Metal Lines Associated with Lyman Alpha Absorbers: A Comparison of Theory and Observations

We study metal line absorption of CIV, CII, SiIV, and NV at redshifts z=3.5 to z=2 within the framework of a cosmological model for the Lyman alpha forest, comparing the results of numerical simulations to recent observations by Songaila & Cowie (1996, SC). In agreement with Rauch, Haehnelt & Steinmetz (1996), we find that the observed mean value of the CIV/HI ratio around z=3 is reproduced if a uniform metallicity of [C/H] = -2.5 is assumed in our model, but that the observed scatter in this ratio is larger than predicted, implying a scatter in the metallicity of the absorbing systems of roughly an order of magnitude. We further argue that absorbers with relatively low column densities (log(N_HI) < 15) likely have a mean metallicity [C/H] less than -2.5, a result which is basically independent of the model considered, and similar to what is predicted in the simulations by Gnedin & Ostriker (1997). Our model predicts no significant evolution in the mean values of metal line column densities between z=3.5 and z=2. Comparison of the predicted and observed numbers of SiIV and NV systems suggests that the photoionizing background radiation field around z=3 is somewhat softer than that proposed by Haardt & Madau (1996). Our model does not account for the increase in the SiIV/CIV ratio at z=3.2 found by SC. While SC suggested that the increase could be explained by a softening of the radiation field due to HeII absorption at z > 3, such a modification does not raise the mean value of SiIV/CIV in our simulation because it shifts numerous weak SiIV systems to just above the detection limit, thus keeping the mean column density of observable SiIV systems low.