We present results from a large volume simulation of Hydrogen reionization. We combine 3d radiative transfer calculations and an N-body simulation, describing structure formation in the intergalactic medium (IGM), to detail the growth of HII regions around high redshift galaxies. Our N-body simulation tracks 1024 3 dark matter particles, in a cubical box of co-moving side length L box = 65.6 Mpc h −1 . This large volume allows us to accurately characterize the size distribution of HII regions throughout most of the reionization process. At the same time, our simulation resolves many of the small galaxies likely responsible for reionization. It confirms a picture anticipated by analytic models: HII regions grow collectively around highly-clustered sources, and have a well-defined characteristic size, which evolves from a sub-Mpc scale at the beginning of reionization to R 10 co-moving Mpc towards the end. We show that in order to obtain this qualitative picture, source resolution must not be sacrificed at too great a level. We present a detailed statistical description of our results, and compare them with a numerical hybrid scheme based on the analytic model by Furlanetto, Zaldarriaga, and Hernquist. This model associates HII regions with large-scale overdensities and is based on the excursion set formalism. We find that the analytic calculation reproduces the size distribution of HII regions, the power spectrum of the ionization field, and the 21 cm power spectrum of the full radiative transfer simulation remarkably well. The ionization field from the radiative transfer simulation, however, has more small scale structure than the analytic calculation, owing to Poisson scatter in the simulated abundance of galaxies on small scales. We propose and validate a simple scheme to incorporate this scatter into our calculations. Our results suggest that analytic calculations are sufficiently accurate to aid in predicting and interpreting the results of future 21 cm surveys. In particular, our fast numerical scheme is useful for forecasting constraints from future 21 cm surveys, and in constructing mock surveys to test data analysis procedures. Subject headings: cosmology: theory -intergalactic medium -large scale structure of universe

It is possible that the properties of H ii regions during reionization depend sensitively on many poorly constrained quantities [the nature of the ionizing sources, the clumpiness of the gas in the intergalactic medium (IGM), the degree to which photoionizing feedback suppresses the abundance of low‐mass galaxies, etc.], making it extremely difficult to interpret upcoming observations of this epoch. We demonstrate that the actual situation is more encouraging, using a suite of radiative transfer simulations, post‐processed on outputs from a 10243, 94‐Mpc N‐body simulation. Analytic prescriptions are used to incorporate small‐scale structures that affect reionization, yet remain unresolved in the N‐body simulation. We show that the morphology of the H ii regions for reionization by POPII‐like stars is most dependent on the global ionization fraction . Changing other parameters by an order of magnitude for fixed often results in similar bubble sizes and shapes. The next most important dependence is on the properties of the ionizing sources. The rarer the sources, the larger and more spherical the H ii regions become. The typical bubble size can vary by as much as a factor of 4 at fixed between different possible source prescriptions. The final relevant factor is the abundance of minihaloes or of Lyman‐limit systems. These systems suppress the largest bubbles from growing, and the magnitude of this suppression depends on the thermal history of the gas as well as the rate at which these systems are photo‐evaporated. We find that neither source suppression owing to photo‐heating nor small‐scale gas clumping significantly affects the large‐scale structure of the H ii regions, with the ionization fraction power spectrum at fixed differing by less than 20 per cent for k < 5 Mpc−1 between all the source suppression and clumping models we consider. Analytic models of reionization are successful at predicting many of the features seen in our simulations. We discuss how observations of the 21‐cm line with the Mileura Widefield Array (MWA) and the Low Frequency Array (LOFAR) can constrain properties of reionization, and we study the effect patchy reionization has on the statistics of Lyα emitting galaxies.

We study the origin and properties of "extra" or "excess" central light in the surface brightness profiles of cusp or power-law elliptical galaxies. Dissipational mergers give rise to two-component profiles: an outer profile established by violent relaxation acting on stars already present in the progenitor galaxies prior to the final stages of the merger, and an inner stellar population comprising the extra light, formed in a compact central starburst. By combining a large set of hydrodynamical simulations with data that span a broad range of profiles at various masses, we show that observed cusp ellipticals appear consistent with the predicted "extra light" structure, and we use our simulations to motivate a two-component description of the observations that allows us to examine how the properties and mass of this component scale with e.g. the mass, gas content, and other properties of the galaxies. We show how to robustly separate the physically meaningful extra light and outer, violently relaxed profile, and demonstrate that the observed cusps and "extra light" are reliable tracers of the degree of dissipation in the spheroid-forming merger. We show that the typical degree of dissipation is a strong function of stellar mass, roughly tracing the observed gas fractions of disks of the same mass over the redshift range z ∼ 0 − 2. We demonstrate a correlation between the strength of this component and effective radius at fixed mass, in the sense that systems with more dissipation are more compact, sufficient to explain the discrepancy in the maximum phase-space and mass densities of ellipticals and their progenitor spirals. We show that the outer shape of the light profile in simulated and observed systems (when fit to properly account for the central light) does not depend on mass, with a mean outer Sérsic index ∼ 2.5. We also explore how this relates to e.g. the shapes, kinematic properties, and stellar population gradients of ellipticals. Extra light contributes to making remnants rounder and diskier, and imprints stellar population gradients. Simulations with the gas content needed to match observed surface brightness profiles reproduce the observed age, metallicity, and color gradients of cusp ellipticals, and we make predictions for how these can be used as tracers of the degree of dissipation in spheroid formation.

We use numerical simulations to study the kinematic structure of remnants formed from mergers of equal-mass disk galaxies. In particular, we show that remnants of dissipational mergers, which include the radiative cooling of gas, star formation, feedback from supernovae, and the growth of supermassive black holes, are smaller, rounder, have, on average, a larger central velocity dispersion, and show significant rotation compared to remnants of dissipationless mergers. The increased rotation speed of dissipational remnants owes its origin to star formation that occurs in the central regions during the galaxy merger. We have further quantified the anisotropy, three-dimensional shape, minor axis rotation, and isophotal shape of each merger remnant, finding that dissipational remnants are more isotropic, closer to oblate, have the majority of their rotation along their major axis, and are more disky than dissipationless remnants. Individual remnants display a wide variety of kinematic properties. A large fraction of the dissipational remnants are oblate isotropic rotators. Many dissipational, and all of the dissipationless, are slowly rotating and anisotropic. The remnants of gas-rich major mergers can well-reproduce the observed distribution of projected ellipticities, rotation parameter (V /σ) * , kinematic misalignments, Ψ, and isophotal shapes. The dissipationless remnants are a poor match to this data. We also investigate the properties of merger remnants as a function of initial disk gas fraction, orbital angular momentum, and the mass of the progenitor galaxies. Our results support the merger hypothesis for the origin of low-luminosity elliptical galaxies provided that the progenitor disks are sufficiently gas-rich, however our remnants are a poor match to the bright ellipticals that are slowly rotating and uniformly boxy.

Observations of the intergalactic medium (IGM) suggest that quasars reionize He ii in the IGM at z ≈ 3. We have run a set of 190 and 430 comoving Mpc simulations of He ii being reionized by quasars to develop an understanding of the nature of He ii reionization and its potential impact on observables. We find that He ii reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ∼ 12,000 K and with large temperature fluctuations on ∼ 50 Mpc scales. Much of this heating occurs far from quasars by photons with long mean free path. We find a temperature-density equation of state of γ − 1 ≈ 0.3 during He ii reionization, but with a wide dispersion in this relation having σ T ∼ 10 4 K. He ii reionization by the observed population of quasars cannot produce an inverted relation (γ − 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the He ii Lyα forest. We argue that the heat input from He ii reionization is unable to cause the observed depression at z ≈ 3.2 in the H i Lyα forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of He ii Lyman limit systems influence our predictions.