Constraining UV Continuum Slopes of Active Galactic Nuclei With CLOUDY Models of Broad Line Region EUV Emission Lines

Abstract: Understanding the composition and structure of the broad-line region (BLR) of active galactic nuclei (AGN) is important for answering many outstanding questions in supermassive black hole evolution, galaxy evolution, and ionization of the intergalactic medium. We used single-epoch UV spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to measure EUV emission-line fluxes from four individual AGN with $0.49 \le z \le 0.64$, two AGN with $0.32 \le z \le 0.40$, and a composite of 159 AGN. With the Cloudy photoionization code, we calculated emission-line fluxes from BLR clouds with a range of density, hydrogen ionizing flux and incident continuum spectral indices. The photoionization grids were fit to the observations using single-component and locally optimally emitting cloud (LOC) models. The LOC models provide good fits to the measured fluxes, while the single-component models do not. The UV spectral indices preferred by our LOC models are consistent with those measured from COS spectra. EUV emission lines such as N IV \lambda 765, O II \lambda 833, and O III \lambda 834 originate primarily from gas with electron temperatures between 37000 K and 55000 K. This gas is found in BLR clouds with high hydrogen densities (n_H \ge 10^12 cm^-3) and hydrogen ionizing photon fluxes (\Phi_H \ge 10^22 cm^-2 s^-1).