The extreme ultraviolet ( EUV ) spectra of distant star-forming regions can not 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 : L _ { \nu } \propto \nu ^ { -4.5 \pm 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 \gamma \simeq 4.5 \pm 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 \lambda \leq 912 \mathring { \mathrm { 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 .