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

A standard ruler akin in application to standard candles, Baryon Acoustic Oscillations (BAO) are a main feature of the matter power spectrum, depending on fundamental quantities cosmologists are striving to measure. The SKA Observatory (SKAO) will look for their signal in the post-reionisation universe via neutral hydrogen Intensity Mapping, a task made challenging by astrophysical foregrounds and resolution limitations. To tackle these issues, we investigate the results emerging from the multipole expansion approach as a mitigation of such limitations and we showcase the gains made from cross-correlating the HI intensity mapping with spectroscopic galaxy observations. This is to be intended as an anticipation of future synergies between the SKAO Project and Euclid- or DESI-like instruments. Adding then the information from the quadrupole term allows for a detection of the BAO feature up to $4.5\mbox{--}6$ sigmas at the lowest redshifts, providing robust constraints on the radial Alcock-Paczynski parameter (a proxy for the Hubble parameter). On the other hand, the corresponding perpendicular parameter remains unconstrained and prior dominated due to beam effects.