H I absorption spectra of 4C 52.37. The unresolved WSRT spectrum is shown in red. The two spatially integrated VLBI profiles (in blue) were extracted from a region of 60 mas × 40 mas centered on E1. The dashed blue line corresponds to the VLBI spectrum that was obtained by integrating only pixels with a value of ≤ − 3σ_VLBI, cube, while for the solid blue line the integration was only limited to the region marked by the 3σ_VLBI, cont contour.