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