evolution of taubar_heII over the observed redshift range, 2.2 < z < 3.3, if the HeII photoionization rate remains roughly constant. HeII absorption in the CDM simulations is produced by a diffuse, fluctuating, intergalactic medium, which also gives rise to the HI ly-alpha forest. Much of the HeII opacity arises in underdense regions where the HI optical depth is very low. We compute statistical properties of the HeII and HI absorption that can be used to test the CDM models and distinguish them from an alternative scenario in which the HeII absorption is caused by discrete, compact clouds. The CDM scenario predicts that a substantial amount of baryonic material resides in underdense regions at high redshift. HeII absorption is the only sensitive probe of such extremely diffuse, intergalactic gas, so it can provide a vital test of this fundamental prediction.Comment: Accepted for publication in ApJ, 36 pages, LaTeX (aaspp4), 12 figures. Changes include addition of more information on statistical uncertainties and on the adopted UV background. Also available at http://www-astronomy.mps.ohio-state.edu/~racc

Characterization of Lyman Alpha Spectra and Predictions of Structure Formation Models: A Flux Statistics Approach

In gravitational instability models, \lya absorption arises from a continuous fluctuating medium, so that spectra provide a non-linear one-dimensional ``map'' of the underlying density field. We characterise this continuous absorption using statistical measures applied to the distribution of absorbed flux. We describe two simple members of a family of statistics which we apply to simulated spectra in order to show their sensitivity as probes of cosmological parameters (H$_{0}$, $\Omega$, the initial power spectrum of matter fluctuations) and the physical state of the IGM. We make use of SPH simulation results to test the flux statistics, as well as presenting a preliminary application to Keck HIRES data.Comment: Contribution to proceedings of the 18th Texas Symposium on Relativistic Astrophysics (eds A. Olinto, J. Frieman and D. Schramm, World Scientific),Chicago, December 1996, 3 pages, LaTeX (sprocl), 2 figures. Also available at http://www-astronomy.mps.ohio-state.edu/~racc

Sub-mm Galaxies in Cosmological Simulations

We study the predicted sub-mm emission from massive galaxies in a Lambda-CDM universe, using hydrodynamic cosmological simulations. Assuming that most of the emission from newly formed stars is absorbed and reradiated in the rest-frame far-IR, we calculate the number of galaxies that would be detected in sub-mm surveys conducted with SCUBA. The predicted number counts are strongly dependent on the assumed dust temperature and emissivity law. With plausible choices for SED parameters (e.g., T=35 K, beta=1.0), the simulation predictions reproduce the observed number counts above ~ 1 mJy. The sources have a broad redshift distribution with median z ~ 2, in reasonable agreement with observational constraints. However, the predicted count distribution may be too steep at the faint end, and the fraction of low redshift objects may be larger than observed. In this physical model of the sub-mm galaxy population, the objects detected in existing surveys consist mainly of massive galaxies (several M_*) forming stars fairly steadily over timescales ~ 10^8-10^9 years, at moderate rates ~100 Msun/yr. The typical descendants of these sub-mm sources are even more massive galaxies, with old stellar populations, found primarily in dense environments. While the resolution of our simulations is not sufficient to determine galaxy morphologies, these properties support the proposed identification of sub-mm sources with massive ellipticals in the process of formation. The most robust and distinctive prediction of this model, stemming directly from the long timescale and correspondingly moderate rate of star formation, is that the far-IR SEDs of SCUBA sources have a relative high 850 micron luminosity for a given bolometric luminosity. [Abridged]Comment: Submitted to ApJ. 34 pages including 8 PS figure

Symmetry-surfing the moduli space of Kummer K3s.

A maximal subgroup of the Mathieu group M24 arises as the combined holomorphic symplectic automorphism group of all Kummer surfaces whose Kaehler class is induced from the underlying complex torus. As a subgroup of M24, this group is the stabilizer group of an octad in the Golay code. To meaningfully combine the symmetry groups of distinct Kummer surfaces, we introduce the concepts of Niemeier markings and overarching maps between pairs of Kummer surfaces. The latter induce a prescription for symmetry-surfing the moduli space, while the former can be seen as a first step towards constructing a vertex algebra that governs the elliptic genus of K3 in an M24-compatible fashion. We thus argue that a geometric approach from K3 to Mathieu Moonshine may bear fruit.Comment: 20 pages; minor changes; accepted for publication in the Proceedings Volume of String-Math 201

Robo-line storage: Low latency, high capacity storage systems over geographically distributed networks

Rapid advances in high performance computing are making possible more complete and accurate computer-based modeling of complex physical phenomena, such as weather front interactions, dynamics of chemical reactions, numerical aerodynamic analysis of airframes, and ocean-land-atmosphere interactions. Many of these 'grand challenge' applications are as demanding of the underlying storage system, in terms of their capacity and bandwidth requirements, as they are on the computational power of the processor. A global view of the Earth's ocean chlorophyll and land vegetation requires over 2 terabytes of raw satellite image data. In this paper, we describe our planned research program in high capacity, high bandwidth storage systems. The project has four overall goals. First, we will examine new methods for high capacity storage systems, made possible by low cost, small form factor magnetic and optical tape systems. Second, access to the storage system will be low latency and high bandwidth. To achieve this, we must interleave data transfer at all levels of the storage system, including devices, controllers, servers, and communications links. Latency will be reduced by extensive caching throughout the storage hierarchy. Third, we will provide effective management of a storage hierarchy, extending the techniques already developed for the Log Structured File System. Finally, we will construct a protototype high capacity file server, suitable for use on the National Research and Education Network (NREN). Such research must be a Cornerstone of any coherent program in high performance computing and communications

Top-Down and Bottom-Up Controls on Microeukaryotic Diversity (i.e., Amplicon Analyses of SAR Lineages) and Function (i.e., Metatranscriptome Analyses) Assessed in Microcosm Experiments

The availability of high-throughput sequencing (HTS) has transformed our understanding of the diversity of microbial eukaryotes (i.e., protists) across diverse habitats. Yet relating this biodiversity to function remains a challenge, particularly in the context of microbial food webs. Here we perform a set of microcosm experiments to evaluate the impact of changing predator and prey concentrations on a marine protist community, focusing on SAR (Stramenopila, Alveolata, and Rhizaria) lineages. We combine an estimate of taxonomic diversity through analysis of SSU-rDNA amplicons with metatranscriptomics, a proxy for function. We assess changes in a community sampled from New England waters with varying concentrations of predators (copepods) and prey (phytoplanktonμm in size). The greatest impact observed is on the diversity and function of the small plankton (2–10 μm, nanoplankton) community in the presence of high prey abundance (i.e., bloom conditions). Many SAR taxa in the nanosized fraction decrease with increasing phytoplankton abundance, while ciliates (from both the nano- and microsized fractions) increase. A large number of transcripts and function estimates in the nanoplankton decreased during our simulated phytoplankton bloom. We also find evidence of an interaction between increasing phytoplankton and copepod abundances on the microsized planktonic community, consistent with the hypothesis that phytoplankton and copepods exert bottom-up control and top-down control on the microsized protists, respectively. Together our analyses suggest that community function [i.e., diversity of gene families (GFs)] remains relatively stable, while the functions at the species level (i.e., transcript diversity within GFs) show a substantial reduction of function under bloom conditions. Our study demonstrated that interactions within plankton food webs are complex, and that the relationships between diversity and function for marine microeukaryotes remain poorly understood