7.1. Comparison with other Observations
The total mass processed by stars is not a directly observable quantity because some fraction of the processed mass will be hidden in stellar remnants or recycled back into the ISM. Estimates of recycling fraction range from 30-50% for various IMF models (see discussions in Pagel 1997), but the cumulative effect of many generations of star formation and repeated recycling is difficult to estimate. Firm lower and upper limits for * are, however, directly observable: the observed mass in stars and stellar remnants at z ~ 0 is a lower limit to the total mass which has been processed through stars, and the total baryon fraction from Big Bang nucleosynthesis is an upper limit. FHP98 estimate the mass fraction in stars and stellar remnants at z ~ 0 to be stars = 0.0025(± 0.001)h-1, corresponding to a mass-to-light ratio of (M / L)B ~ 5.9(M / L). In units of B, this lower limit is stars = 0.13(± 0.05) hB. Our estimate of the total mass fraction processed through stars, * = 0.33(± 0.07)B, is comfortably above this lower limit and is, obviously, less than the upper limit - the total baryon mass fraction from Big Bang nucleosynthesis and deuterium measurements.
We can also compare the metal mass fraction predicted by the EBL with the observed mass fraction in gas, stars, and stellar remnants in the local universe. Based on recent estimates by FHP99, ~ 80% of the observed baryons at z ~ 0 are located in the intracluster gas of groups and clusters, 17% are in stars and stellar remnants, and only 3% are in neutral atomic and molecular gas. The observed metal mass fraction in hot intracluster gas has been estimated to be at least 0.33Z for rich galaxy clusters and 0.25 - 1Z in groups based on X-ray observations of Fe features (Renzini 1997, Mushotzky & Loewenstein 1997). More recent estimates from Buote (1999, 2000) based on more detailed models of the temperature distribution of the intracluster gas have found values closer to 1Z in several clusters and elliptical galaxies. For a total cluster and group gas mass density of gas = 0.011+0.013-0.005 h-1 (FHP98) and assuming a metal mass fraction of 0.65± 0.35 Z for clusters and groups of all masses, the observed metal mass fraction in clusters is Z, gas = 0.007+0.009-0.005 h-1 Z. Repeating this exercise for the stellar component, we assume that the metallicity of stars at z ~ 0 is roughly solar (1.0± 0.25 Z) and that the mass density in stars is stars = 0.0025(± 0.001)h-1. The total metal mass in stars and stellar remnants locally is then Z, star = 0.0025(± 0.001)h-1 Z. The total metal mass fraction in the local universe is then Z = Z, star + Z, gas = 0.0095+0.010-0.006 h-1 Z, or 0.50+0.52-0.32 hZ B. This estimate is consistent with the value indicated by the bolometric EBL, 0.24(± 0.13) Z B, as calculated above.