Left panel: Formation efficiency, η_HeBH, of He+BH binaries as a function of metallicity, Z, as obtained from the different binary-evolutionary models computed in this study (see the legend). Note that, here, η_HeBH comprises only the population of He+BH binaries that evolve into BBH or NS+BH mergers within a Hubble time. Right panel: Estimated galaxy-wide counts, N_HeBH, obs, of bright, observable X-ray sources (with L_X > 10³⁵  ergs⁻¹) corresponding to the Local Group galaxies (and their individual star formation rates; see the text) named along the upper X-axis. Number estimates from StarTrack are not shown, as only evolutionary data for the first and last time steps of the He+BH phase are available.