Abstract
Studies of quasar absorption lines reveal that the low-density intergalactic medium (IGM) at z ~ 3 is enriched to between 10-3 and 10-2 solar metallicity. This enrichment may have occurred in an early generation of Population III stars at redshift z 10, by protogalaxies at 6 z 10, or by larger galaxies at 3 z 6. This paper addresses the third possibility by calculating the enrichment of the IGM at z 3 by galaxies of baryonic mass 108.5 M☉. We use already completed cosmological simulations, to which we add a prescription for chemical evolution and metal ejection by winds, assuming that the winds have properties similar to those observed in local starbursts and Lyman break galaxies. Results are given for a number of representative models, and we also examine the properties of the galaxies responsible for the enrichment as well as the physical effects responsible for wind escape and propagation. We find that winds of velocity 200-300 km s-1 are capable of enriching the IGM to the mean level observed, although many low-density regions would remain metal free. Calibrated by observations of Lyman break galaxies, our calculations suggest that most galaxies at z 3 should drive winds that can escape and propagate to large radii. The primary effect limiting the enrichment of low-density intergalactic gas in our scenario is then the travel time from high- to low-density regions, implying that the metallicity of low-density gas is a strong function of redshift.
Highlights
III stars at redshift z ∼> 10, by protogalaxies at 6 ∼< z ∼< 10, or by larger galaxies at 3 ∼< z ∼< 6
The mass of star formation is tallied for a given galaxy, for which we compute the mass, star-formation rate (SFR), and an area A = 44(M/1011 M⊙)0.76 kpc2 based on an empirical relation between disk area and mass (Gavazzi, Peirini & Boselli 1996)
Gnedin finds that supernova blowapart of protogalaxies is ineffective, but that dynamical removal of metals can explain the metals in low-density regions – though this requires a metal formation rate at z ∼> 4 that is about 10-50 times that observationally inferred at z ∼ 3 − 4 (e.g., Steidel et al 1999) and implies more efficient dynamical removal of metals than found by Aguirre et al (2001)
Summary
III stars at redshift z ∼> 10, by protogalaxies at 6 ∼< z ∼< 10, or by larger galaxies at 3 ∼< z ∼< 6. The simulation is described in more detail in Aguirre et al (2001) and in Weinberg et al (1999) These simulations do not include winds, but we use a parameterized prescription, described in § 2, to add metals in a way that models their ejection and distribution by supernova-driven galactic outflows. This post-processing method allows us to vary our assumptions and parameters, and we give results for a number of representative models.
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