Lunar Orbit Measurement of Cosmic Dawn 21 cm Global Spectrum

Abstract

A redshifted 21 cm line absorption signature is commonly expected from the cosmic dawn era, when the first stars and galaxies formed. The detailed traits of this signal can provide important insight on the cosmic history. However, high precision measurement of this signal is hampered by the ionosphere refraction and absorption, as well as radio frequency interference (RFI). A space observation can solve the problem of the ionosphere, and the Moon can shield the RFI from the Earth. In this paper, we present simulations of the global spectrum measurement in the 30 -- 120 MHz frequency band on the lunar orbit, from the proposed Discovering the Sky at the Longest wavelength (DSL) project. In particular, we consider how the measured signal varies as the satellite moves along the orbit, take into account the blockage of different parts of the sky by the Moon and the antenna response. We estimate the sensitivity for such a 21 cm global spectrum experiment. An RMS noise level of $\le 0.05$ K is expected at 75 MHz after 10 orbits ($\sim$ 1 day) observation, for a frequency channel width of 0.4 MHz. We also study the influence of a frequency-dependent beam, which may generate complex spectral structures in the spectrum. Estimates of the uncertainties in the foreground and 21 cm model parameters are obtained.

14 pages, 16 figures, accepted for publication in ApJ

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• A new method of reconstructing Galactic three-dimensional structures using ultralong-wavelength radio observations
• STARFIRE: An algorithm for estimating radio frequency interference in orbits around Earth
• Forecast measurement of the 21 cm global spectrum from Lunar orbit with the Vari-Zeroth-Order Polynomial (VZOP) method
• On Measuring the 21 cm Global Spectrum of the Cosmic Dawn with an Interferometer Array
• Detecting Cosmic 21 cm Global Signal Using an Improved Polynomial Fitting Algorithm
• Astrophysical uncertainties challenge 21-cm forecasts: A primordial black hole case study
• Synthesis imaging with a lunar orbit array: I. global sky map and its systematics
• A Multi-Mechanism Hybrid Model of Peaked-Spectrum Radio Sources
• Recovering 21cm Monopole Signals Without Smoothness
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