The metallicity and internal structure of the Lyman-alpha forest clouds

Abstract

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 N_HI_ >= 3 x 10^14^ cm^-2^ have measurable C IV lines with N_ C IV_ > 10^12^ cm^-2^. We find the ratio of the median column density of C IV to the median N_H 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 N_H I_ ~3 X 10^17^ 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.