A large sample of Kohonen-selected SDSS quasars with weak emission lines: selection effects and statistical properties

TL;DR

This study identifies and analyzes a large sample of weak emission line quasars from SDSS, revealing their distinct properties and potential selection biases, and suggests they are linked to high accretion rates and specific evolutionary stages.

Contribution

It provides a large, systematically selected sample of WLQs, compares their properties with normal quasars, and discusses the effects of selection biases and intrinsic differences.

Findings

WLQs have higher luminosities, Eddington ratios, and accretion rates.

Approximately half of the observed excess is due to selection bias.

WLQs exhibit bluer spectra and specific radio properties.

Abstract

We performed a search for WLQs in the spectroscopic data from the Sloan Digital Sky Survey Data Release 7 based on Kohonen self-organising maps for nearly 10^5 quasar spectra. The final sample consists of 365 quasars and includes in particular a subsample of 46 WLQs with equivalent widths W(MgII) < 11 A and W(CIV) < 4.8 A. We compared various properties of the WLQs with those of control samples of ordinary quasars. Particular attention was paid to selection effects. The WLQs have, on average, significantly higher luminosities, Eddington ratios, and accretion rates. About half of the excess comes from a selection bias, but an intrinsic excess remains probably caused primarily by higher accretion rates. The spectral energy distribution shows a bluer continuum at rest-frame wavelengths > 1500 A. The variability in the optical and UV is relatively low, even taking the variability-luminosity anti-correlation into account. The percentage of radio detected quasars and of core-dominant radio sources is significantly higher than for the control sample, whereas the mean radio-loudness is lower. The properties of our WLQ sample can be consistently understood assuming that it consists of a mix of quasars at the beginning of a stage of increased accretion activity and of beamed radio-quiet quasars. (Abstract modified to match the arXiv format)