Abstract
Detection of 21~cm emission of HI from the epoch of reionization, at redshifts z>6, is limited primarily by foreground emission. We investigate the signatures of wide-field measurements and an all-sky foreground model using the delay spectrum technique that maps the measurements to foreground object locations through signal delays between antenna pairs. We demonstrate interferometric measurements are inherently sensitive to all scales, including the largest angular scales, owing to the nature of wide-field measurements. These wide-field effects are generic to all observations but antenna shapes impact their amplitudes substantially. A dish-shaped antenna yields the most desirable features from a foreground contamination viewpoint, relative to a dipole or a phased array. Comparing data from recent Murchison Widefield Array observations, we demonstrate that the foreground signatures that have the largest impact on the HI signal arise from power received far away from the primary field of view. We identify diffuse emission near the horizon as a significant contributing factor, even on wide antenna spacings that usually represent structures on small scales. For signals entering through the primary field of view, compact emission dominates the foreground contamination. These two mechanisms imprint a characteristic "pitchfork" signature on the "foreground wedge" in Fourier delay space. Based on these results, we propose that selective down-weighting of data based on antenna spacing and time can mitigate foreground contamination substantially by a factor ~100 with negligible loss of sensitivity.
Highlights
At the end of the recombination epoch, the universe was completely neutral
We report two important findings: foregrounds that most severely obscure the redshifted 21 cm power spectrum are not caused by emission in the central field of view, but rather by bright objects from near the horizon; and, diffuse Galactic emission plays a significant role hitherto unpredicted
Our primary motivation in this work is to understand how the various bright foregrounds will manifest in three-dimensional power spectrum of H I from 21 cm reionization observations
Summary
At the end of the recombination epoch, the universe was completely neutral. This period, referred to as the Dark Ages in the universe’s history, is characterized by the localized accumulation of matter under the influence of gravity. Morales & Hewitt (2004) show that the inherent isotropy and symmetry of the EoR signal in frequency and spatial wavenumber (k) space make it distinguishable from sources of contamination that are isolated to certain k modes by virtue of their inherent spectral smoothness (Morales et al 2006; Bowman et al 2009; Liu & Tegmark 2011; Parsons et al 2012b; Dillon et al 2013; Pober et al 2013) Since this contamination is expected to be several orders of magnitude stronger than the underlying EoR H I signal, it is critical to characterize foregrounds precisely in order to reduce their impact on EoR H I power spectrum detection sensitivity.
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