Hydrogen intensity mapping with MeerKAT: Preserving cosmological signal by optimising contaminant separation

Removing contaminants is a delicate, yet crucial step in neutral hydrogen (H <ns0:sc>I</ns0:sc>) intensity mapping and often considered the technique’s greatest challenge. Here, we address this challenge by analysing H <ns0:sc>I</ns0:sc> intensity maps of about 100 deg2 at redshift z ≈ 0.4 collected by the MeerKAT radio telescope, an SKA Observatory (SKAO) precursor, with a combined 10.5-hour observation. Using unsupervised statistical methods, we removed the contaminating foreground emission and systematically tested, step-by-step, some common pre-processing choices to facilitate the cleaning process. We also introduced and tested a novel multiscale approach: the data were redundantly decomposed into subsets referring to different spatial scales (large and small), where the cleaning procedure was performed independently. We confirm the detection of the H <ns0:sc>I</ns0:sc> cosmological signal in cross-correlation with an ancillary galactic data set, without the need to correct for signal loss. In the best set-up we achieved, we were able to constrain the H <ns0:sc>I</ns0:sc> distribution through the combination of its cosmic abundance (ΩH <ns0:sc>I</ns0:sc>) and linear clustering bias (bH <ns0:sc>I</ns0:sc>) up to a cross-correlation coefficient (r). We measured ΩH <ns0:sc>I</ns0:sc>bH <ns0:sc>I</ns0:sc>r = [0.93 ± 0.17] × 10−3 with a ≈6σ confidence, which is independent of scale cuts at both edges of the probed scale range (0.04 ≲ k ≲ 0.3 h Mpc−1), corroborating its robustness. Our new pipeline has successfully found an optimal compromise in separating contaminants without incurring a catastrophic signal loss. This development instills an added degree of confidence in the outstanding science we can deliver with MeerKAT on the path towards H <ns0:sc>I</ns0:sc> intensity mapping surveys with the full SKAO.