On the ages of bright galaxies $\sim 500$ Myr after the Big Bang: insights into star formation activity at $z\gtrsim 15$ with JWST

Abstract

With JWST, new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of $z\gtrsim 7$ 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 $z\gtrsim 8$ objects could be detected with Spitzer, making it difficult to robustly constrain activity at $z\gtrsim 10$. Here, we leverage the greatly improved rest-optical sensitivity of JWST at $z\gtrsim 8$ to constrain the ages of seven UV-bright ($M_{UV}\lesssim -19.5$) galaxies selected to lie at $z\sim 8.5-11$, then investigate implications for $z\gtrsim 15$ star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright $z\sim 8.5-11$ galaxies. We find a median age of $\sim 20$ Myr, younger than that inferred at $z\sim 7$ with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only $\sim 3$ percent of bright $z\sim 8.5-11$ galaxies would be similarly luminous at $z\gtrsim 15$, implying that the number density of bright galaxies declines by at least an order of magnitude between $z\sim 8.5-11$ and $z\sim 15$. 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 $z\sim 15$ galaxies are observed during a burst of star formation.

arXiv comment16 pages, 10 figures, 1 appendix, accepted to MNRAS

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