Extreme quasars at high redshift
  Context:Quasars radiating at extreme Eddington ratios (xA) are likely a prime
mover of galactic evolution and have been hailed as potential distance
indicators. Their properties are still scarcely known.
  Aims:We test the effectiveness of the selection criteria defined on the 4D
Eigenvector 1 (4DE1) for identifying xA sources. We provide a quantitative
description of their UV spectra in the redshift range 2<z<2.9.
  Methods:19 extreme quasar candidates were identified using 4DE1 selection
criteria applied to SDSS spectra: AlIII1860/SiIII]1892>0.5 and
CIII]1909/SiIII]1892<1. The emission line spectra was studied using
multicomponent fits of deep spectroscopic observations obtained with the
OSIRIS-GTC.
  Results:Spectra confirm that almost all of these quasars are xA sources with
very similar properties. We provide spectrophotometric and line profile
measurements for the SiIV1397+OIV]1402, CIV1549+HeII1640, and the 1900A blend
composed by AlIII1860, SiIII]1892, FeIII and a weak CIII]1909. The spectra can
be characterized as very low ionization (logU~-3), a condition that explains
the significant FeIII emission. CIV1549 shows low equivalent width (<30 A for
the most sources), and high or extreme blueshift amplitudes (-5000<c(1/2)<-1000
kms-1). Weak-lined quasars appear as extreme xA quasars and not as an
independent class. The CIV1549 high amplitude blueshifts coexists in all cases
save one with symmetric and narrower AlIII and SiIII] profiles. Estimates of
the Eddington ratio using the AlIII FWHM as a virial broadening estimator are
consistent with the ones of a previous xA sample.
  Conclusions:It is now feasible to assemble large samples of xA quasars from
the latest data releases of the SDSS. We provide evidence that AlIII1860 could
be associated with a low-ionization virialized sub-system, supporting previous
suggestions that AlIII is a reliable virial broadening estimator.
