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ABSTRACT We present a comprehensive simulation-based study of the bayeseor code for 21 cm power spectrum recovery when analytically marginalizing over foreground parameters. To account for covariance between the 21 cm signal and contaminating foreground emission, bayeseor jointly constructs models for both signals within a Bayesian framework. Due to computational constraints, the forward model is constructed using a restricted field of view (FoV) in the image domain. When the only Epoch of Reionization contaminants are noise and foregrounds, we demonstrate that bayeseor can accurately recover the 21 cm power spectrum when the component of sky emission outside this forward-modelled region is downweighted by the beam at the level of the dynamic range between the foreground and 21 cm signals. However, when all-sky foreground emission is included along with a realistic instrument primary beam with sidelobes above this threshold extending to the horizon, the recovered power spectrum is contaminated by unmodelled sky emission outside the restricted FoV model. Expanding the combined cosmological and foreground model to cover the whole sky is computationally prohibitive. To address this, we present a modified version of bayeseor that allows for an all-sky foreground model, while the modelled 21 cm signal remains only within the primary FoV of the telescope. With this modification, it will be feasible to run an all-sky bayeseor analysis on a sizeable compute cluster. We also discuss several future directions for further reducing the need to model all-sky foregrounds, including wide-field foreground subtraction, an image-domain likelihood utilizing a tapering function, and instrument primary beam design.
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Low-frequency Radio Recombination Lines Away from the Inner Galactic Plane
Diffuse radio recombination lines (RRLs) in the Galaxy are possible foregrounds for redshifted 21 cm experiments. We use EDGES drift scans centered at −26.°7 decl. to characterize diffuse RRLs across the southern sky. We find that RRLs averaged over the large antenna beam (72° × 110°) reach minimum amplitudes of R.A. = 2–6 hr. In this region, the Cα absorption amplitude is 33 ± 11 mK (1σ) averaged over 50–87 MHz (27 ≳ z ≳ 15 for the 21 cm line) and increases strongly as frequency decreases. Cβ and Hα lines are consistent with no detection with amplitudes of 13 ± 14 and 12 ± 10 mK (1σ), respectively. At 108–124.5 MHz (z ≈ 11) in the same region, we find no evidence for carbon or hydrogen lines at the noise level of 3.4 mK (1σ). Conservatively assuming that observed lines come broadly from the diffuse interstellar medium, as opposed to a few compact regions, these amplitudes provide upper limits on the intrinsic diffuse lines. The observations support expectations that Galactic RRLs can be neglected as significant foregrounds for a large region of sky until redshifted 21 cm experiments, particularly those targeting cosmic dawn, move beyond the detection phase. We fit models of the spectral dependence of the lines averaged over the large beam of EDGES, which may contain multiple line sources with possible line blending, and find that including degrees of freedom for expected smooth, frequency-dependent deviations from local thermodynamic equilibrium (LTE) is preferred over simple LTE assumptions for Cα and Hα lines. For Cα we estimate departure coefficients 0.79 < b_n β_n < 4.5 along the inner Galactic plane and 0 < b_n β_n < 2.3 away from the inner Galactic plane.
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- PAR ID:
- 10477709
- Publisher / Repository:
- The Astronomical Journal
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 167
- Issue:
- 1
- ISSN:
- 0004-6256
- Page Range / eLocation ID:
- 2
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-3881/ad08ba
