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

Abstract

We forecast the sensitivity with which the Murchison Widefield Array (MWA) can measure the 21 cm power spectrum of cosmic hydrogen. The MWA is sensitive to roughly a decade in scale (wavenumbers of k $ 0:1Y1 h Mpc À1 ). This amounts primarily to constraints on two numbers: the amplitude and the slope of the 21 cm power spectrum on the scales probed. We find, however, that the redshift evolution in these quantities can yield important information about reionization. We examine a range of theoretical models, spanning uncertainties in the nature of the ionizing sources and the abundance of minihalos during reionization. Although the power spectrum differs substantially among these models, a generic prediction is that the amplitude of the 21 cm power spectrum on MWA scales (k $ 0:4 h Mpc À1 ) peaks near the epoch when the intergalactic medium (IGM) is %50% ionized. Moreover, the slope of the 21 cm power spectrum flattens as the ionization fraction increases and the sizes of the H ii regions grow. With regards to detection sensitivity, we show that the optimal MWA antenna configuration for power spectrum measurements would pack all 500 antenna tiles as closely as possible in a compact core. Detecting the characteristic redshift evolution of our models will help to confirm that observed 21 cm fluctuations originate from the IGM, and not from foregrounds, and will provide an indirect constraint on the evolution of the volume-filling factor of H ii regions during reionization. After two years of observations, the MWA can constrain the filling factor at an epoch when x i h i$ 0:5 to within roughly AE x i h i$ 0:1 at 2 confidence. Subject headingg s: cosmology: theory -intergalactic medium -large-scale structure of universe

PAPER-64 CONSTRAINTS ON REIONIZATION: THE 21 cm POWER SPECTRUM ATz= 8.4

“…The triangles indicate 2 σ upper limits from GMRT (Paciga et al 2011) (yellow) at z = 8.6, MWA ) at z = 9.5 (magenta), and the previous PAPER upper limit (P14) at z = 7.7 (green). The magenta curve shows a predicted model 21 cm power spectrum at 50% ionization (Lidz et al 2008). the error bars on the spherically averaged power spectrum P(k), where positive and negative k  measurements are kept separate for diagnostic purposes. In the estimation of the dimensionless power spectrum…”

Probing the first galaxies with the Square Kilometer Array

“…Given a detailed model and small error bars this could be measured directly from the shape of the power spectrum. More generally, if measured over only a small range of wave-numbers, the redshift dependence of x α could be extracted by looking at the redshift evolution of the amplitude and slope in a manner analogous to that suggested by Lidz et al (2008) for reionization. Note that, although during Lyα domination different models have similar power spectra for the same x α , its evolution with redshift could be used to distinguish these models.…”

Reionization by UV or X-ray sources

“…With this amplitude, and ignoring the issue of foreground cleaning 1 LOw Frequency ARray, http://www.lofar.org 2 Murchison Widefield Array, http://mwatelescope.org/ 3 Giant Metrewave Radio Telescope, http://www.gmrt.ncra. tifr.res.in 4 21 Centimeter Array, http://21cma.bao.ac.cn/ 5 Square Kilometre Array, http://www.skatelescope.org/ residuals, statistical quantities such as the three-dimensional power spectrum should be measurable with LOFAR or MWA with a few 100 h integration (Morales & Hewitt 2004;Lidz et al 2008). In absorption however, the amplitude of the fluctuations may exceed 100 mK (Gnedin 2004;Santos et al 2008;Baek et al 2009), the exact level depending on the relative contribution of the X-ray and UV sources to the process of cosmic reionization.…”