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We present a comprehensive set of forecasts for the cross-correlation signal between 21 cm intensity mapping and galaxy redshift surveys. We focus on the data sets that will be provided by the SKAO for the 21 cm signal, DESI and Euclid for galaxy clustering. We build a likelihood which takes into account the effect of the beam for the radio observations, the Alcock–Paczynski effect, a simple parametrization of astrophysical nuisances, and fully exploit the tomographic power of such observations in the range z  = 0.7–1.8 at linear and mildly non-linear scales (k < 0.25 h  Mpc−1). The forecasted constraints, obtained with Monte Carlo Markov Chains techniques in a Bayesian framework, in terms of the six base parameters of the standard ΛCDM model, are promising. The predicted signal-to-noise ratio for the cross-correlation can reach ∼50 for z ∼ 1 and k ∼ 0.1 h  Mpc−1. When the cross-correlation signal is combined with current Cosmic Microwave Background (CMB) data from Planck, the error bar on |$\Omega _{\rm c}\, h^2$| and H 0 is reduced by factors 3 and 6, respectively, compared to CMB only data, due to the measurement of matter clustering provided by the two observables. The cross-correlation signal has a constraining power that is comparable to the autocorrelation one and combining all the clustering measurements a sub-per cent error bar of 0.33 per cent on H 0 can be achieved, which is about a factor 2 better than CMB only measurements. Finally, as a proof of concept, we test the full pipeline on the real data measured by the MeerKat collaboration (Cunnington et al. 2022) presenting some (weak) constraints on cosmological parameters. [ABSTRACT FROM AUTHOR]

The measurement of the large-scale distribution of neutral hydrogen in the late Universe, obtained with radio telescopes through the hydrogen 21 cm line emission, has the potential to become a key cosmological probe in the upcoming years. We explore the constraining power of 21 cm intensity mapping observations on the full set of cosmological parameters that describe the ΛCDM model. We assume a single-dish survey for the SKA Observatory and simulate the 21 cm linear power spectrum monopole and quadrupole within six redshift bins in the range |$z$|  = 0.25–3. Forecasted constraints are computed numerically through Markov Chain Monte Carlo techniques. We extend the sampler CosmoMC by implementing the likelihood function for the 21 cm power spectrum multipoles. We assess the constraining power of the mock data set alone and combined with Planck 2018 CMB observations. We find that 21 cm multipoles observations alone are enough to obtain constraints on the cosmological parameters comparable with other probes. Combining the 21 cm data set with CMB observations results in significantly reduced errors on all the cosmological parameters. The strongest effect is on Ω c h 2 and H 0, for which the error is reduced by almost a factor four. The percentage errors we estimate are |$\sigma _{\Omega _ch^2} = 0.25~{{\ \rm per\ cent}}$| and |$\sigma _{H_0} = 0.16~{{\ \rm per\ cent}}$|⁠ , to be compared with the Planck only results |$\sigma _{\Omega _ch^2} = 0.99~{{\ \rm per\ cent}}$| and |$\sigma _{H_0} = 0.79~{{\ \rm per\ cent}}$|⁠. We conclude that 21 cm SKAO observations will provide a competitive cosmological probe, complementary to CMB and, thus, pivotal for gaining statistical significance on the cosmological parameters constraints, allowing a stress test for the current cosmological model. [ABSTRACT FROM AUTHOR]

We present a detection of correlated clustering between MeerKAT radio intensity maps and galaxies from the WiggleZ Dark Energy Survey. We find a 7.7σ detection of the cross-correlation power spectrum, the amplitude of which is proportional to the product of the |${\rm H}\, {\small I}$| density fraction (⁠|$\Omega _{{\rm H}\, {\small I}}$|⁠), |${\rm H}\, {\small I}$| bias (⁠|$b_{{\rm H}\, {\small I}}$|⁠), and the cross-correlation coefficient (r). We therefore obtain the constraint |$\Omega _{{\rm H}\, {\small I}}b_{{\rm H}\, {\small I}}r\, {=}\, [0.86\, {\pm }\, 0.10\, ({\rm stat})\, {\pm }\, 0.12\, ({\rm sys})]\, {\times }\, 10^{-3}$|⁠ , at an effective scale of |$k_{\rm eff}\ {\sim }\ 0.13\, h\, \text{Mpc}^{-1}$|⁠. The intensity maps were obtained from a pilot survey with the MeerKAT telescope, a 64-dish pathfinder array to the SKA Observatory (SKAO). The data were collected from 10.5 h of observations using MeerKAT 's L -band receivers over six nights covering the 11 h field of WiggleZ, in the frequency range 1015–973 MHz (0.400 |$\, {\lt }\, z\, {\lt }\,$| 0.459 in redshift). This detection is the first practical demonstration of the multidish autocorrelation intensity mapping technique for cosmology. This marks an important milestone in the roadmap for the cosmology science case with the full SKAO. [ABSTRACT FROM AUTHOR]

We present a new halo model of neutral hydrogen (H  i) calibrated to galaxy formation simulations at redshifts z ∼ 0.1 and z ∼ 1.0 that we employ to investigate the constraining power of interferometric H  i intensity mapping on H  i astrophysics. We demonstrate that constraints on the small-scale H  i power spectrum can break the degeneracy between the H  i density |$\Omega _{\rm H\, \small {I}}$| and the H  i bias |$b_{\rm H\, \small {I}}$|⁠. For z ∼ 0.1, we forecast that an accurate measurement of |$\Omega _{\rm H\, \small {I}}$| up to 6 per cent level precision and the large-scale H  i bias |$b_{\rm H\, \small {I}}^0$| up to 1 per cent level precision can be achieved using Square Kilometre Array (SKA) pathfinder data from MeerKAT and Australian SKA Pathfinder (ASKAP). We also propose a new description of the H  i shot noise in the halo model framework in which a scatter of the relation between the H  i mass of galaxies and their host halo mass is taken into account. Furthermore, given the number density of H  i galaxies above a certain H  i mass threshold, future surveys will also be able to constrain the H  i mass function using only the H  i shot noise. This will lead to constraints at the 10 per cent level using the standard Schechter function. This technique will potentially provide a new way of measuring the H  i mass function, independent from existing methods. We predict that the SKA will be able to further improve the low-redshift constraints by a factor of 3, as well as pioneering measurements of H  i astrophysics at higher redshifts. [ABSTRACT FROM AUTHOR]