Constraining cosmology and ionization history with combined 21 cm power spectrum and global signal measurements

MNRAS 000, 1–12 (2015) Preprint 30 October 2015 Compiled using MNRAS L A TEX style file v3.0 Constraining cosmology and ionization history with combined 21 cm power spectrum and global signal measurements Adrian Liu, 1, 2? † Aaron R. Parsons 1, 3 1 Department of Astronomy, University of California Berkeley, Berkeley, CA 94720, USA Center for Cosmological Physics, University of California Berkeley, Berkeley, CA 94720, USA 3 Radio Astronomy Laboratory, University of California Berkeley, Berkeley, CA 94720, USA arXiv:1510.08815v1 [astro-ph.CO] 29 Oct 2015 2 Berkeley 30 October 2015 ABSTRACT Improvements in current instruments and the advent of next-generation instruments will soon push observational 21 cm cosmology into a new era, with high significance measurements of both the power spectrum and the mean (“global”) signal of the 21 cm brightness temperature. In this paper we use the recently commenced Hydrogen Epoch of Reionization Array as a worked example to provide forecasts on astrophysical and cosmological parameter constraints. In doing so we improve upon previous forecasts in a number of ways. First, we provide updated forecasts using the latest best-fit cosmo- logical parameters from the Planck satellite, exploring the impact of different Planck datasets on 21 cm experiments. We also show that despite the exquisite constraints that other probes have placed on cosmological parameters, the remaining uncertain- ties are still large enough to have a non-negligible impact on upcoming 21 cm data analyses. While this complicates high-precision constraints on reionization models, it provides an avenue for 21 cm reionization measurements to constrain cosmology. We additionally forecast HERA’s ability to measure the ionization history using a com- bination of power spectrum measurements and semi-analytic simulations. Finally, we consider ways in which 21 cm global signal and power spectrum measurements can be combined, and propose a method by which power spectrum results can be used to train a compact parameterization of the global signal. This parameterization reduces the number of parameters needed to describe the global signal, increasing the likelihood of a high significance measurement. Key words: dark ages, reionization, first stars – methods: data analysis – techniques: interferometric – radio lines: general INTRODUCTION In coming years, experiments targeting the highly redshifted 21 cm line will transform both astrophysics and cosmology. Large scale cosmological surveys of the 21 cm brightness temperature probe the density, ionization state, and spin temperature of hydrogen atoms in the intergalactic medium (IGM), making them powerful direct probes of the reioniza- tion epoch, when the first generation of galaxies systemati- cally ionized the IGM. Such measurements promise to place exquisite constraints on the parameters governing these first galaxies that have hitherto been essentially unconstrained (Pober et al. 2014; Greig & Mesinger 2015). 21 cm cosmology Hubble Fellow † E-mail: acliu@berkeley.edu c 2015 The Authors also has the potential to probe the pre-reionization epoch, providing insight into heating mechanisms in our early Uni- verse, whether from X-ray sources (Christian & Loeb 2013, Mesinger et al. 2014; Ewall-Wice et al., submitted), struc- ture formation shocks (Gnedin & Shaver 2004), or exotic phenomena such as dark matter annihilation (Vald´es et al. 2013; Evoli et al. 2014). From the standpoint of cosmology, a better understanding of reionization via 21 cm cosmology removes a key nuisance from cosmic microwave background (CMB) datasets, allowing improved constraints on cosmo- logical parameters such as the amplitude of primordial scalar fluctuations (Clesse et al. 2012; Liu et al. 2015). With the 21 cm line potentially capable of accessing larger fractions of our observable Universe than any other cosmological probe, futuristic surveys will not only further refine our limits on fundamental parameters (McQuinn et al. 2006; Mao et al.