A deep search for metals near redshift 7: the line of sight towards ULAS J1120+0641

Abstract

We present a search for metal absorption line systems at the highest redshifts to date using a deep (30 h) Very Large Telescope/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 (Wrest < 0.3 Å) 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.