Using cosmological hydrodynamic simulations that dynamically incorporate enriched galactic outflows together with analytical modelling, we study the origin of the stellar mass-gas-phase metallicity relation (MZR). We find that metallicities are driven by an equilibrium between the rate of enrichment owing to star formation and the rate of dilution owing to infall of unenriched gas. This equilibrium is in turn governed by the outflow strength. As such, the MZR provides valuable insights and strong constraints on galactic outflow properties across cosmic time. We compare three outflow models: no outflows, a `constant-wind model that emulates the popular Dekel & Silk scenario, and a `momentum-driven wind' model that best reproduces z >~ 2 intergalactic medium metallicity data. Only the momentum-driven wind scaling simulation is able to reproduce the observed z ~ 2 MZR's slope, amplitude, and scatter. In order to understand why, we construct a one-zone chemical evolution model guided by simulations. This model shows that the MZR in our outflow simulations can be understood in terms of three parameters: (i) the equilibrium metallicity Zg,eq = yṀSFR/ṀACC (where y = net yield), reflecting the enrichment balance between star formation rate ṀSFR and gas accretion rate ṀACC (ii) the dilution time , representing the time-scale for a galaxy to return to Zg,eq after a metallicity-perturbing interaction; and (iii) the blowout mass Mblowout, which is the galaxy stellar mass above which winds can escape its halo. Without outflows, galaxy metallicities exceed observations by approximately two to three times, although the slope of the MZR is roughly correct owing to greater star formation efficiencies in larger galaxies. When outflows with mass-loading factor ηW are present, galaxies below Mblowout obey Zg,eq ~ y/(1 + ηW), while above Mblowout, Zg,eq -> y. Our constant-wind model has Mblowout ~ 1010 Msolar, which yields a sharp upturn in the MZR above this scale and a flat MZR with large scatter below it, in strong disagreement with observations. Our momentum-driven wind model naturally reproduces the observed Zg ~ M0.3* because Zg,eq ~ η-1W ~ M1/3* when ηW >> 1 (i.e. at low masses). The flattening of the MZR at M* >~ 1010.5 Msolar observed by Tremonti et al. is reflective of the mass-scale where ηW ~ 1 rather than a characteristic outflow speed; in fact, the outflow speed plays little role in the MZR except through Mblowout. The tight observed MZR scatter is ensured when td <~ dynamical time, which is only satisfied at all masses in our momentum-driven wind model. We also discuss secondary effects on the MZR, such as baryonic stripping from neighbouring galaxies' outflows.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- April 2008
- galaxies: abundances;
- galaxies: evolution;
- cosmology: theory;
- 28 pages, 15 figures, accepted by MNRAS with minor revisions. The definitive version is available at www.blackell-synergy.com