Primordial globular clusters, X-ray binaries and cosmological reionization

Abstract

Globular clusters are dense stellar systems that have typical ages of ∼13 billion years, implying that they formed during the early epochs of galaxy formation at redshifts of z≳ 6. Massive stars in newly formed or primordial globular clusters could have played an important role during the epoch of cosmological reionization (z≳ 6) as sources of energetic, neutral hydrogen ionizing UV photons. We investigate whether or not these stars could have been as important in death as sources of energetic X-ray photons as they were during their main sequence (MS) lives. Most massive stars are expected to form in binaries, and an appreciable fraction of these (as much as ∼30 per cent) will evolve into X-ray luminous (LX∼ 1038 erg s−1) high-mass X-ray binaries (HMXBs). These sources would have made a contribution to the X-ray background at z≳ 6. Using Monte Carlo models of a globular cluster, we estimate the total X-ray luminosity of a population of HMXBs. We compare and contrast this with the total UV luminosity of the massive stars during their MS lives. For reasonable estimates, we find that the bolometric luminosity of the cluster peaks at ∼1042 erg s−1 during the first few million years, but declines to ∼1041 erg s−1 after ∼5 Myr as the most massive stars evolve off the MS. From this time onwards, the total bolometric luminosity is dominated by HMXBs and falls gradually to ∼1040 erg s−1 after ∼50 Myr. Assuming a power-law spectral energy distribution for the HMXBs, we calculate the effective number of neutral hydrogen ionizations per HMXB and show that HMXBs can be as important as sources of ionizing radiation as massive stars. Finally, we discuss the implications of our results for modelling galaxy formation at high redshift and the prospects of using globular clusters as probes of reionization