Detecting neutral hydrogen at z > 3 in large spectroscopic surveys of
  quasars
  We present a pipeline based on a random forest classifier for the
identification of high column-density clouds of neutral hydrogen (i.e. the
Lyman limit systems, LLSs) in absorption within large spectroscopic surveys of
z>3 quasars. We test the performance of this method on mock quasar spectra that
reproduce the expected data quality of the Dark Energy Spectroscopic Instrument
(DESI) and the WHT Enhanced Area Velocity Explorer (WEAVE) surveys, finding
>90% completeness and purity for N(HI)> 10^17.2 cm^-2 LLSs against quasars of g
< 23 mag at z~3.5-3.7. After training and applying our method on 10,000 quasar
spectra at z~3.5-4.0 from the Sloan Digital Sky Survey (Data Release 16), we
identify ~6600 LLSs with N(HI)>10^17.5 cm^-2 between z~3.1-4.0 with a
completeness and purity of >90% for the classification of LLSs. Using this
sample, we measure a number of LLSs per unit redshift of 2.32 +/- 0.08 at
z=[3.3,3.6]. We also present results on the performance of random forest for
the measurement of the LLS redshifts and HI column densities, and for the
identification of broad absorption line quasars.
