The Ultraviolet Emission Properties of Five Low-Redshift Active Galactic Nuclei at High Signal-to-Noise Ratio and Spectral Resolution
Abstract
We analyze the ultraviolet (UV) emission line and continuum properties of five low-redshift active galactic nuclei (four luminous quasars: PKS 0405 - 123, H1821 +643, PG 0953+414, and 3C 273, and one bright Seyfert 1 galaxy: Mrk 205). The HST spectra have higher signal-to-noise ratios (typically ~60 per resolution element) and spectral resolution (A = 1300) than all previously published UV spectra used to study the emission characteristics of active galactic nuclei. We include in the analysis ground-based optical spectra covering Hβ and the narrow [O III] λλ4959,5007 doublet. The following new results are obtained: Lyβ/Lyα = 0.03-0.12 for the four quasars, which is the first accurate measurement of the long-predicted Lyβ intensity in QSOs. The cores of Lyα and C IV are symmetric to an accuracy of better than 2.5% within about 2000 km s^-1^ of the line peak. This high degree of symmetry of Lyα argues against models in which the broad line cloud velocity field has a significant radial component. The observed smoothness of the Lyα and C IV line profiles requires at least ~10^4^ individual clouds if bulk velocity is the only line-broadening mechanism. The overall similarity of the Lyα and C IV λ1549 profiles rules out models for the broad-line region (BLR) with a radial distribution of virialized clouds having an ionization parameter U is proportional to Radius^-1^. The measured high values of O VI λ1034/Lyα and low values of C III λ977/O VI λ1034 imply a BLR component with U ~ 1. The red-wing flux in O VI relative to Lyα suggests the presence of an inner, high-velocity, optically thin component with U > 1 in the BLR. The N V/Lyα ratio is 0.135 +/- 0.01 for the four quasars, which may be an indication of higher than solar N abundance and metallicity. The maximum contribution of a narrow ([O III]-like) component is about 3%-6% of the total broad-line flux; this limit is generally highest for C III]. This result constrains the covering factor of the narrow line region or indicates the presence of dust. An unresolved component having full width at half-maximum less than 230 km s^-1^ typically contributes less than 0.5% of the observed broad lines flux. The HST data permit the first relatively accurate measurements of the Lyγ, C III λ977, S VI λλ933, 945, and the N III λ991 emission lines, as well as the measurement of a number of other weak or strongly blended lines at λ > 1216 A. In agreement with observations of high-redshift quasars, the peaks of Lyα, C IV, and C III] are blueshifted by ~200 km s^-1^ relative to [O III] λ5007, while He II λ1640 is shifted by about 500 km s^-1^. The low ionization lines, Mg II, Hβ, and O I λ1304, are in most cases only marginally shifted to the red.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1994
- DOI:
- 10.1086/173545
- arXiv:
- arXiv:astro-ph/9305026
- Bibcode:
- 1994ApJ...420..110L
- Keywords:
-
- Absorption Spectra;
- Active Galactic Nuclei;
- Red Shift;
- Seyfert Galaxies;
- Signal To Noise Ratios;
- Spectral Resolution;
- Abundance;
- Emission Spectra;
- Lyman Alpha Radiation;
- Lyman Beta Radiation;
- Quasars;
- Stellar Luminosity;
- Ultraviolet Astronomy;
- Astrophysics;
- GALAXIES: ACTIVE;
- GALAXIES: NUCLEI;
- GALAXIES: QUASARS: EMISSION LINES;
- GALAXIES: SEYFERT;
- LINE: PROFILES;
- ULTRAVIOLET: GALAXIES;
- Astrophysics
- E-Print:
- 39 pages (+ 6 pages of tables + 16 of figures), AST 93/27