The missing metals problem – II. How many metals are in z ⋍ 2.2 galaxies?

Abstract

Abstract In the context of the ‘missing metals problem’, the contributions of the ultraviolet-selected z ⋍ 2.2‘BX’ galaxies and z ⋍ 2.5‘distant red galaxies’ (DRGs) have not been discussed previously. Here we show that: (i) DRGs make only a marginal contribution to the metal budget (∼5 per cent); (ii) BX galaxies contribute as much as 18 per cent to the metal budget; and (iii) the K-bright subsample (K < 20) of the BX sample (roughly equivalent to the ‘BzK’ selected samples) contributes roughly half of this 18 per cent, owing to both their larger stellar masses and higher metallicities, implying that the rare K-bright galaxies at z > 2 are a major source of metals in the budget. We showed in the first paper of this series that submillimetre galaxies (SMGs) brighter than 3 mJy contribute ∼5 per cent (≲9 per cent as an upper limit) to the metal budget. Adding the contribution of SMGs and damped Lyα absorbers to the contribution of ultraviolet-selected galaxies implies that at least 30 per cent of the metals (in galaxies) have been accounted for at z ⋍ 2. The cosmic metal density thus accounted for is ρZ,galaxies ⋍ 1.3 × 106 M⊙ Mpc-3 or, in terms of the closure density, ΩZ= 9.6 × 10−6. This is a lower limit given that galaxies on the faint end of the luminosity function are not included. An estimate of the distribution of metals in local galaxies as a function of luminosity suggests that galaxies with luminosity <L⋆ contribute about half of the total mass of metals. If the metals in galaxies at z ∼ 2 are similarly distributed then faint galaxies alone cannot solve the ‘missing metals problem’. Galaxy populations at z ∼ 2 account for only about 50 per cent of the total metals predicted.