ULTRAVIOLET EMISSION-LINE CORRELATIONS INHST/COS SPECTRA OF ACTIVE GALACTIC NUCLEI: SINGLE-EPOCH BLACK HOLE MASSES

Abstract

Effective methods of measuring supermassive black hole masses in active galactic nuclei (AGNs) are of critical importance to studies of galaxy evolution. While there has been much success in obtaining masses through reverberation mapping, the extensive observing time required by this method has limited the practicality of applying it to large samples at a variety of redshifts. This limitation highlights the need to estimate these masses using single-epoch spectroscopy of ultraviolet (UV) emission lines. We use UV spectra of 44 AGNs from HST/COS, the International Ultraviolet Explorer, and the Far Ultraviolet Spectroscopic Explorer of the C iv λ1549, O vi λ1035, O iii] λ1664, He ii λ1640, C ii λ1335, and Mg ii λ2800 emission lines and explore their potential as tracers of the broad-line region and supermassive black hole mass. The higher signal-to-noise ratio and better spectral resolution of the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) resolve AGN intrinsic absorption and produce more accurate line widths. From these, we test the viability of mass-scaling relationships based on line widths and luminosities and carry out a principal component analysis based on line luminosities, widths, skewness, and kurtosis. At L1450 ⩽ 1045 erg s−1, the UV line luminosities correlate well with Hβ, as does the 1450 Å continuum luminosity. We find that C iv, O vi, and Mg ii can be used as reasonably accurate estimators of AGN black hole masses, while He ii and C ii are uncorrelated.