4.2. Element Abundances in the Interstellar Gas
The ESI spectrum of cB58, which covers the wavelength region from 1075 to 2800Å at a resolution of 58 km s-1, is a real treasure trove of information on this galaxy. For example, it includes 48 interstellar absorption lines of elements from H to Zn in a variety of ionisation stages, from neutral (H I, C I, O I, N I) to highly ionised species (Si IV, C IV, N V). The lines are fully resolved so that column densities can be derived from the analysis of their profiles. From these data Pettini et al. (2002b) were able to piece together for the first time a comprehensive picture of the chemical composition of the interstellar gas in a Lyman break galaxy (Figure 25) and examine the clues it provides on its evolutionary status and past history of star formation.
As can be seen from Figure 25, the ambient interstellar medium of cB58 is highly enriched in the elements released by Type II supernovae; O, Mg, Si, P, and S all have abundances of ~ 2/5 solar. Thus, even at this relatively early epoch (z = 2.7276 corresponds to 2.5 Gyr after the Big Bang in our adopted cosmology - see Table 1), this galaxy had already processed more than one third of its gas into stars.
Figure 25. Pattern of chemical abundances in the ambient interstellar medium of cB58 deduced by Pettini et al. (2002b). The vertical height of the boxes shows the typical uncertainty in the abundance determinations. Blue boxes denote elements thought to be synthesised by massive stars which explode as Type II supernovae, while red boxes are for the Fe-peak elements predominantly produced by Type Ia SN. Their release into the ISM, as well as that of N from intermediate mass stars, lags behind that of the Type II SN products by several 100 Myr.
Furthermore, cB58 appears to be chemically young, in that it is relatively deficient in elements produced by stars of intermediate and low mass with longer lifetimes than those of Type II SN progenitors. N and the Fe-peak elements we observe (Mn, Fe, and Ni) are all less abundant than expected by factors of between 0.4 and 0.75dex. Depletion onto dust, which is known to be present in cB58, probably accounts for some of the Fe-peak element underabundances, but this is not likely to be an important effect for N. On the basis of current ideas of the nucleosynthesis of N, discussed in Section 2.4.3, it would appear that much of the ISM enrichment in cB58 has taken place within the last 250 Myr, the lifetime of the intermediate mass stars believed to be the main source of N. For comparison, the starburst episode responsible for the UV and optical light we see is estimated to be younger than ~ 35 Myr, on the basis of theoretical models of the spectral energy distribution at these wavelengths (Ellingson et al. 1996).
Taken together, these two findings are highly suggestive of a galaxy caught in the act of converting its interstellar medium into stars on a few dynamical timescales - quite possibly in cB58 we are witnessing the formation of a galactic bulge or an elliptical galaxy. The results of the chemical analysis are consistent with the scenario proposed by Shapley et al. (2001), whereby galaxies whose UV spectra are dominated by strong, blueshifted absorption lines, as is the case here, are the youngest in the range of ages of LBGs. These findings also lend support to models of structure formation which predict that, even at z 3, near-solar metallicities should in fact be common in galaxies with masses greater than ~ 1010 M (e.g. Nagamine et al. 2001). The baryonic mass of cB58 is deduced to be mbaryons 1 × 1010 M, from consideration of its star formation history, metallicity, and the velocity dispersion of its ionised gas.