Hi intensity mapping with MeerKAT: A measurement of the 21-cm auto-spectrum on cosmological scales

We report a direct measurement of the 21-cm signal from neutral hydrogen (Hi) via the intensity mapping technique at redshift z = 0.42 through its power spectrum at cosmological scales 0.095 h Mpc−1 < k < 0.245 h Mpc−1. We analyse intensity maps observed in the frequency range 971.2 MHz < ν < 1023.6 MHz during the 2021 observational campaign of the MeerKAT Large Area Synoptic Survey (MeerKLASS) spanning an area of approximately 236 °2. Capitalising on the depth of this survey, we divide the data set into multiple observationally independent subsets. After performing blind contaminant separation independently on each subset, we exploit internal cross-correlations to suppress the instrumental noise plateau and isolate the cosmological signal from residual systematics. Although the subsets’ auto-spectra show the presence of excess power that we deem linked to still unresolved systematics, the cross-spectra measurements satisfactorily pass a battery of null and robustness tests. We introduce a multi-subset formalism to infer the amplitude of the Hi auto-spectrum from our internal cross-spectra measurements while marginalising the potential signal loss that occurs during data processing, reaching a detection with signal-to-noise ratio of 6.2. We constrain the clustering amplitude of the signal in terms of the Hi cosmic abundance and its linear bias, measuring 103 ΩHibHi = 1.271-0.598+1.080. This direct measurement marks a milestone in demonstrating the effectiveness of the Hi intensity mapping technique for future surveys with the SKA Observatory, of which MeerKAT is a precursor.