Baryon acoustic oscillations from H i intensity mapping: The importance of cross-correlations in the monopole and quadrupole

Cosmological parameter estimation in the post-reionization era via neutral hydrogen radio emission (HI) is among the key science goals of the forthcoming SKA Observatory (SKAO). This paper explores detection capability for baryon acoustic oscillations (BAO) with a suite of 100 simulations introducing the main limitations from foreground contamination and poor angular resolution caused by the radio telescope beam. Such broad single-dish beam representing a serious challenge for BAO detection with HI intensity mapping, we investigate a multipole expansion approach as a means for mitigating such limitations. We also showcase the gains made from cross-correlating the HI intensity mapping data with an overlapping spectroscopic galaxy survey, aiming to test potential synergies between the SKA project and other future cosmological experiments at optical/near-infrared wavelengths. For our similar to 4 000 deg(2) data set at z = 0.9, replicating the essential features of an SKAO HI intensity mapping survey, we were able to achieve a similar to 4.5 sigma detection of BAO features in auto-correlation despite the dominant beam effect. Cross-correlation with an overlapping galaxy survey can increase this to a similar to 6 sigma detection. Furthermore, including the power spectrum quadrupole besides the monopole in a joint fit can approximately double the BAO detection significance. Despite not implementing a radial-only P(k(parallel to)) analysis in favour of the three-dimensional P(k) and its multipoles, we were still able to obtain robust constraints on the radial Alcock-Paczynski parameter, whereas the perpendicular parameter remains unconstrained and prior dominated due to beam effects.