The Paschen Jump as a Diagnostic of the Diffuse Nebular Continuum Emission in Active Galactic Nuclei

Hengxiao Guo, Aaron J. Barth, Kirk T. Korista, Michael R. Goad, Edward M. Cackett, Misty C. Bentz, William N. Brandt, D. Gonzalez-Buitrago, Gary J. Ferland, Jonathan M. Gelbord, Luis C. Ho, Keith Horne, Michael D. Joner, Gerard A. Kriss, Ian McHardy, Missagh Mehdipour, Daeseong Park, Raymond Remigio, U. Vivian, Marianne Vestergaard

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


Photoionization modeling of active galactic nuclei (AGN) predicts that diffuse continuum (DC) emission from the broad-line region makes a substantial contribution to the total continuum emission from ultraviolet through near-infrared wavelengths. Evidence for this DC component is present in the strong Balmer jump feature in AGN spectra, and possibly from reverberation measurements that find longer lags than expected from disk emission alone. However, the Balmer jump region contains numerous blended emission features, making it difficult to isolate the DC emission strength. In contrast, the Paschen jump region near 8200 Å is relatively uncontaminated by other strong emission features. Here, we examine whether the Paschen jump can aid in constraining the DC contribution, using Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of six nearby Seyfert 1 nuclei. The spectra appear smooth across the Paschen edge, and we find no evidence of a Paschen spectral break or jump in total flux. We fit multicomponent spectral models over the range 6800-9700 Å and find that the spectra can still be compatible with a significant DC contribution if the DC Paschen jump is offset by an opposite spectral break resulting from blended high-order Paschen emission lines. The fits imply DC contributions ranging from ∼10% to 50% at 8000 Å, but the fitting results are highly dependent on assumptions made about other model components. These degeneracies can potentially be alleviated by carrying out fits over a broader wavelength range, provided that models can accurately represent the disk continuum shape, Fe ii emission, high-order Balmer line emission, and other components.

Original languageEnglish
Article number60
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2022

Bibliographical note

Funding Information:
We thank the anonymous referee for providing helpful suggestions. We thank Matthew Malkan and Ari Laor for useful discussions about the physics of the Paschen jump. We acknowledge the contributions of additional co-investigators to the proposal for HST program 15124, including Rick Edelson, Michael Fausnaugh, Jelle Kaastra, and Bradley Peterson. Research at UCI has been supported in part by the NSF grant No. AST-1907290. Support at UCI for HST programs 14744 and 15124 was provided by NASA through grants from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. E.C. acknowledges funding support from HST program number 15413 (for Mrk 110) and NSF grant No. AST-1909199. L.C.H. is supported by the National Science Foundation of China (grant Nos. 11721303, 11991052) and the National Key R&D Program of China (2016YFA0400702). M.C.B. gratefully acknowledges support from the NSF through grant AST-2009230. V.U. acknowledges funding support from the NASA Astrophysics Data Analysis Program grant # 80NSSC20K0450. M.V. gratefully acknowledges support from the Independent Research Fund Denmark via grant No. DFF 8021-00130. G.J.F. acknowledges support by the NSF (grant Nos. 1816537, 1910687), NASA (grant Nos. ATP 17-ATP17-0141, 19-ATP19-0188), and STScI (grant Nos. HST-AR-15018 and HST-GO-16196.003-A). J.M.G. gratefully acknowledges support from NASA under the awards 80NSSC17K0126 and 80NSSC19K1638. W.N.B. acknowledges support from NSF grant No. AST-2106990.

Funding Information:
Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions.

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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