Simulating cosmic metal enrichment by the first galaxies

Abstract

We study cosmic metal enrichment via AMR hydrodynamical simulations in a (10\nMpc/h)$^3$ volume following the Pop III-Pop II transition and for different Pop\nIII IMFs. We have analyzed the joint evolution of metal enrichment on galactic\nand intergalactic scales at z=6 and z=4. Galaxies account for <9% of the\nbaryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e.\nregions with extremely low density ($\\Delta$<1), (b) the true intergalactic\nmedium (IGM, 1<$\\Delta$<10) and (c) the circumgalactic medium (CGM,\n10<$\\Delta<10^{2.5}$), the interface between the IGM and galaxies. By z=6 a\ngalactic mass-metallicity relation is established. At z=4, galaxies with a\nstellar mass $M_*=10^{8.5}M_\\odot$ show log(O/H)+12=8.19, consistent with\nobservations. The total amount of heavy elements rises from\n$\\Omega^{SFH}_Z=1.52\\, 10^{-6}$ at z=6 to 8.05 $10^{-6}$ at z=4. Metals in\ngalaxies make up to ~0.89 of such budget at z=6; this fraction increases to\n~0.95 at z=4. At z=6 (z=4) the remaining metals are distributed in\nCGM/IGM/voids with the following mass fractions: 0.06/0.04/0.01\n(0.03/0.02/0.01). Analogously to galaxies, at z=4 a density-metallicity\n($\\Delta$-Z) relation is in place for the diffuse phases: the IGM/voids have a\nspatially uniform metallicity, Z~$10^{-3.5}$Zsun; in the CGM Z steeply rises\nwith density up to ~$10^{-2}$Zsun. In all diffuse phases a considerable\nfraction of metals is in a warm/hot (T>$10^{4.5}$K) state. Due to these\nphysical conditions, CIV absorption line experiments can probe only ~2% of the\ntotal carbon present in the IGM/CGM; however, metal absorption line spectra are\nvery effective tools to study reionization. Finally, the Pop III star formation\nhistory is almost insensitive to the chosen Pop III IMF. Pop III stars are\npreferentially formed in truly pristine (Z=0) gas pockets, well outside\npolluted regions created by previous star formation episodes.\n