Detection of Rest-frame Optical Lines from X-shooter Spectroscopy of Weak Emission Line Quasars

TL;DR

This study presents optical and near-infrared spectroscopy of seven weak emission line quasars, revealing that their broad emission lines are weaker but not absent, and suggesting they are wind-dominated quasars with unusual BELR properties.

Contribution

First detailed optical and near-infrared spectroscopic analysis of a larger sample of WLQs, comparing high- and low-ionization lines to understand their BELR characteristics.

Findings

Broad Hb and Ha detected in all WLQs, weaker than typical quasars.

WLQs show strong optical FeII emission and significant CIV blueshifts.

BELRs may have unusual high-ionization components or atypical photoionization states.

Abstract

Over the past 15 years, examples of exotic radio-quiet quasars with intrinsically weak or absent broad emission line regions (BELRs) have emerged from large-scale spectroscopic sky surveys. Here, we present spectroscopy of seven such weak emission line quasars (WLQs) at moderate redshifts (z=1.4-1.7) using the X-shooter spectrograph, which provides simultaneous optical and near-infrared spectroscopy covering the rest-frame ultraviolet through optical. These new observations effectively double the number of WLQs with spectroscopy in the optical rest-frame, and they allow us to compare the strengths of (weak) high-ionization emission lines (e.g., CIV) to low-ionization lines (e.g., MgII, Hb, Ha) in individual objects. We detect broad Hb and Ha emission in all objects, and these lines are generally toward the weaker end of the distribution expected for typical quasars (e.g., Hb has rest-frame equivalent widths ranging from 15-40 Ang.). However, these low-ionization lines are not exceptionally weak, as is the case for high-ionization lines in WLQs. The X-shooter spectra also display relatively strong optical FeII emission, Hb FWHM <4000 km/s, and significant CIV blueshifts (1000-5500 km/s) relative to the systemic redshift; two spectra also show elevated ultraviolet FeII emission, and an outflowing component to their (weak) MgII emission lines. These properties suggest that WLQs are exotic versions of "wind-dominated" quasars. Their BELRs either have unusual high-ionization components, or their BELRs are in an atypical photoionization state because of an unusually soft continuum.