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Published December 10, 2013 | Published
Journal Article Open

Submillimeter Recombination Lines in Dust-obscured Starbursts and Active Galactic Nuclei


We examine the use of submillimeter (submm) recombination lines of H, He, and He^+ to probe the extreme ultraviolet (EUV) luminosity of starbursts (SBs) and active galactic nuclei (AGNs). We find that the submm recombination lines of H, He, and He^+ are in fact extremely reliable and quantitative probes of the EUV continuum at 13.6 eV to above 54.6 eV. At submm wavelengths, the recombination lines originate from low energy levels (n = 20-50). The maser amplification, which poses significant problems for quantitative interpretation of the higher n, radio frequency recombination lines, is insignificant. Lastly, at submm wavelengths, the dust extinction is minimal. The submm line luminosities are therefore directly proportional to the emission measures (EM_(ION) = n_e × n_(ion) × volume) of their ionized regions. We also find that the expected line fluxes are detectable with ALMA and can be imaged at ~0."1 resolution in low redshift ultraluminous infrared galaxies. Imaging of the H I lines will provide accurate spatial and kinematic mapping of the star formation distribution in low-z IR-luminous galaxies, and the relative fluxes of the H I and He II recombination lines will strongly constrain the relative contributions of SBs and AGNs to the luminosity. The H I lines should also provide an avenue to constraining the submm dust extinction curve.

Additional Information

© 2013 American Astronomical Society. Received 2013 August 8; accepted 2013 September 26; published 2013 November 26. We thank Chris Hirata for discussions during this work, Zara Scoville for proofreading the manuscript, and Jin Koda and Min Yun for comments. We thank the Aspen Center for Physics and the NSF grant 1066293 for hospitality during the writing of this paper. We also thank the referee for suggesting we include a discussion of radiative line excitation (Section 2.5).

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