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The extreme ultraviolet (EUV) spectra of distant star-forming regions cannotbe probed directly using either ground- or space-based telescopes due to thehigh cross-section for interaction of EUV photons with the interstellar medium.This makes EUV spectra poorly constrained. The mm/submm recombination lines ofH and He, which can be observed from the ground, can serve as a reliable probeof the EUV. Here we present a study based on ALMA observations of threeGalactic ultra-compact HII regions and the starburst region Sgr B2(M), in whichwe reconstruct the key parameters of the EUV spectra using mm recombinationlines of HI, HeI and HeII. We find that in all cases the EUV spectra between13.6 and 54.4 eV have similar frequency dependence: L_{\nu}~ \nu^{-4.5 +/-0.4}. We compare the inferred values of the EUV spectral slopes with the valuesexpected for a purely single stellar evolution model (Starburst99) and theBinary Population and Spectral Synthesis code (BPASS). We find that theobserved spectral slope differs from the model predictions. This may imply thatthe fraction of interacting binaries in HII regions is substantially lower thanassumed in BPASS. The technique demonstrated here allows one to deduce the EUVspectra of star forming regions providing critical insight into photonproduction rates at \lambda < 912 A and can serve as calibration to starburstsynthesis models, improving our understanding of star formation in distantuniverse and the properties of ionizing flux during reionization.

Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII