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 x 10¹4^ cm^-2^ have measurable C IV lines with N_ C IV_ > 10¹2^ cm^-2^. We find the ratio of the median column density of C IV to the median NH I_ to be 2 X 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 NH I_ ~3 X 10¹7^ cm^-2^ and to the mean value in lower column density samples. The kinematic structure of the C IV 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.