Uncovering the Spectral Energy Distribution in Active Galaxies Using High Ionization Mid-infrared Emission Lines

TL;DR

This study models the EUV-soft X-ray spectral energy distribution of active galaxies using photoionization models, showing a broken power-law suffices to explain mid-infrared emission line ratios without needing a big bump component.

Contribution

It demonstrates that a broken power-law continuum can explain mid-infrared emission lines in AGN, constraining the EUV--soft X-ray slope and challenging the necessity of a big bump in the spectrum.

Findings

A broken power-law model reproduces observed line ratios.

EUV--soft X-ray slope alpha_i is between 1.5 and 2.0.

Peak of the big bump, if present, spans 600A to 1900A.

Abstract

The shape of the spectral energy distribution of active galaxies in the EUV--soft X-ray band (13.6 eV to 1 keV) is uncertain because obscuration by dust and gas can hamper our view of the continuum. To investigate the shape of the spectral energy distribution 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 AGN. Our calculations show that a broken power-law continuum model is sufficient to reproduce the [Ne V]14.32 mm/[NeIII], [Ne V]24.32mm/[O IV]25.89mm and [O IV] 25.89mm/[Ne III] ratios, and does not require the addition of a "big bump" EUV model component. We constrain the EUV--soft X-ray slope, alpha_i, to be between 1.5 -- 2.0 and derive a best fit of alpha_i ~ 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}=10^{5.18} K to 10^{5.7} K, suggesting that the peak of this component spans a large range of energies extending from ~ lambda 600A to lambda 1900A. In this case, the best fitting peak energy that matches the mid-infrared line ratios of Seyfert 1 galaxies occurs between ~ lambda 700--1000A. 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 > 4 Ry to generate the observed amount of mid-infrared emission in our sample of BAT AGN.