Can UVB variations reconcile simulated quasar absorption lines at high
  redshift?
  In this work, we present new calculations of the observables associated with
synthetic metal and HI absorption lines in the spectra of high redshift
quasars, inspired by questions and limitations raised in work with a uniform
Haardt-Madau 2012 ultraviolet background (UVB). We introduce variations at $z
\sim$ 6 to the UVB and HI self--shielding and explore the sensitivity of the
absorption features to modifications of the hardness of the UVB. We find that
observed SiIV and low ionization states (e.g. CII, SiII, OI) are well
represented by a soft UV ionizing field at $z =$ 6 but, this same prescription,
fails to reproduce the statistical properties of the observed ion CIV absorber
population. Therefore, we recommend a moderate reduction of the UVB at this
redshift, an emissivity change between the UVB models that lies in between the
Haardt-Madau 2012 emissivity J$_{\nu}$ and one with a dex below J$_{\nu} -$ 1
at 1 Ryd. On the other hand, variations in the HI self--shielding (SSh)
prescription leave a non--negligible imprint in the calculated HI column
density distribution function (CDDF) at $z =$ 4 and the comoving mass density
of neutral Hydrogen (and the associated calculation with damped Lyman--$\alpha$
absorber systems) at 4 $< z <$ 6. We conclude that small variations in the UVB
and HI SSh at $z \sim$ 6 play an important role in improving the estimation of
metal ions and HI statistics at this redshift.
