The correlation of line strength with luminosity and redshift from composite quasi-stellar object spectra

Abstract

We have generated a series of composite quasi-stellar object (QSO) spectra using over 22 000 individual low-resolution (∼8-Å) QSO spectra obtained from the 2dF (18.25 b_J 20.85) and 6dF (16b_J≤18.25) QSO Redshift Surveys. The large size of the catalogue has enabled us to construct composite spectra in relatively narrow redshift (Δz = 0.25) and absolute magnitude (ΔM_B = 0.5) bins. The median number of QSOs in each composite spectrum is ∼200, yielding typical signal-to-noise ratios of ∼100. For a given redshift interval, the composite spectra cover a factor of over 25 in luminosity. For a given luminosity, many of the major QSO emission lines (e.g. Mg iiλ2798, [Oii]λ3727) can be observed over a redshift range of 1 or greater. Using the composite spectra we have measured the line strengths (equivalent widths) of the major broad and narrow emission lines. We have also measured the equivalent width of the Ca iiλ3933 K absorption feature caused by the host galaxy of the active galactic nuclei (AGN). Under the assumption of a fixed host galaxy spectral energy distribution (SED), the correlation seen between Ca ii K equivalent width and source luminosity implies L_gal∝L^0.42±0.05_QSO. We find strong anticorrelations with luminosity for the equivalent widths of [O ii]λ3727 and [Ne v]λ3426. These provide hints to the general fading of the NLR in high-luminosity sources, which we attribute to the NLR dimensions becoming larger than the host galaxy. This could have important implications for the search for type 2 AGN at high redshifts. If average AGN host galaxies have SEDs similar to average galaxies, then the observed narrow [O ii] emission could be solely a result of the host galaxy at low luminosities (M_B∼−20). This suggests that the [O ii] line observed in high-luminosity AGN may be emitted, to a large part, by intense star-forming regions. The AGN contribution to this line could be weaker than previously assumed. We measure highly significant Baldwin effects for most broad emission lines (C ivλ1549, C iii]λ1909, Mg iiλ2798, Hγ, Hβ) and show that they are predominantly caused by correlations with luminosity, not redshift. We find that the Hβ and Hγ Balmer lines show an inverse Baldwin effect and are positively correlated with luminosity, unlike the broad ultraviolet lines. We postulate that this previously unknown effect is caused by a luminosity-dependent change in the ratio of disc to non-disc continuum components.