Metal Enrichment of the Intergalactic Medium and Production of Massive Black Holes

Abstract

A model for the chemical evolution of the intergalactic medium (IGM) is presented using theoretical yields of very massive stars (VMSs; M_(VMS) > 100 M_☉) and Type II supernovae (SNe II). It is shown that if [Si/C] is indeed as high as ~0.7 in the IGM, then VMSs (M_(VMS) ≈ 140-260 M_☉) associated with pair-instability supernovae (PI-SNe) in low-mass (~10^5 M_☉) halos at high redshift must produce at least 50% of the Si. The remainder is from later galactic outflows of SN II debris, which also provide most of the C and O. Both sources are required to account for the metal inventory in the IGM. The early VMS production must continue until redshift z ~ 15 so that the efficiency of VMS formation per low-mass halo is significantly below unity. Contributions from the later galactic outflows mainly occur at z ~ 4-6. Using a Salpeter initial mass function, we infer that the number of VMSs (M_(VMS) ≈ 260-2000 M_☉) producing massive black holes (MBHs) with an average mass 〈M_(MBH)〉 ~ 270-550 M_☉ is ≈0.72 times the number of VMSs associated with PI-SNe. The amount of metals (particularly Si) in the IGM that is attributable to PI-SNe is thus closely coupled with the total mass of MBHs produced in epochs prior to galaxy formation. Production of ~50% of the Si in the IGM by PI-SNe corresponds to an early inventory of MBHs that constitutes a fraction ~(4-8) × 10^(-5) of the total baryonic mass in the universe. This is comparable to the global mass budget of the central supermassive black holes (SMBHs) in present-day galaxies. The corresponding occurrence rates in each halo of ~10^5 M_☉ during the epoch of VMS formation at z ≳ 15 are ~0.9 Gyr^(-1) for VMSs associated with PI-SNe and ~0.6 Gyr^(-1) for the concomitant more massive stars producing MBHs. These rates may be of use to studies of H_2 dissociation and reionization and to models of SMBH formation.