Signatures of Galactic Superwinds: Inhomogeneous Metal Enrichment of the Lyman Alpha Forest

Abstract

We investigate possible signatures of feedback from galactic superwinds on the metallicity of the Lyman alpha forest, using a set of high resolution hydrodynamic simulations of a Lambda cold dark matter model. Simulations produce metals self-consistently, based on one single parameter, the metal yield, which in turn is constrained by metallicity in the intra-cluster gas. We follow metals as a separate density species. For the metallicity of Lyman alpha clouds with column density of N_{HI}~10^{14.5}-10^{15.5}/cm^2 at z=2-4 we find reasonable agreement between simulations, both with and without GSW, and observations (Schaye et al). A unique signature and sensitive test of GSW is, however, provided by lower density regions with gas density of ρ/<ρ> = 0.01-1.0 and a corresponding column density of 10^{12}-10^{14}/cm^2. Without GSW we predict that both the mean and median metallicity of \lya clouds in this column density range at z=2-4 should have Z<10^{3}Zsun, since these small systems support little star formation. GSW contaminate these regions, however, and also there is a significant fraction (~25%) of Lyman alpha clouds in this column density range which have a high metallicity excess of 10^{-2}Zsun, resulting in a mean metallicity of ~10^{-2}Zsun. In addition, we find that there is a minimum in the median metallicity for clouds of N_{HI}~10^{13}-10^{14}/cm^2 in the case with GSW, whereas without GSW the metallicity decreases monotonically and rapidly with decreasing column density. Finally, we predict that the ratio of secondary (e.g., N) to primary metals (e.g., O,C) is expected to be smaller by a factor of 10 in clouds of N_{HI}~10^{14.5}/cm^2 compared to that in large galaxies; this factor increases to >50 for N_{HI}< 10^{13.5}/cm^2.