Tracing Quasar Narrow-Line Regions Across Redshift: A Library of High S/N Optical Spectra

TL;DR

This study constructs high S/N composite spectra of quasars from SDSS data to analyze narrow-line region properties across redshifts, revealing stable high-ionization lines and increased star formation indicators at higher redshifts.

Contribution

It provides a new library of composite spectra and investigates the evolution of narrow emission lines and star formation in quasar host galaxies up to redshift 0.75.

Findings

High ionization line luminosities show no evolution with redshift.

[OII] emission increases with redshift, indicating enhanced star formation.

Inverse correlation between FeII strength and [OIII] EW and Hbeta width.

Abstract

In a single optical spectrum, the quasar narrow-line region (NLR) reveals low density, photoionized gas in the host galaxy interstellar medium, while the immediate vicinity of the central engine generates the accretion disk 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 (SDSS-DR7). We divide the sample into bins of continuum luminosity and Hbeta FWHM that are used to construct median composites at different redshift steps up to 0.75. We measure the luminosities of the narrow-emission lines [NeV]3427, [NeIII]3870, [OIII]5007, and [OII]3728 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 SED or the host galaxy ISM illuminated by the continuum. In contrast, we find that the [OII] line becomes stronger at higher redshifts, and we interpret this as a consequence of enhanced star formation contributing to the [OII] emission in host galaxies at higher redshifts. The SFRs estimated from the [OII] luminosities show a flatter increase with z than non-AGN galaxies given our assumed AGN contribution to the [OII] luminosity. Finally, we confirm an inverse correlation between the strength of the FeII4570 complex and both the [OIII] EW (though not the luminosity) and the width of the Hbeta line as known from the eigenvector 1 correlations.