On the selection of damped Lyman alpha systems using MgII absorption at
  2<z<4
  The XQ-100 survey provides optical and near infrared coverage of 36 blindly
selected, intervening damped Lyman alpha systems (DLAs) at 2 < z < 4,
simultaneously covering the MgII doublet at 2796A, 2803A, and the Ly-alpha
transition. Using the XQ-100 DLA sample, we investigate the completeness of
selecting DLA absorbers based on their MgII rest-frame equivalent width (W2796)
at these redshifts. Of the 29 DLAs with clean MgII profiles, we find that six
(20% of DLAs) have W2796 < 0.6A. The DLA incidence rate of W2796 < 0.6A
absorbers is a factor of ~5 higher than what is seen in z~1 samples, indicating
a potential evolution in the MgII properties of DLAs with redshift. All of the
W2796 < 0.6A DLAs have low metallicities (-2.5 < [M/H] < -1.7), small velocity
widths (v90 < 50 km/s), and tend to have relatively low N(HI). We demonstrate
that the exclusion of these low W2796 DLAs results in a higher mean N(HI) which
in turn leads to a ~7% increase in the cosmological gas density of HI of DLAs
at 2 < z < 4; and that this exclusion has a minimal effect on the HI-weighted
mean metallicity.
