High-Resolution Observations of the Lyman-α Forest in the Redshift Interval 0.9 ≤ z ≤ 1.9

Abstract

High-resolution spectroscopy of the bright QSOs HE 0515-4414 (z _em = 1.7, B = 15.0) and HS 0747+4259 (z _em = 1.9, V = 15.8) with HST/STIS combined with VLT/UVES and Keck/HIRES observations, respectively, offers for the first time the opportunity to study the Lyman-α forest in the scarcely examined redshift range 0.9 < z < 1.9 at a resolution of ≤ 10 kms^-1. The number density evolution of the Lyman-α lines is well described by the power law approach dn/dz α (1 + z)^γ. We find γ = -0.13 ± 0.33 for the weak lines (13.10 ≤ log N _HI ≤ 14.00) indicating a constant comoving density of these objects, while the high column density lines (13.64 ≤ log N _HI ≤ 16.00) show evolution with γ = 1.22 ± 0.59. The expected slow-down in their evolution does not appear earlier than z ∼ 1.