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 ionosphere refraction and absorption, as well as radio frequency interference (RFI). Space observation can solve the problem of the ionosphere, and the Moon can shield the RFI from 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 project. In particular, we consider how the measured signal varies as the satellite moves along the orbit and 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 ≤0.05 K is expected at 75 MHz after 10 orbits (∼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 structures in the spectrum. Estimates of the uncertainties in the foreground and 21 cm model parameters are obtained.
The cross-correlation of optical galaxies with the neutral hydrogen (HI) radiation intensity can enhance the signal-to-noise ratio (SNR) of the H i intensity measurement. In this paper, we investigate the cross-correlation of the galaxy samples obtained by the spectroscopic survey of the China Space Station Telescope (CSST) with the H i Intensity mapping (IM) survey of the Five-hundred-meter Aperture Spherical Telescope (FAST). Using the IllusitrisTNG simulation result at redshift 0.2 ∼ 0.3, we generate mock data of the CSST survey and a FAST L-band drift scan survey. The CSST spectroscopic survey can yield a sample of galaxies with a high comoving number density of $10^{-2} ({\ \rm Mpc}/h)^{-3}$ at z ∼ 0.3. We cross-correlate the foreground-removed radio intensity with the CSST galaxies, including both the whole sample, and red and blue galaxy sub-samples separately. We find that in all cases the H i and optical galaxies are well correlated. The total H i abundance can be measured with a high precision from this correlation. A relative error of $\sim 0.6{{\%}}$ for $\Omega _{\rm H\, \small {I}}$ could be achieved at z ∼ 0.3 for an overlapping survey area of $10000 {\ \rm deg}^2$.
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