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Comparative Analysis of the High- and Low-Ionization Lines in the Broad-Line Region of Active Galactic Nuclei

The Astrophysical Journal Supplement Series1996Vol. 104, pp. 37–37

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P. Marziani, J. W. Sulentic, D. Dultzin‐Hacyan, M. Calvani, M. Moles

Abstract

We present the first comparative study of high- and low-ionization lines in a large sample of active galactic nuclei (AGNs). We report measurements of C IV λ1549, HeII λ1640, and O III] λ1663 paired to measurements of Hβ, HeII λ4686, and Fe II_opt_ in 52 low-redshift AGNs (z <~0.8). Ultraviolet archival spectra covering the region of C IV λ1549 were obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST). Moderate- resolution optical spectra (~4-7 A) were obtained in most cases within two years of the HST observations. We present an atlas of matching C IV λ1549 and Hβb broad-line profiles. Velocity centroids at different profile heights and other profile properties were measured after removal of narrow-line components and contaminating satellite lines of other elements. Our results add refinements to the "standard view" of nebular conditions in the broad-line region (BLR). (1) The average electron density is most probably weighted toward values higher than expected in standard BLR models. The weakness of O III] λ1663 and N IV] λ1486 relative to He II λ1640 suggest that the semiforbidden lines are collisionally de-excited at n_e_ >~ 10^10.5^ cm^- 3^. (2) There is evidence, supporting earlier work, that photoionization via Lyα fluorescence plays a major role in the production of Fe II_UV_. We identify and discuss briefly the Fe II_UV_ multiplets in the range 1550-1750 A. (3) The observational distinction between the narrow- and broad-line regions is blurred by the contribution of a region of intermediate density (10^6^ cm^-3^ <~ n_e_ <~ 10^7^ cm^-3^). Best evidence for this emission comes from the [O III] λ4363 and C IV λ1549 profiles. It is most likely associated with the innermost part of the narrow-line region. Comparisons between the broad-line profile of C IV λ1549, Hβ and, whenever possible, HeII λ1640 suggest (1) that radio- loud and radio-quiet AGNs are very different classes of objects that should be treated separately, and (2) that ionization structure varies within the velocity field. The extreme radial velocity emission is characterized by a high-ionization spectrum dominated by C IV λ 1549 and HeII. Several basic results are: (i) C IV λ1549 is almost always blueshifted with respect to Hβ. (ii) There is a fundamental kinematic difference between radio loud and radio-quiet objects relative to the rest frame of the AGN. (iii) Radio-loud objects show predominantly redshifted and asymmetric Hβ and unshifted symmetric C IV λ1549. (iv) Radio-quiet objects show predominantly blueshifted and asymmetric C IV profiles with Hβ usually symmetric and unshifted. Correlation analysis of the radio-quiet sample suggests that C IV λ1549 EW, C IV λ1549 shift/asymmetry, and the strength of Fe II_opt_ are related. C IV λ1549 profiles with large blueshift/asymmetry show lower EW and stronger Fe II_opt_ emission. The results suggest that C IV λ1549 and Hβ arise in different velocity/geometry regimes. C IV λ1549 may be predominantly emitted in an outflowing bicone of large aperture (~45^deg^), while Fe II_opt_ is most probably emitted in a strongly flattened geometry (possibly an accretion disk). Bulk Hβ emission may come from a less flattened configuration of clouds. In this interpretation, the flux ratio Fe II_opt_/Hβ_BC_ is an aspect indicator for radio-quiet objects. Radio-loud objects are more enigmatic, and the existence of a reliable optical aspect indicator remains uncertain. There is evidence that line profile properties of C IV λ1549 and Hβ are "coupled" in the radio-loud sample, suggesting that a common origin for the high- and low- ionization line emission is a possibility. If this is true, however, we do not have a reasonable explanation for the predominance of Hβ redshifts and red asymmetries in that population.

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