Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII. (arXiv:2006.15153v1 [astro-ph.GA])

Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII. (arXiv:2006.15153v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Murchikova_L/0/1/0/all/0/1">Lena Murchikova</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Murphy_E/0/1/0/all/0/1">Eric J. Murphy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lis_D/0/1/0/all/0/1">Dariusz C. Lis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Armus_L/0/1/0/all/0/1">Lee Armus</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mink_S/0/1/0/all/0/1">Selma de Mink</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sheth_K/0/1/0/all/0/1">Kartik Sheth</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zakamska_N/0/1/0/all/0/1">Nadia Zakamska</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tramper_F/0/1/0/all/0/1">Frank Tramper</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bongiorno_A/0/1/0/all/0/1">Angela Bongiorno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Elvis_M/0/1/0/all/0/1">Martin Elvis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kewley_L/0/1/0/all/0/1">Lisa Kewley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sana_H/0/1/0/all/0/1">Hugues Sana</a>

The extreme ultraviolet (EUV) spectra of distant star-forming (SF) 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. 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. We
present a study based on ALMA 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 54.4eV of the
objects studied here have similar frequency dependence: L_nu~nu^{-4.5 pm 0.4}.
We compare the inferred values of the EUV spectral slopes with the values
expected for a single stellar evolution model (Starburst99) and the Binary
Population and Spectral Synthesis code (BPASS v2.1). We find that the observed
spectral slope gamma~4.5 pm 0.4 differs from the model predictions. For an
instantaneous starburst model with age <5Myrs the observed spectral slopes are
consistently steeper than predictions of either of the models. For an
instantaneous starburst with age >5Myrs 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 SF regions providing critical insight into photon production
rates at lambda>912A and can serve as calibration to starburst synthesis
models, improving our understanding of SF and the properties of ionizing flux
during reionization.

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