Metal lines associated with Ly alpha absorbers: A comparison of theory and observations

We study the metal-line absorption of C IV, C II, Si IV, and N V at redshifts z = 3.5 to z = 2 within the framework of a cosmological model for the Ly alpha forest, comparing the results of numerical simulations to recent observations by Songaila & Cowie (SC). We find that the observed mean value of the C IV/H I ratio at z similar or equal to 3 is reproduced if a uniform metallicity of [C/H] similar to -2.5 is assumed in our model, but that the observed scatter in this ratio is larger than predicted, implying a scatter of roughly an order of magnitude in the metallicity of the absorbing systems. The enrichment pattern that is required for our model to match SC's C IV observations is very similar to that predicted by other simulations of reionization and early metal enrichment in this type of cosmological scenario. 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 Si IV and N V systems suggests that the photoionizing background radiation held at z similar to 3 is somewhat softer than that proposed earlier. Our model does not account for the increase in the Si IV/C IV ratio at z greater than or similar to 3.2 found by SC. While the latter study suggested that the increase could be explained by a softening of the radiation field caused by He II absorption at z greater than or similar to 3, such a modification does not raise the mean value of Si IV/C IV significantly in our simulation, because it shifts numerous weak Si IV systems to just above the detection limit, thus keeping the mean column density of observable Si IV systems low.