Detecting redshift space distortions with forthcoming 21cm intensity maps from MeerKLASS

Redshift space distortions (RSD) provide a powerful measurement of structure formation in modern cosmology. In this work, we explore the potential of 21cm intensity mapping with the MeerKAT Large Area Synoptic Survey (MeerKLASS) to detect RSD and probe gravity within the redshift range of MeerKAT's UHF band (0.4<z<1.45). Using simulated sky observations, we examine the impact from observational effects caused by blind foreground removal and MeerKAT's primary beam, plus k-cuts to the measurements. With a multipole expansion formalism, we demonstrate how the quadrupole of the HI power spectrum can be modelled and also show how a foreground transfer function can reconstruct the distorted signal. With 1,115 deg squared of simulated MeerKLASS observations at 0.4<z<1.0, our proposed analysis pipeline reveals a 6.7σ detection of an RSD-induced quadrupole relative to one caused by anisotropic observational effects alone. We also explore parameter inference with the full MeerKLASS 10,000 deg squared survey. Within forecasts, we demonstrate how a tomographic analysis across the full UHF redshift range can be leveraged to address certain parameter degeneracies with e.g. the HI bias (b HI) and cosmic density of HI (Ω HI). By adopting a redshift-dependent model for the bias and HI density combined with simulation-informed priors, we demonstrate how degeneracies can be alleviated, and indicate the potential for MeerKLASS to reach competitive precision on simple probes of modified gravity.