Uncovering the spectral energy distribution in active galaxies using high-ionization mid-infrared emission lines

Abstract

The shape of the spectral energy distribution (SED) of active galaxies in the extreme-ultraviolet (EUV)-soft X-ray band (13.6eV-1keV) is uncertain because obscuration by dust and gas can hamper our view of the continuum. To investigate the shape of the SED in this energy band, we have generated a set of photoionization models which reproduce the small dispersion found in correlations between high-ionization mid-infrared emission lines in a sample of hard X-ray-selected active galactic nuclei (AGNs). Our calculations show that a broken power-law continuum model is sufficient to reproduce the [Ne V] 14.32 μm/[Ne III], [Ne V]24.32 μm/[O IV] 25.89 μm, and [O IV]25.89 μm/[Ne III] ratios and does not require the addition of a "big bump" EUV model component. We constrain the EUV-soft X-ray slope, αi, to be between 1.5 and 2.0 and derive a best fit of αi 1.9 for Seyfert 1 galaxies, consistent with previous studies of intermediate-redshift quasars. If we assume a blue bump model, most sources in our sample have derived temperatures between T BB = 105.18 K and 105.7 K, suggesting that the peak of this component spans a large range of energies extending from ∼600 to 1900 Å. In this case, the best-fitting peak energy that matches the mid-infrared line ratios of Seyfert 1 galaxies occurs between ∼700 and 1000 Å. Despite the fact that our results do not rule out the presence of an EUV bump, we conclude that our power-law model produces enough photons with energies >4Ry to generate the observed amount of mid-infrared emission in our sample of Burst Alert Telescope AGNs. © 2011. The American Astronomical Society. All rights reserved.