A deep search for metals near redshift 7: the line-of-sight towards ULAS
  J1120+0641
  We present a search for metal absorption line systems at the highest
redshifts to date using a deep (30h) VLT/X-Shooter spectrum of the z = 7.084
quasi-stellar object (QSO) ULAS J1120+0641. We detect seven intervening systems
at z > 5.5, with the highest-redshift system being a C IV absorber at z = 6.51.
We find tentative evidence that the mass density of C IV remains flat or
declines with redshift at z < 6, while the number density of C II systems
remains relatively flat over 5 < z < 7. These trends are broadly consistent
with models of chemical enrichment by star formation-driven winds that include
a softening of the ultraviolet background towards higher redshifts. We find a
larger number of weak ( W_rest < 0.3A ) Mg II systems over 5.9 < z < 7.0 than
predicted by a power-law fit to the number density of stronger systems. This is
consistent with trends in the number density of weak Mg II systems at z = 2.5,
and suggests that the mechanisms that create these absorbers are already in
place at z = 7. Finally, we investigate the associated narrow Si IV, C IV, and
N V absorbers located near the QSO redshift, and find that at least one
component shows evidence of partial covering of the continuum source.
