A New Iron Emission Template for Active Galactic Nuclei. I. Optical
  Template for the H$\beta$ region
  We present a new empirical template for iron emission in active galactic
nuclei (AGN) covering the $4000-5600$ A 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 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 multi-component iron emission
templates. The new template generally provides the best fit (lowest $\chi^2$)
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$\beta$,
H$\gamma$, H$\delta$), 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$\beta$ line width, which
consequently impacts single-epoch black hole mass estimates by $\sim0.1$ dex on
average and possibly up to $\sim0.3-0.5$ dex individually.
