The Metallicity of Galaxy Disks: Infall versus Outflow

Abstract

Both gas accretion (infall) and winds (outflow) change a galaxy's metallicity and gas fraction, lowering the effective yield. Low effective yields in galaxies with rotation speeds <120 km s À1 have been widely interpreted as due to the onset of supernova-driven winds below a characteristic galaxy mass, but gas accretion is also a viable explanation. However, calculations presented here prove that (1) metal-enriched outflows are the only mechanism that can significantly reduce the effective yield, but only for gas-rich systems; (2) it is nearly impossible to reduce the effective yield of a gas-poor system, no matter how much gas is lost or accreted; and (3) any subsequent star formation drives the effective yield back to the closed-box value. Thus, only gas-rich systems with low star formation rates (such as dwarf irregulars) can produce and maintain low effective yields. The drop in effective yield seen in low-mass galaxies is therefore due to these galaxies' gas richnesses and low star formation rates, which result from their surface densities falling entirely below the Kennicutt SF threshold. Additional calculations confirm that the fraction of baryonic mass lost through winds varies only weakly with galaxy mass, shows no sharp upturn at any mass scale, and does not require that >15% of the baryons have been lost by galaxies of any mass. Supernova feedback is therefore unlikely to be effective for removing large amounts of gas from low-mass disk galaxies. In addition, the dependence between metal loss and galaxy mass is sufficiently weak that massive galaxies dominate metal enrichment of the IGM. The calculations in this paper provide limiting cases for any arbitrary chemical evolution history, as is proven in an Appendix.

Insights into the Origin of the Galaxy Mass-Metallicity Relation

“…The solid line denotes the "combined" model, the dash-dotted line shows the effect of mass-dependent SFE only, and the dashed line shows the contribution of the mass-dependent metal-enriched winds. consistent with the conclusions independently drawn, for example, by Dalcanton (2007) and Ellison et al (2008).…”

Galaxy metallicity scaling relations in the EAGLE simulations

“…The trends predicted for y eff provide clues about the role of gas infall and/or winds on the evolution of galaxies (e.g. Dalcanton 2007). The decrease of the simulated y eff towards lower masses suggests that these systems have been affected by efficient outflows of metalenriched SN ejecta.…”

“…These models and simulations reproduce qualitatively the observed trends but they show discrepancies regarding the exact value of the slope and level of evolution of the predicted relations. Supernova (SN)-driven outflows have often been invoked as a key ingredient for establishing a MZR (Larson 1974;Tremonti et al 2004;Dalcanton 2007;Kobayashi, Springel & White 2007). Given the shallower potential wells of low-mass galaxies, metal-enriched material can be more efficiently ejected from these systems, keeping their metallicities low.…”

Gravitational and mass distribution effects on stationary superwinds

“…In terms of a closed-box enrichment model, Peeples et al (2008) explain that the only possibility is that these galaxies might have low gas fractions for their masses. In such a case, only a small pollution would be enough to enrich the gas (Dalcanton 2007). On similar grounds, Ellison et al (2008) analyzed a large galaxy sample from SDSS and determined that at fixed mass, galaxies with smaller half-light radii tend to have higher abundances.…”

“…Such a decrease of the velocity can lead to high densities and, in consequence, the flow could become both gravitationally and radiatively unstable and eventually enter into the outpouring or even the inpouring regime. Since this time, the gas could have been polluted by mixing with material external to the generating (proto-) galaxy before being reinserted, the inpouring (inflow) regime corresponds to an open-box metal enrichment scenario, or perhaps impoverishment or neither of both; see the general theorems presented by Edmunds (1990) and the work of Dalcanton (2007).…”