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Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II

Murchikova, Lena and Murphy, Eric J. and Lis, Dariusz C. and Armus, Lee and de Mink, Selma E. and Sheth, Kartik and Zakamska, Nadia and Tramper, Frank and Bongiorno, Angela and Elvis, Martin and Kewley, Lisa and Sana, Hugues (2020) Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II. Astrophysical Journal, 903 (1). Art. No. 29. ISSN 1538-4357.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Murchikova, Lena0000-0001-8986-5403
Murphy, Eric J.0000-0001-7089-7325
Lis, Dariusz C.0000-0002-0500-4700
Armus, Lee0000-0003-3498-2973
de Mink, Selma E.0000-0001-9336-2825
Sheth, Kartik0000-0002-5496-4118
Zakamska, Nadia0000-0001-6100-6869
Tramper, Frank0000-0002-5891-0950
Bongiorno, Angela0000-0002-0101-6624
Elvis, Martin0000-0001-5060-1398
Kewley, Lisa0000-0001-8152-3943
Sana, Hugues0000-0001-6656-4130
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.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Friends of the Institute for Advanced StudyUNSPECIFIED
National Radio Astronomy ObservatoryUNSPECIFIED
David and Barbara Groce Startup Fund, CaltechUNSPECIFIED
Subject Keywords:Astrophysicists ; Interstellar medium ; Star-forming regions ; Star formation ; Radiative recombination ; Compact H II region
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Astrophysicists (105); Interstellar medium (847); Star forming regions (1565); Star formation (1569); Radiative recombination (2057); Compact H II region (286)
Record Number:CaltechAUTHORS:20201102-074155453
Persistent URL:
Official Citation:Lena Murchikova et al 2020 ApJ 903 29
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106373
Deposited By: Tony Diaz
Deposited On:04 Nov 2020 18:30
Last Modified:04 Nov 2020 18:30

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