Modelling the diffuse continuum emission: the NLS1 Mrk 110

Abstract

We present detailed model diffuse continuum (DC) plus hydrogen emission line templates from a summation over a broad range in hydrogen gas densities. We address the effect of finite gas densities and the presence of weak higher-order emission-line transitions on the strength and location of the Balmer and Paschen jumps which act to shift the jumps red-ward of their respective vacuum wavelength positions and substantially soften the gradient of the jump. Our photoionisation model-based DC template favours a lower optical depth and lower electron temperature than traditionally employed for the Balmer continuum, directly impacting empirical estimates of the strength of UV Fe ii. Microturbulent velocities increase the emission-line-continuum contrast, suppressing the DC contribution and weakening the jump heights, though the spectral shape of the DC remains broadly similar. Even with these additional complexities, a spectral decomposition of the UV-Optical-IR spectrum of the NLS1 Mrk 110, which includes a substantial DC contribution (30–50% of the total light at 3600 Å), reveals a significant deficit in emission just longward of the Balmer jump. Interestingly, significant thermal DC emission acts to flatten the SED through the UV-optical, negating oft-employed intrinsic reddening. Our best fit 1000 Å–3$\umu$m model requires a black hole mass ~108 M⊙, similar to that inferred when considering features in the H/He broad emission line profiles that suggest the presence of gravitational redshift. Finally, we provide diagnostic plots that may assist the spectral modeler to construct a physically useful DC spectrum and quantify its contribution to AGN UV-optical spectra.