THE METALLICITY AND INTERNAL STRUCTURE OF THE LYMAN-ALPHA FOREST CLOUDS

Using very high S/N, high spectral resolution data obtained with the HIRES spectrograph on the Keck 10 m telescope, we show that roughly half of Lyman-alpha forest clouds with NHI≥3×1014 cm-2 have measurable C IV lines with NCIV>1012 cm-2. We find the ratio of the median column density of C IV to the median NHI to be 2×10-3, roughly corresponding to a metallicity of 10-2 solar. This is shown to be comparable to that of a sample of Lyman-limit system clouds with NHI∼3×1017 cm-2 and to the mean value in lower column density samples. The kinematic structure of the CIV lines shows that the Lyα clouds are blended complexes of cloudlets individually having T≈35,000-70,000 K and velocity dispersions of ∼18 km s-1; these complexes require a gravitational confinement mechanism.