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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$_ν$ and one with a dex below J$_ν -$ 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--$α$ 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.