On the ages of bright galaxies ∼500 Myr after the big bang: insights into star formation activity at z ≳ 15 with JWST

With JWST, new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of z greater than or similar to 7 galaxies demonstrated the power of using galaxy stellar masses and star formation histories (SFHs) to indirectly infer the cosmic star formation history. However, only the brightest individual z (SIC) 8 objects could be detected with Spitzer , making it difficult to robustly constrain activity at z greater than or similar to 10. Here, we leverage the greatly improved rest-optical sensitivity of JWST at z greater than or similar to 8 to constrain the ages of seven UV-bright ( M-uv less than or similar to -19.5) galaxies selected to lie at z similar to 8.5-11, then investigate implications for z (SIC) 15 star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright z similar to 8.5-11 galaxies. We find a median age of similar to 20 Myr, younger than that inferred at z similar to 7 with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only similar to 3 per cent of bright z similar to 8.5-11 galaxies would be similarly luminous at z > 15, implying that the number density of bright galaxies declines by at least an order of magnitude between z similar to 8.5-11 and z similar to 15. This evolution is challenging to reconcile with some early JWST results suggesting the abundance of bright galaxies does not significantly decrease towards very early times, but we suggest this tension may be eased if young stellar populations form on top of older stellar components, or if bright z similar to 15 galaxies are observed during a burst of star formation.

Automated summary

With JWST, new opportunities to study the evolution of galaxies in the early Universe are emerging. The study uses JWST to constrain the ages of UV-bright galaxies at z similar to 8.5-11 and investigates the implications for z similar to 15 star formation. The results suggest an evolution towards larger specific star formation rates at early times, but also a significant decline in the number density of bright galaxies between z similar to 8.5-11 and z similar to 15.