Zusammenfassung
The star formation rate (SFR) depends sensitively on the gas metallicity,
because metals are crucial to mediating the transition from
intermediate-temperature atomic gas to cold molecular gas, a necessary
precursor to star formation. We study the impact of this effect on the star
formation history of galaxies. We incorporate metallicity-dependent star
formation and metal enrichment in a simple model that follows the evolution of
a halo main progenitor. Our model shows that including the effect of
metallicity leads to suppression of star formation at redshift z>2 in dark
halos with masses >~ 10^11 Msun, with the suppression becoming near total for
halos below ~10^9.5-10 Msun. We find that at high redshift, till z~2, the SFR
cannot catch up with the gas inflow rate (IR), because the SFR is limited by
the free-fall time, and because it is suppressed further by a lack of metals in
small halos. As a result, in each galaxy the SFR is growing in time faster than
the IR, and the integrated cosmic SFR density is rising with time. The
suppressed in situ SFR at high z makes the growth of stellar mass dominated by
ex situ SF, which implies that the specific SFR (sSFR) remains constant with
time. The intensely accreted gas at high $z$ is accumulating as an atomic gas
reservoir. This provides additional fuel for star formation in 10^10 - 10^12
Msun halos at z~1-3, which allows the SFR to exceed the instantaneous IR, and
may enable an even higher outflow rate. At z<1, following the natural decline
in IR with time due to the universal expansion, the SFR and sSFR are expected
to drop. We specify the expected dependence of sSFR and metallicity on stellar
mass and redshift. At a given z, and below a critical mass, these relations are
predicted to be flat and rising respectively. Our model predictions
qualitatively match some of the puzzling features in the observed star
formation history.
Beschreibung
[1106.0301] Metallicity-Dependent quenching of Star Formation at High Redshift in Small Galaxies
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