Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II
Abstract
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 millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: L_ ν ∝ 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). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. 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 Å, and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization.
Additional Information
© 2020 The American Astronomical Society. Received 2020 June 26; revised 2020 August 26; accepted 2020 September 7; published 2020 October 29. We are grateful to Nick Scoville for cowriting the ALMA proposals and collaboration on the initial stages of the paper, to J.J. Eldridge, Susan Clark, David Guszejnov, and to the anonymous referee for their thoughtful comments, which helped improve the manuscript. L.M.'s stipend at the IAS is provided by the Friends of the Institute for Advanced Study. A part of this work was conducted while L.M. was supported by the SOS NRAO program and as a Groce Fellow at Caltech. L.M. is grateful to Dr. David and Barbara Groce for their kindness and support. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00111.S and ADS/JAO.ALMA#2016.1.01015.S ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.Attached Files
Published - Murchikova_2020_ApJ_903_29.pdf
Accepted Version - 2006.15153.pdf
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Additional details
- Eprint ID
- 106373
- Resolver ID
- CaltechAUTHORS:20201102-074155453
- Friends of the Institute for Advanced Study
- National Radio Astronomy Observatory
- David and Barbara Groce Startup Fund, Caltech
- NASA/JPL/Caltech
- Created
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2020-11-04Created from EPrint's datestamp field
- Updated
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2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)