Abstract
We present an empirical template spectrum suitable for fitting/subtracting and studying the FeII and FeIII line emission in the restframe UV spectra of active galatic nuclei (AGNs), the first empirical UV iron template to cover the full 1250 - 3090 A range. Iron emission is often a severe contaminant in optical--UV spectra of AGNs. 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 AGNs. Despite the wealth of work on modeling the AGN FeII emission and the need to account for it in observed AGN spectra, there is no UV template electronically available to aid this process. The iron template we present is based on HST spectra of the Narrow Line Seyfert 1, IZw1. Its intrinsic narrow lines (~900 km/s) and rich iron spectrum make the template particularly suitable for use with most AGN spectra. The iron emission spectrum and the line identifications and measurements are presented and compared with the work of Laor et al. We illustrate the application of the derived FeII and FeIII templates by fitting and subtracting the iron emission from UV spectra of four high-z quasars and of the nearby quasar, 3C273. We briefly discuss the small discrepancies between this observed iron emission and the UV template, and compare the template with previously published ones. We discuss the advantages and limitations of the 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 AGNs, at least until accurate theoretical iron emission models are developed. (Abridged)
Highlights
Quasar5 ultraviolet (UV) spectra characteristically contain broad emission lines originating in the central ([light year) region, the broad line region (BLR)
With the availability of high signal-to-noise ratio (S/N), high-resolution spectra, and in particular data taken with Hopkins Ultraviolet T elescope (e.g., Kriss et al 1992 ; Zheng et al 1995 ; Zheng, Kriss, & Davidsen 1996), Hubble Space T elescope (HST ) (e.g., Laor et al 1994, 1995 ; Zheng et al 1997 ; Kriss et al 2000 ; Kraemer & Crenshaw 2000), and the Keck telescope (e.g., Tran, Cohen, & Goodrich 1995 ; Brotherton et al 1997 ; Barlow & Sargent 1997 ; Larkin et al 2000 ; Carson et al 2000), it has become obvious that quasar spectra contain a plethora of weak lines as well
We have presented a UV iron template based on HST archival data of I Zw 1, and described the method with which the template was generated
Summary
Quasar ultraviolet (UV) spectra characteristically contain broad emission lines originating in the central ([light year) region, the broad line region (BLR). For a number of years after the discovery of quasars these lines and a few iron transitions (Greenstein & Schmidt 1964 ; Wampler & Oke 1967) were the only lines detected in their UV spectra. With the availability of high signal-to-noise ratio (S/N), high-resolution spectra, and in particular data taken with Hopkins Ultraviolet T elescope (e.g., Kriss et al 1992 ; Zheng et al 1995 ; Zheng, Kriss, & Davidsen 1996), Hubble Space T elescope (HST ) (e.g., Laor et al 1994, 1995 ; Zheng et al 1997 ; Kriss et al 2000 ; Kraemer & Crenshaw 2000), and the Keck telescope (e.g., Tran, Cohen, & Goodrich 1995 ; Brotherton et al 1997 ; Barlow & Sargent 1997 ; Larkin et al 2000 ; Carson et al 2000), it has become obvious that quasar spectra contain a plethora of weak lines as well
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