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Balancing Outflows and Gas Dilution: The Mass-Metallicity Relation at z=0 | | Molly S. Peeples
; Francesco Shankar
; | | Date: |
21 Jul 2010 | | Abstract: | We present a new formalism with which to understand the relation between
galaxy stellar mass and gas-phase oxygen abundance that explicitly considers
the mass-dependence of galaxy gas fractions and outflows. By assuming that
galaxies populate zero-scatter relations between their stellar masses, gas
fractions, metallicities, outflow efficiencies, and halo properties, we show
that if metal-accretion is negligible, then a galaxy’s gas-phase metallicity Zg
can be simply expressed as Zg=y[zetaw+alpha*Fg+1]^-1, where y is the
nucleosynthetic yield, zetaw is a term describing the efficiency with which the
galaxy expels its metals, Fg is the gas-to-stellar mass ratio, and alpha is a
factor of order unity. We apply this formalism to z~0 observations to show that
reproducing observed oxygen abundances simultaneously with observed galaxy gas
fractions requires efficient outflows. Without winds, models that match the
mass-metallicity relation have Fg>=0.3 dex higher than observed. Moreover, gas
fractions at z=0 are small enough the mass-metallicity relation does not depend
sensitively on the exact slope of the Fg-Mstar relation. Successful models
require metal-expulsion efficiencies that are high and scale steeply with mass.
Specifically, most reasonable models require zetaw>1 and zetaw proportional to
vvir^-3 or steeper, where zetaw=(Zw/Zg)(Mw/MSFR) is the metallicity-weighted
mass-loading parameter, Zw is the metallicity of the outflowing material, Mw is
the mass outflow rate, and MSFR is the star formation rate. If the unweighted
mass-loading factor etaw=Mw/MSFR scales as vvir^-1 or vvir^-2 as has been
suggested from momentum- or energy-driven models, then a steep mass-dependence
of zetaw implies that the Zw-Mstar relation should be shallower than the
Zg-Mstar relation. | | Source: | arXiv, 1007.3743 | | Services: | Forum | Review | PDF | Favorites | |
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