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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$_{ u}$ and one with a dex below J$_{ u} -$ 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.
Aims. We have searched for temporal variations of narrow absorption lines in high resolution quasar spectra. A sample of 5 distant sources have been assembled, for which 2 spectra - VLT/UVES or Keck/HIRES - taken several years apart are available. Me
We have monitored 12 intrinsic narrow absorption lines (NALs) in five quasars and seven mini-broad absorption lines (mini-BALs) in six quasars for a period of 4-12 years (1-3.5 years in the quasar rest-frame). We present the observational data and th
We wish to study the extent and subparsec scale spatial structure of intervening quasar absorbers, mainly those involving neutral and molecular gas. We have selected quasar absorption systems with high spectral resolution and good S/N data, with some
We report on the highly variable SiIV and CIV broad absorption lines in SDSS J113831.4+351725.2 across four observational epochs. Using the SiIV doublet components, we find that the blue component is usually saturated and non-black, with the ratio of
We identify a sample of 74 high-redshift quasars (z>3) with weak emission lines from the Fifth Data Release of the Sloan Digital Sky Survey and present infrared, optical, and radio observations of a subsample of four objects at z>4. These weak emissi