An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zwicky 1

By M. Vestergaard, B. Wilkes. 2001

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We present an empirical template spectrum suitable for fitting and subtracting/studying the Fe II and Fe III emission lines in the rest frame ultraviolet spectra of quasars and active galactic nuclei, the first empirical ultraviolet iron template to cover the full range of 1250-3090 Å. Iron emission is often a severe contaminant in optical-ultraviolet spectra of active galactic nuclei and quasars. Its presence complicates and limits the accuracy of measurements of both strong and weak emission lines and the continuum emission, affecting studies of line and continuum interrelations, the ionization structure, and elemental abundances in active galaxies and quasars. Despite the wealth of work on modeling the quasar Fe II emission and the need to account for this emission in observed quasar spectra, there is no ultraviolet template electronically available to aid this process. The iron template we present is based on Hubble Space Telescope spectra of the narrow line Seyfert 1 galaxy, I Zwicky 1 (I Zw 1, z = 0.061). The intrinsic narrow lines (≳ 900 km s-1) of this source and its rich iron spectrum make the template particularly suitable for use with most active galactic nuclei and quasar spectra. The iron emission spectrum, the line identifications, and the measurements of absorption and emission lines are presented and compared with the work of Laor et al. Comments on each individual line feature and the line fitting are available in Appendix A. The methods used to develop and apply the template are also described. We illustrate the application of the derived Fe II and Fe III templates by fitting and subtracting iron emission from the spectra of four high-redshift quasars and of the nearby quasar, 3C 273, confirming their general applicability to active galaxies despite the somewhat unusual properties of I Zw 1. We briefly discuss the small discrepancies between the observed iron emission of these quasars and the ultraviolet template, and compare the template with previously published ones. We discuss the advantages and limitations of the UV Fe II and Fe III templates and of the template fitting method. We conclude that the templates work sufficiently well to be a valuable and important tool for eliminating and studying the iron emission in active galaxies, at least until accurate theoretical iron emission models are developed. The Si IV+O IV] λ1400 feature in I Zw 1 is clearly strong relative to C IV λ1549, and C IV and C III] λ1909 are both relatively weak. This may partially be due to the higher densities and lower ionization parameter prevailing in narrow line Seyfert 1 galaxies and to the big blue bump shifting toward lower energies in more luminous Seyferts, such as I Zw 1. In I Zw 1 the narrow line width reveals that C III] is heavily blended with Si III] λ1892, Al III λλ1854,1863, and Fe III transitions. This suggests that the C III] line strength and width may be overestimated in many quasar line studies where the lines are broader and deblending is not possible. This affects density estimates of the broad line region. Photoionization modeling, including all these line features, and subsequent fitting to the spectra are required to estimate the true C III] strength. We also argue, based on earlier work, that (strong) iron emission may be connected with high densities and associated with outflows.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships (2002, background): “2 of Marziani et al. 1996; Vestergaard & Wilkes 2001; M. Vestergaard et al. 2002, in preparation).”
Now You See It, Now You Don't: The Disappearing Central Engine of the Quasar J1011+5442 (2015, background): “…law of the form fλ = Aλ −α to characterize the quasar continuum, Gaussians to characterize the emission lines, optical (Boroson & Green 1992) and UV (Vestergaard & Wilkes 2001) Fe ii templates that have been combined into a single template that covers our full wavelength range, and Charlot &…”
ESTIMATING BLACK HOLE MASSES IN ACTIVE GALACTIC NUCLEI USING THE Mg ii λ2800 EMISSION LINE (2009, methodology): “With the recent availability of a more refined UV Fe ii template (Tsuzuki et al. 2006; cf. Vestergaard & Wilkes 2001), we have higher confidence that the linewidth measurements of Mg ii are reasonably robust.”
CONSTRAINTS ON BLACK HOLE GROWTH, QUASAR LIFETIMES, AND EDDINGTON RATIO DISTRIBUTIONS FROM THE SDSS BROAD-LINE QUASAR BLACK HOLE MASS FUNCTION (2010, methodology): “Vestergaard et al. (2008) modeled the observed quasar spectra using a power-law continuum, an optical–UV iron line spectrum based on I Zw I (Vestergaard & Wilkes 2001; Véron-Cetty et al. 2004), a Balmer continuum, and host galaxy template (Bruzual & Charlot 2003) for objects at z < 0.5.”
Iron Emission in the z = 6.4 Quasar SDSS J114816.64+525150.3 (2003, methodology): “We used an iron template derived fromHST ultraviolet observations of I Zw 1 as described by Vestergaard & Wilkes (2001)5; the iron emission is dominated by FeII blends with a contribution from FeIII .”
A Systematic Analysis of Fe II Emission in Quasars: Evidence for Inflow to the Central Black Hole (2008, background): “Vestergaard & Wilkes (2001) measured the velocity of each UV Fe line and showed that their shifts can vary by as much as 100 km s 1.”
ANALYSIS OF OPTICAL Fe ii EMISSION IN A SAMPLE OF ACTIVE GALACTIC NUCLEUS SPECTRA (2010, background): “A number of authors have created an Fe ii template in the UV and optical range (see Vestergaard & Wilkes 2001, and references therein).”
THE DISCOVERY OF THE FIRST “CHANGING LOOK” QUASAR: NEW INSIGHTS INTO THE PHYSICS AND PHENOMENOLOGY OF ACTIVE GALACTIC NUCLEI (2014, background): “…a power-law component for the AGN continuum; Fe II and UV Fe templates over a range of velocities to account for blends in Fe emission lines in the broad line spectrum (Boroson & Green 1992; Vestergaard & Wilkes 2001); and a suite of Gaussian components to model the broad and narrow emission lines.”
EVIDENCE FOR NON-EVOLVING Fe ii/Mg ii RATIOS IN RAPIDLY ACCRETING z ∼ 6 QSOs (2011, methodology): “…Shen et al. (2010) fit the spectra considering the power-law non–stellar contin- 8 De Rosa et al. 2011 9 uum, the Fe ii line forest (modeled using the template by Vestergaard & Wilkes 2001), and both the narrow (simple Gaussian) and broad (simple/double Gaussian) components of the Mg ii doublet.”
Broad Emission-Line Shifts in Quasars: An Orientation Measure for Radio-Quiet Quasars? (2002, methodology): “Alternatively, the changes could be due to Fe III (UV34); see Vestergaard & Wilkes (2001), § 5.2.”
ON THE COSMIC EVOLUTION OF THE SCALING RELATIONS BETWEEN BLACK HOLES AND THEIR HOST GALAXIES: BROAD-LINE ACTIVE GALACTIC NUCLEI IN THE zCOSMOS SURVEY (2009, background): “We also thank R. Decarli, F. Fontanot, O. Gerhard, J. Greiner, P. Hopkins, S. Komossa, N. Menci, P. Monaco, H. Netzer, R. Saglia, L. Wisotzki, J.H. Woo for their useful comments and suggestions.”
The evolution of the broad-line region among SDSS quasars (2005, background): “Sometimes at ∼ 1600Å, the Feii feature is seen in emission in quasars (e.g., Marziani et al. 1996; Vestergaard & Wilkes 2001; Vestergaard & Peterson 2005) or in absorption in low-ionization BALs (e.g., Hall et al. 2002).”
COMPARING SINGLE-EPOCH VIRIAL BLACK HOLE MASS ESTIMATORS FOR LUMINOUS QUASARS (2012, methodology): “Templates for Fe II and Fe III emission have been constructed from the spectrum of the narrow-line Seyfert 1 galaxy, I Zw 1 (e.g., Boroson & Green 1992; Vestergaard & Wilkes 2001; Tsuzuki et al. 2006).”
The Sloan Digital Sky Survey Reverberation Mapping Project: Sample Characterization (2018, methodology): “In addition to the continuum, we fit the optical and UV Fe II emission using empirical templates from the literature (e.g., Boroson & Green 1992; Vestergaard & Wilkes 2001; Tsuzuki et al. 2006; Salviander et al. 2007).”
Biases in Virial Black Hole Masses: An SDSS Perspective (2007, methodology): “The iron template for Hβ is taken from Boroson & Green (1992) and the iron template for MgII is a modified version of the Vestergaard & Wilkes (2001) template (Salviander et al. 2007) which extends under the MgII line itself.”
A Catalog of Broad Absorption Line Quasars from the Sloan Digital Sky Survey Early Data Release (2003, methodology): “For Mg ii BAL identification we use the window 2910± 10 Å, the first minimum in the Fe ii flux redward of Mg ii (Vestergaard & Wilkes 2001).”
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift (2003, methodology): “…FWHM(C iv), was measured reliably for this sample on a smooth representation of the emission profile, obtained by modeling with 2 to 3 Gaussian functions (following Laor et al. 1994), above a global power-law continuum fit to line-free windows (e.g., Francis et al. 1991; Vestergaard & Wilkes 2001).”