We study the abundance of silicon in the intergalactic medium by analyzing the statistics of Si iv , C iv , and H i pixel optical depths in a sample of 19 high-quality quasar absorption spectra , which we compare with realistic spectra drawn from a hydrodynamical simulation .
Simulations with a constant and uniform Si/C ratio , a C distribution as derived in Paper II of this series , and a UV background ( UVB ) model from Haardt & Madau reproduce the observed trends in the ratio of Si iv and C iv optical depths , \tau _ { SiIV } / \tau _ { CIV } .
The ratio \tau _ { SiIV } / \tau _ { CIV } depends strongly on \tau _ { CIV } , but it is nearly independent of redshift for fixed \tau _ { CIV } , and is inconsistent with a sharp change in the hardness of the UVB at z \approx 3 .
Scaling the simulated optical depth ratios gives a measurement of the global Si/C ratio ( using our fiducial UVB , which includes both galaxy and quasar contributions ) of [ Si/C ] = 0.77 \pm 0.05 , with a possible systematic error of \sim 0.1 dex .
The inferred [ Si/C ] depends on the shape of the UVB ( harder backgrounds leading to higher [ Si/C ] ) , ranging from [ Si/C ] \simeq 1.5 for a quasar-only UVB , to [ Si/C ] \simeq 0.25 for a UVB including both galaxies and artificial softening ; this provides the dominant uncertainty in the overall [ Si/C ] .
Examination of the full \tau _ { SiIV } / \tau _ { CIV } distribution yields no evidence for inhomogeneity in [ Si/C ] and constrains the width of a lognormal probability distribution in [ Si/C ] to be much smaller than that of [ C/H ] ; this implies a common origin for Si and C. Since the inferred [ Si/C ] depends on the UVB shape , this also suggests that inhomogeneities in the hardness of the UVB are small .
There is no evidence for evolution in [ Si/C ] .
Variation in the inferred [ Si/C ] with density depends on the UVB and rules out the quasar-only model unless [ Si/C ] increases sharply at low density .
Comparisons with low-metallicity halo stars and nucleosynthetic yields suggest either that our fiducial UVB is too hard or that supermassive Population III stars might have to be included .
The inferred [ Si/C ] , if extrapolated to low density , corresponds to a contribution to the cosmic Si abundance of [ Si/H ] = -2.0 , or \Omega _ { Si } \simeq 3.2 \times 10 ^ { -7 } , a significant fraction of all Si production expected by z \approx 3 .