21-cm line intensity mapping
The 21-cm line emission arises from quantum spin-flip transitions in neutral hydrogen atoms (HI), with a rest-frame frequency of 1420 MHz. It is the most important spectral line in radio astronomy for probing matter, given the abundance of hydrogen in the Universe. The 21-cm line intensity mapping is a novel observational technique to map fluctuations in the large-scale distribution of matter in the Universe, using the redshifted 21-cm line emission from HI (e.g., Battye et al., MNRAS 2004). By measuring temperature fluctuations of the HI 21-cm signal across different radio frequencies, this method enables a tomographic reconstruction of the matter distribution at different redshifts through spectroscopic observations of the redshifted line. It provides a competitive alternative to optical galaxy surveys for mapping large-scale structure across wider redshift volumes. HI intensity mapping experiments such as BINGO, a SKAO pathfinder, have the potential to detect baryon acoustic oscillations (BAO) in the HI power spectrum at low redshifts (𝑧 ≤ 1) and constrain the properties of dark energy. However, HI observations from BINGO and SKAO will be heavily contaminated by astrophysical foregrounds, in particular Galactic synchrotron emission, which is at least four orders of magnitude stronger than the cosmological signal. Component separation is therefore recognised as the key challenge for future radio astronomy surveys such as SKAO and BINGO.
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Extracting HI cosmological signal with generalized needlet internal linear combination
Olivari, Remazeilles, Dickinson, MNRAS 456, 2749 (2016)
With a former PhD student, Lucas Olivari, we explored the adaptation of CMB component-separation methods to line intensity mapping for the clean extraction of the cosmological HI 21-cm signal. We extended the GNILC method (Remazeilles et al., MNRAS 2011) to mitigate radio foregrounds and recover the cosmological HI 21-cm signal at different redshifts. Because the HI 21-cm emission frequency spectrum is largely random and unknown, the GNILC method relies on prior knowledge of the overall shape of the HI power spectrum (i.e., the matter power spectrum) to disentangle the HI signal from Galactic foregrounds—similar to the separation of CIB and Galactic dust in CMB observations (Planck Collaboration XLVIII, A&A 2016). Our results show that the method is robust to the complexity of radio foregrounds (synchrotron, free-free, extragalactic radio sources) and relatively insensitive to the choice of priors on the theoretical HI power spectrum. Using realistic simulations of the radio sky, we demonstrated that GNILC can reconstruct BAOs in the HI power spectrum at the different redshifts probed by the BINGO telescope, currently under construction in Brazil. The GNILC method for 21-cm intensity mapping, developed in this work, has since been applied to forecast the performance of future radio telescopes, including BINGO (Fornazier et al., A&A 2022; de Mericia et al., A&A 2023) and SKAO (De Caro et al., 2025).