Anisotropic AGN Outflows and Enrichment of the Intergalactic Medium. II. Metallicity

TL;DR

This study uses cosmological simulations to show that anisotropic AGN outflows significantly enrich the intergalactic medium with metals, especially in underdense regions, with enrichment levels increasing over cosmic time.

Contribution

It introduces a detailed anisotropic outflow model in cosmological simulations to quantify AGN-driven metal enrichment of the IGM across redshifts.

Findings

AGN with L > 10^9 L_sun dominate IGM enrichment.

Average IGM metallicity reaches [O/H] = -2.8 at z=0.

Enriched volume fractions increase with decreasing redshift.

Abstract

We investigate the large-scale influence of outflows from AGNs in enriching the IGM with metals in a cosmological context. We combine cosmological simulations of large scale structure formation with a detailed model of metal enrichment, in which outflows expand anisotropically along the direction of least resistance, distributing metals into the IGM. The metals carried by the outflows are generated by two separate stellar populations: stars located near the central AGN, and stars located in the greater galaxy. Using this algorithm, we performed a series of 5 simulations of the propagation of AGN-driven outflows in a cosmological volume of size (128/h Mpc)^3 in a Lambda-CDM universe, and analyze the resulting metal enrichment of the IGM. We found that the metallicity induced in the IGM is greatly dominated by AGNs having bolometric luminosity L > 10^9 L_sun, sources with 10^8 < L / L_sun < 10^9 having a negligible contribution. Our simulations produced an average IGM metallicity of [O/H] = -5 at z = 5.5, which then rises gradually, and remains relatively flat at a value [O/H] = -2.8 between z = 2 and z = 0. The ejection of metals from AGN host galaxies by AGN-driven outflows is found to enrich the IGM to > 10 - 20% of the observed values, the number dependent on redshift. The enriched IGM volume fractions are small at z > 3, then rise rapidly to the following values at z = 0: 6 - 10% of the volume enriched to [O/H] > -2.5, 14 - 24% volume to [O/H] > -3, and 34 - 45% volume to [O/H] > -4. At z > 2, there is a gradient of the induced enrichment, the metallicity decreasing with increasing IGM density, enriching the underdense IGM to higher metallicities, a trend more prominent with increasing anisotropy of the outflows. This can explain observations of metal-enriched low-density IGM at z = 3 - 4.