Number of RH as a function of redshift. Left panel: integral number of RHs within a given redshift N_H(< z). The black arrows show the upper boundaries to the total number of RH within that redshift bin that are obtained by adding RH and U cases. Right panel: number of RHs in redshift bins N_H(z, Δz); the percentage of RHs with ultra-steep spectra (i.e. ν_s < 610 MHz) is reported in each redshift bin. In both panels, red lines and regions are the expected values, and black line and dots are the observed values.