Matthew McQuinn

Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization

A number of radio interferometers are currently being planned or constructed to observe 21 cm emission from reionization. Not only will such measurements provide a detailed view of that epoch, but, since the 21 cm emission also traces the distribution of matter in the universe, this signal can be used to constrain cosmological parameters. The sensitivity of an interferometer to the cosmological information in the signal may depend on how precisely the angular dependence of the 21 cm three-dimensional power spectrum can be measured. Using an analytic model for reionization, we quantify all the effects that break the spherical symmetry of the three-dimensional 21 cm power spectrum. We find that upcoming observatories will be sensitive to the 21 cm signal over a wide range of scales, from larger than 100 to as small as 1 comoving Mpc. Next, we consider three methods to measure cosmological parameters from the signal: (1) direct fitting of the density power spectrum to the signal, (2) using only the velocity field fluctuations in the signal, and (3) looking at the signal at large enough scales that all fluctuations trace the density field. With the foremost method, the first generation of 21 cm observations should moderately improve existing constraints on cosmological parameters for certain low-redshift reionization scenarios, and a 2 yr observation with the second-generation interferometer MWA5000 in combination with the CMB telescope Planck could improve constraints on Ω_w, Ω_(m)h^2, Ω_(b)h^2, Ω_ν, n_s, and α_s. If the universe is substantially ionized by z ~ 12 or if spin temperature fluctuations are important, we show that it will be difficult to place competitive constraints on cosmological parameters with any of the considered methods.

The morphology of H ii regions during reionization

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 1024 3 , 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 x i . Changing other parameters by an order of magnitude for fixed x i 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 x i 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 x i 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.

Simulations and analytic calculations of bubble growth during hydrogen reionization

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, to detail the growth of H II regions around high-redshift galaxies. Our simulation tracks 10243 dark matter particles, in a box of comoving side length 65.6 Mpc h-1. This large volume allows us to accurately characterize the size distribution of H II 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: H II 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 Mpc toward the end. We present a detailed statistical description of our results and compare them with a numerical scheme based on the analytic model by Furlanetto and coworkers. We find that the analytic calculation reproduces the size distribution of H II regions and the 21 cm power spectrum of the radiative transfer simulation remarkably well. The ionization field from the 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.

Studying reionization with Lyα emitters

We show that observations of high-redshift Lyα emitters (LAEs) have the potential to provide definitive evidence for reionization in the near future. Using 200-Mpc radiative transfer simulations, we calculate the effect that patchy reionization has on the line profile, on the luminosity function, and, most interestingly, on the clustering of emitters for several realistic models of reionization. Reionization increases the measured clustering of emitters, and we show that this enhancement would be essentially impossible to attribute to anything other than reionization. Our results motivate looking for the signature of reionization in existing LAE data. We find that for stellar reionization scenarios the angular correlation function of the 58 LAEs in the Subaru Deep Field 2 = 6.6 photometric sample is more consistent with a fully ionized universe (mean volume ionized fraction x i ≈ 1) than a universe with x i 2σ confidence level. Measurements in the next year on Subaru will increase their z = 6.6 LAE sample by a factor of 5 and tighten these limits. If the clustering signature of reionization is detected in an LAE survey, a comparison with a Lyman-break or an Ha survey in the same field would confirm the reionization hypothesis. We discuss the optimal LAE survey specifications for detecting reionization, with reference to upcoming programmes.

He II reionization and its effect on the intergalactic medium

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 ? ? 1 0.3 during He?II reionization, but with a wide dispersion in this relation having ? T ~ 104 K. He?II reionization by the observed population of quasars cannot produce an inverted relation (? ? 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the He?II Ly? 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 Ly? 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.

How accurately can 21 cm tomography constrain cosmology

There is growing interest in using 3-dimensional neutral hydrogen mapping with the redshifted 21 cm line as a cosmological probe. However, its utility depends on many assumptions. To aid experimental planning and design, we quantify how the precision with which cosmological parameters can be measured depends on a broad range of assumptions, focusing on the 21 cm signal from

Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array

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ON LYMAN-LIMIT SYSTEMS AND THE EVOLUTION OF THE INTERGALACTIC IONIZING BACKGROUND

. We cover assumptions related to modeling of the ionization power spectrum, to the experimental specifications like array layout and detector noise, to uncertainties in the reionization history, and to the level of contamination from astrophysical foregrounds. We derive simple analytic estimates for how various assumptions affect an experiments sensitivity, and we find that the modeling of reionization is the most important, followed by the array layout. We present an accurate yet robust method for measuring cosmological parameters that exploits the fact that the ionization power spectra are rather smooth functions that can be accurately fit by 7 phenomenological parameters. We find that for future experiments, marginalizing over these nuisance parameters may provide constraints almost as tight on the cosmology as if 21 cm tomography measured the matter power spectrum directly. A future square kilometer array optimized for 21 cm tomography could improve the sensitivity to spatial curvature and neutrino masses by up to 2 orders of magnitude, to

QUASAR PROXIMITY ZONES AND PATCHY REIONIZATION

\ensuremath{\Delta}{\ensuremath{\Omega}}_{k}\ensuremath{\approx}0.0002

The kinetic Sunyaev–Zel'dovich effect from reionization

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