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

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross- section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The mm/submm recombination lines of H and He, which can be observed from the ground can serve as a reliable probe of the EUV. These lines are weekly impacted by dust absorption, their fluxes are independent of metallicity, and depend linearly on the number of EUV continuum photons. Here we present a study based on Atacama Large Millimeter/submillimeter Array observations of three Galactic ultra-compact HII regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra of these sources using mm recombination lines of HI, HeI and HeII. We find that the EUV spectra between 13.6 and 54.4 eV of the objects studied here have similar frequency dependence: Lν ∝ ν−4.5±0.4. We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS v2.1). We find that the observed spectral slope γ ≃ 4.5 ± 0.4 differs from the model predictions. For an instantaneous starburst model with age < 5 Myrs the observed spectral slopes are consistently steeper than predictions of either of the models. For an instantaneous starburst with age > 5 Myrs the observed spectral slope is in-between the values predicted by the two models. This may imply that the fraction of interacting binaries in HII regions is substantially lower than assumed in BPASS v2.1. The technique demonstrated here allows one to deduce the EUV spectra of star forming regions providing critical insight into photon production rates at λ ≤ 912 ̊A and can serve as calibration to starburst synthesis models, improving our understanding of star formation in distant universe and the properties of ionizing flux during reionization.