On the population of primordial star clusters in the presence of ultraviolet background radiation

We use the algorithm of Cole et al. to generate merger trees for the first star clusters in a Λ cold dark matter (ΛCDM) cosmology under an isotropic ultraviolet background radiation field, parametrized by J 21 . We have investigated the problem in two ways: a global radiation background and local radiative feedback surrounding the first star clusters. Cooling in the first haloes at high redshift is dominated by molecular hydrogen, H 2 – we call these Generation 1 objects. At lower redshift and higher virial temperature, T vir ≳ 10 4 K , electron cooling dominates – we call these Generation 2. Radiation fields act to photodissociate H 2 , but also generate free electrons that can help to catalyze its production. At modest radiation levels, J 21 /(1 + z ) 3 ∼ 10 −12 –10 −7 , the nett effect is to enhance the formation of Generation 1 star clusters. At higher fluxes, the heating from photoionization dominates and halts their production. With a realistic build‐up of flux over time, the period of enhanced H 2 cooling is so fleeting as to be barely discernable and the nett effect is to move primordial star cluster formation towards Generation 2 objects at lower redshift. A similar effect is seen with local feedback. Provided that enough photons are produced to maintain ionization of their host halo, they will suppress the cooling in Generation 1 haloes and boost the numbers of primordial star clusters in Generation 2 haloes. Significant suppression of Generation 1 haloes occurs for specific photon fluxes in excess of about 10 43  ph s −1  M −1 ⊙ .