Metal Factories in the Early Universe

We have measured the mass of metals in 13 submillimetre galaxies at z~4 in which the gas, based on previous observations, lies in a cold rotating disk. We measured the metal masses using either the submillimetre line or continuum emission from three tracers of the overall metal content - carbon atoms, carbon monoxide molecules and dust grains - using the first calibration of this technique that treats all three tracers simultaneously (Dunne et al. 2022). We obtain very similar mass estimates from the different tracers, which are similar to the entire metal content of a present-day massive early-type galaxy. We used the dynamical masses of these galaxies to set an upper limit on the mass of the interstellar medium in each galaxy, allowing us to set a lower limit on the metal abundance of the ISM, finding values for many of the galaxies well above the solar value. The only caveat to this result is that we cannot be certain for all galaxies that the emission from the tracer comes from within the region covered by the dynamical analysis. Our high values for the metal abundance are supported by a recent JWST study of one galaxy which found a super-solar metal abundance approximately 3 times greater than our lower limit. Using two chemical evolution models, we show that the high metal masses and metal abundances are what is expected shortly after the formation of a galaxy for a top-heavy IMF. We suggest a scenario for galaxy evolution in which massive galaxies reach a high metal abundance during their formation phase, which is then gradually reduced by dry mergers with lower mass galaxies. We use the chemical-evolution models to show that the metals in the outflows from massive early-type galaxies in their formation phase can quantitatively explain the long-standing puzzle that approximately 75% of the metals in clusters of galaxies is in the intracluster gas rather than in the galaxies.