Tracing quasar narrow-line regions across redshift: a library of high-S/N optical spectra

In a single optical spectrum, the quasar narrow-line region (NLR) reveals low-density, photoionized gas in the host galaxy interstellar medium (ISM), while the immediate vicinity of the central engine generates the accretion disc continuum and broad emission lines. To isolate these two components, we construct a library of high-S/N optical composite spectra created from the Sloan Digital Sky Survey Data Release 7. We divide the sample into bins of continuum luminosity and Hβ full width at half-maximum that are used to construct median composites at different redshift steps up to 0.75. We measure the luminosities of the narrow-emission lines [Ne v] λ3427, [Ne iii] λ3870, [O iii] λ5007, and [O ii] λ3728 with ionization potentials (IPs) of 97, 40, 35, and 13.6 eV, respectively. The high IP lines’ luminosities show no evidence of increase with redshift consistent with no evolution in the AGN spectral energy distribution or the host galaxy ISM illuminated by the continuum. In contrast, we find that the [O ii] line becomes stronger at higher redshifts, and we interpret this as a consequence of enhanced star formation contributing to the [O ii] emission in host galaxies at higher redshifts. The SFRs estimated from the [O ii] luminosities show a flatter increase with z than non-AGN galaxies given our assumed AGN contribution to the [O ii] luminosity. Finally, we confirm an inverse correlation between the strength of the Fe ii λ4570 complex and both the [O iii] equivalent width (though not the luminosity) and the width of the Hβ line as known from the eigenvector 1 correlations.