A New Iron Emission Template for Active Galactic Nuclei. I. Optical Template for the Hβ Region*
Daeseong Park, Aaron J. Barth, Luis C. Ho, Ari Laor
Abstract
Abstract We present a new empirical template for iron emission in active galactic nuclei (AGNs) covering the 4000–5600 Å range. The new template is based on a spectrum of the narrow-line Seyfert 1 galaxy Mrk 493 obtained with the Hubble Space Telescope. In comparison with the canonical iron template object I Zw 1, Mrk 493 has narrower broad-line widths, lower reddening, and a less extreme Eddington ratio, making it a superior choice for template construction. We carried out a multicomponent spectral decomposition to produce a template incorporating all the permitted and forbidden lines of Fe ii identified in the Mrk 493 spectrum over this wavelength range, as well as lines from Ti ii , Ni ii , and Cr ii . We tested the template by fitting it to AGN spectra spanning a broad range of iron emission properties, and we present a detailed comparison with fits using other widely used monolithic and multicomponent iron emission templates. The new template generally provides the best fit (lowest χ 2 ) compared to other widely used monolithic empirical templates. In addition, the new template yields more accurate spectral measurements including a significantly better match of the derived Balmer line profiles (H β , H γ , H δ ), in contrast with results obtained using the other templates. Our comparison tests show that the choice of iron template can introduce a systematic bias in measurements of the H β line width, which consequently impacts single-epoch black hole mass estimates by ∼0.1 dex on average and possibly up to ∼0.3–0.5 dex individually.