Large-scale clustering of Lyman-alpha emission intensity from SDSS/BOSS

(Abridged) We detect the large-scale structure of Lya emission in the Universe at redshifts z=2-3.5 by measuring the cross-correlation of Lya surface brightness with quasars in SDSS/BOSS. We use a million spectra targeting Luminous Red Galaxies at z<0.8, after subtracting a best fit model galaxy spectrum from each one, as an estimate of the high-redshift Lya surface brightness. The quasar-Lya emission cross-correlation we detect has a shape consistent with a LambdaCDM model with Omega_M =0.30^+0.10-0.07. The predicted amplitude of this cross-correlation is proportional to the product of the mean Lya surface brightness, <mu_alpha>, the amplitude of mass fluctuations, and the quasar and Lya emission bias factors. Using known values, we infer <mu_alpha>(b_alpha/3) = (3.9 +/- 0.9) x 10^-21 erg/s cm^-2 A^-1 arcsec^-2, where b_alpha is the Lya emission bias factor. If the dominant sources of Lya emission are star forming galaxies, we infer rho_SFR = (0.28 +/- 0.07) (3/b_alpha) /yr/Mpc^3 at z=2-3.5. For b_alpha=3, this value is a factor of 21-35 above previous estimates from individually detected Lya emitters, although consistent with the total rho_SFR derived from dust-corrected, continuum UV surveys. 97% of the Lya emission in the Universe at these redshifts is therefore undetected in previous surveys of Lya emitters. Our measurement is much greater than seen from stacking analyses of faint halos surrounding previously detected Lya emitters, but we speculate that it arises from similar Lya halos surrounding all luminous star-forming galaxies. We also detect redshift space anisotropy of the quasar-Lya emission cross-correlation, finding evidence at the 3.0 sigma level that it is radially elongated, consistent with distortions caused by radiative-transfer effects (Zheng et al. (2011)). Our measurements represent the first application of the intensity mapping technique to optical observations.

Gravitational redshifts from large-scale structure

The recent measurement of the gravitational redshifts of galaxies in galaxy clusters by Wojtak et al. has opened a new observational window on dark matter and modified gravity. By stacking clusters this determination effectively used the line of sight distortion of the cross-correlation function of massive galaxies and lower mass galaxies to estimate the gravitational redshift profile of clusters out to 4 Mpc/h. Here we use a halo model of clustering to predict the distortion due to gravitational redshifts of the cross-correlation function on scales from 1 - 100 Mpc/h. We compare our predictions to simulations and use the simulations to make mock catalogues relevant to current and future galaxy redshift surveys. Without formulating an optimal estimator, we find that the full BOSS survey should be able to detect gravitational redshifts from large-scale structure at the ~4 sigma level. Upcoming redshift surveys will greatly increase the number of galaxies useable in such studies and the BigBOSS and Euclid experiments should be capable of measurements with precision at the few percent level. As has been recently pointed out by McDonald, Kaiser and Zhao et al, other interesting effects including relativistic beaming and transverse Doppler shift can add additional asymmetric distortions to the correlation function. While these contributions are subdominant to the gravitational redshift on large scales, they represent additional opportunities to probe gravitational physics and indicate that many qualitatively new measurements should soon be possible using large redshift surveys.

Terapixel imaging of cosmological simulations

The increasing size of cosmological simulations has led to the need for new visualization techniques. We focus on Smoothed Particle Hydrodynamical (SPH) simulations run with the GADGET code and describe methods for visually accessing the entire simulation at full resolution. The simulation snapshots are rastered and processed on supercomputers into images that are ready to be accessed through a web interface (GigaPan). This allows any scientist with a web-browser to interactively explore simulation datasets in both in spatial and temporal dimensions, datasets which in their native format can be hundreds of terabytes in size or more. We present two examples, the first a static terapixel image of the MassiveBlack simulation, a P-GADGET SPH simulation with 65 billion particles, and the second an interactively zoomable animation of a different simulation with more than one thousand frames, each a gigapixel in size. Both are available for public access through the GigaPan web interface. We also make our imaging software publicly available.

Galaxy morphology, kinematics and clustering in a hydrodynamic simulation of a LambdaCDM universe

We explore galaxy properties and their link with environment and clustering using a population of ~1000 galaxies formed in a high resolution hydrodynamic simulation of the Lambda CDM cosmology. At the redshift we concentrate on, z=1, the spatial resolution is 1.4 proper kpc/h and Milky-way sized disk galaxies contain ~10^5 particles within their virial radii. We include supermassive black hole accretion and feedback as well as a multiphase model for star formation. We find that a number of familiar qualitative relationships hold approximately between galaxy properties, for example, galaxies lie between two broad extremes of type, where ``late types tend to be smaller in size, have lower circular velocities, younger stars, higher star formation rates, larger disk to bulge ratios and lower Sersic indices than ``early types. As in previous studies the stellar component of disk galaxies is not as rotationally supported as in observations. Bulges contain too much of the stellar mass, although disks do have scale lengths compatible with observations. The addition of black hole physics to the simulations does not appear to have an impact on the angular momentum results, nor do we find that it is affected in an identical simulation with significantly lower mass resolution. Despite this, we can profitably use the rank order of either disk to total ratio, Sersic index, or galaxy age to separate galaxies into morphological classes and examine the density-morphology relation and morphology dependence of clustering. We find that while at redshift z=0, the well known preponderance of early types in dense environments is seen, at z=1 the density-morphology relation becomes flatter and late type galaxies are even seen to have a higher clustering amplitude than early types (abridged).

Radiation-induced large-scale structure during the reionization epoch: the autocorrelation function

The structures produced during the epoch of reionization by the action of radiation on neutral hydrogen are in principle different to those that arise through gravitational growth of initially small perturbations. We explore the difference between the two mechanisms using high resolution cosmological radiative transfer. Our computations use a Monte Carlo code which raytraces directly through SPH kernels without a grid, preserving the high spatial resolution of the underlying hydrodynamic simulation. Because the properties of the first sources of radiation are uncertain, we simulate a range of models with different source properties and recombination physics. We examine the morphology of the neutral hydrogren distribution and the reionization history in these models. We find that at fixed mean neutral fraction, structures are visually most affected by the existence of a lower limit in source luminosity, then by galaxy mass to light ratio, and are minimally affected by changes in the recombination rate and amplitude of mass fluctuations. We concentrate on the autocorrelation function of the neutral hydrogen, xi_HI(r) as a basic quantitive measure of Radiation Induced Structure (RIS.) All the models we test exhibit a characteristic behaviour, with xi_HI(r) becoming initially linearly antibiased with respect to the matter correlation function, reaching a minimum bias factor b~0.5 when the universe is ~10-20% ionized. After this xi_HI(r) increases rapidly in amplitude, overtaking the matter correlation function. It keeps a power law shape, but flattens considerably, reaching an asymptotic logarithmic slope of gamma ~-0.5. The growth rate of HI fluctuations is exponentially more rapid than gravitational growth over a brief interval of redshift Deltaz ~ 2-3.