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

Kavli Affiliate: Luis C. Ho

| First 5 Authors: Hengxiao Guo, Aaron J. Barth, Kirk T. Korista, Michael R. Goad, Edward M. Cackett

| Summary:

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 r{A} 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 STIS 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 multi-component spectral models over the range $6800-9700$ r{A} 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 $sim$10% to 50% at 8000 r{A}, 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.

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