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 reionisation (z � 6) as sources of energetic, neutral hydrogen ionising 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 lives. Most massive stars are expected to form in binaries, and an appreciable fraction of these (as much as � 30%) will evolve into X-ray luminous (LX � 10 38 erg/s) 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 main sequence lives. For reasonable estimates, we find that the bolometric luminosity of the cluster peaks at � 10 42 erg/s during the first few million years, but declines to � 10 41 erg/s after � 5 million years as the most massive stars evolve off the main sequence. From this time onwards, the total bolometric luminosity is dominated by HMXBs and falls gradually to � 10 40 erg/s after � 50 million years. Assuming a power-law spectral energy distribution for the HMXBs, we calculate the effective number of neutral hydrogen ionisations per HMXB and show that HMXBs can be as important as sources of ionising 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 reionisation.
Highlights
Hydrogen is the most abundant element in the Universe and it is fundamental to galaxy formation, representing the principal raw material from which stars form
Less mass is lost during the supernova when the remnant is a black hole rather than a neutron star, and black holes may suffer less violent natal kicks. These arguments suggest that binaries that survive to become high-mass X-ray binaries (HMXBs) in primordial globular clusters are more likely to contain black holes, which suggests in turn that they could be more X-ray luminous than typical HMXBs in the local Universe, which are dominated by neutron star systems
Dense and relatively simple stellar systems whose dynamical properties and evolution are well understood. These properties have led to increasing interest in globular clusters as probes of the conditions under which galaxies formed at high redshifts
Summary
Hydrogen is the most abundant element in the Universe and it is fundamental to galaxy formation, representing the principal raw material from which stars form. In this paper we consider what fraction of massive stars in primordial globular clusters must evolve into HMXBs for these sources to make a significant contribution to the Xray ionising background.
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