Authors: Katz & Ricotti
In this paper the contribution of old globular clusters to reionization is discussed. Observations of globular clusters in the Milky Way (and a few extra-galactic ones) reveal two distinct categories: metal-poor and metal-rich. Age determinations of these two populations are highly uncertain (errors of 1 Gyr) and there is a significant spread in ages (1 Gyr for the metal-poor and 6 Gyr for the metal-rich population). However, the age gap between metal-rich and metal-poor globular clusters is greater than the age range within each population, suggesting that there are two distinct epochs of globular cluster formation. Likely 55% (worst case: 20%) of the globular clusters in the Milky Way have formed at z>4. The authors try to constrain the formation rate of globular clusters at hight redshift by computing synthesised luminosity functions.
The following assumptions are made on globular cluster formation:
1) At the time of formation, the globular clusters were 9.1 times more massive than at the time they were observed in the Milky Way
2) The globular cluster initial mass function is lognormal or a power law
3) From the frequency of globular clusters in galaxies in the local Universe, the mass density and the formation rate can be inferred
The unknown is the fraction of todays globular clusters that formed at high redshift
The most massive globular clusters can be observed with HST up to redshift 6, and with JWST up to redshift 7.5. At z>0.01 globular clusters are unresolved and their light will dominate a high-z dwarf galaxy for about 10 Myr after their formation.
The authors constrain the fraction of globular clusters forming as a function of redshift using the luminosity function and continuum slope. They use two approaches. First assuming only one globular forms per halo, which requires the least amount of assumptions but is unrealistic. Second, allowing for multiple globular clusters per halo, assuming a linear relationship between the halo mass and globular cluster mass, a Press-Schechter mass function and a minimum halo mass in which star formation takes place (independent of redshift).
With these upper limits on the formation rate of globular clusters the authors conclude there are two distinct formation epochs of globular clusters: near z~2.5 and around the epoch of reionisation. Although the authors can only give upper limits, they argue that these upper limits are close to the actual values. These formation rates of globular clusters during reionisation imply that if the escape fraction is close to 1, the number of ionising photons produced is sufficient for reionisation.
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