Chemical enrichment across the cosmic epochs

The metallicity of a galaxy (i.e. the content of elements heavier than helium) is a powerful tracer of the processes involved in galaxy evolution. Indeed, the metallicity is directly connected to the history of star formation (which is responsible for the production of metals), but also to the presence of outflows (which generally eject metal-enriched gas) and accretion of (~pristine) gas from the intergalactic medium, which generally dilutes the galaxy metallicity.

By exploiting an extensive Large Programme ("AMAZE") at the European Southern Observatory , targeting distant star forming galaxies, it was found for the first time that the metallicity of galaxies evolves significantly at z>3 (Maiolino et al. 2008, Troncoso, Maiolino et al. 2014). The first metallicity maps of such primeval galaxies have revealed that the origin of such evolution is associated with massive gas inflows and outflows in the early universe (Cresci, Mannucci, Maiolino et al. 2010, Troncoso, Maiolino et al. 2014).

More recently the investigation of metallicity gradients is being extended to a large sample of galaxies at z~1.5-2.5 throguh the ESO Large Programme KLEVER, which is providing a detailed description of the chemical enrichment of galaxies at high redshift by using multiple diagnostics accessible in the near-IR bands at these redshifts. Preliminary results are reported in Curti, Maiolino et al. (2019)

Line flux map (~proportional to SFR), velocity field and metalilcity map of a galaxy of the AMAZE Large Programme at z~3.1 (from Cresci et al. 2010)
Average metallicity gradients in local galaxies in bins of stellar mass (Belfiore, Maiolino et al. 2017).

The detailed, spatially resolved metal budget of nearby galaxies reveals that such gas flows, which occurred throughout the galaxy lifetime, leave their clear imprint on the metallicity distribution of local galaxies (Belfiore, Maiolino & Bothwell 2015). By measuring spatially resolved metallicity maps of thousands of galaxies (by exploiting the Manga SDSSIV survey) we have found a clear dependence of radial metallicity gradients on stellar mass, becoming steeper in massive galaxies, and providing precious information on the internal circulation of metals during galaxy evolution (Belfiore, Maiolino et al. 2017).

Three-dimentional relation between gas metallicity, stellar mass and star formation (Mannucci, Cresci, Maiolino et al. 2010)

We have further shown that the metallicity of galaxies follow well defined scaling relations with other galaxy properties, such as stellar mass, star formation rate, atomic gas content and molecular gas content, often with small scatter, and that such relations seem to persist at high redshift (Mannucci, Cresci, Maiolino et al. 2010; Bothwell, Maiolino, et al. 2013; Bothwell, Maiolino et al. 2016; Maiolino et al. 2008; Troncoso, Maiolino et al. 2014, Curti, Maiolino et al. 2019, Curti et al. 2020b). Such tight scaling relations have been modeled by several theoretical groups and generally support a scenario in which the evolution of the bulk of the galaxy population evolves following smooth evolutionary processes, in which star formation, inflows and outflows are generally close to equilibrium (e.g. Peng & Maiolino 2014a).

Metallicity dependence on galaxy overdensity for central and satellite galaxies, in bins of stellar mass (Peng & Maiolino 2014b). Central galaxies show no evidence for any dependence on environment while satellite galaxies show a clear dependence on galaxy overdensity.

We also found that the gas metallicity of satellite galaxies depends strongly on the environment in which they live, with galaxies living in overdense environments being systematically characterized by higher metallicity (Peng & Maiolino 2014b); this has been interpreted as evidence that galaxies in dense environments accrete gas that has been pre-enriched (and expelled into the intergalactic medium) by other galaxies. Indication for a similar effect has been found in distant galaxies at z~1.5 (Williams, Maiolino et al. 2013), suggesting that the intergalactic medium is already highly enriched at these early epochs.

Interestingly, the stellar metallicity of satellite galaxies is much less dependent on environment indicating that the gas-phase enrichment is a phenomenon occurring on short timescales (Trussler, Maiolino, et al. 2020).

Mass-stellar metallicity relation for local star forming galaxies, local passive galaxies and their star forming projenitors at high redshift. The metallicity difference between passive galaxies and their star forming projenitors is an indication of starvation during the quenching process (Trussler, Maiolino et al. 2019)

By investigating the stellar metallicities of several thousands galaxies, and by comparing them with their star forming progenitors at high redshift, we could infer that the quenching of star formation in galaxies involves an extensive phase of starvation (halt of fresh gas supply), although outflows also play a role, especially in low mass galaxies.

Trussler, Maiolino et al. 2019,

Peng, Maiolino, Cochrane 2015

Top: maps of Lyalpha, CIV, HeII and CIII] emission obtained from the stacking of 80 quasars at z~2. Bottom radial surface brighness of these lines indicating the enrichment of metals at least out to 50 kpc (Guo, Maiolino et al. 2020).

By using deep 3D spectroscpic data of a large sample of 80 quasars at z ∼ 3 obtained with the MUSE instrument at the Very Large Telescope we have detected emission of metal lines in the circumgalactic medium (CGM) of these galaxies on scales of ~50-60 kpc. Through a detailed analysis we have revealed that the circumgalactic medium of these massive primeval galaxies is composed of two phases: a metal-rich component located within the central 30–50 kpc and associated with chemical pre-enrichment by past quasar-driven outflows and a more extended component of the CGM that has much lower metallicity and likely associated with near-pristine gas accreted from the intergalactic medium.

Guo, Maiolino et al. 2020

An extensive review of the chemical enrichment of galaxies across the cosmic epochs is given in Maiolino & Mannucci (2019)