Time to Sparkler. Accurate ages of lensed globular clusters at z=1.4 with JWST photometry

Determining reliable ages for old stellar objects at different redshifts offers a powerful means to constrain cosmology without relying on a specific cosmological model. This is known as the ‘cosmic clocks’ method. Globular clusters (GCs), long recognised as hosts of the Universe's oldest stars, have served as archetypical cosmic clocks. However, their age estimates have traditionally been confined to redshift z=0, limiting their role to constraining the present-day age of the Universe t(z=0)=t_0. Here, we explore how to measure reliable ages of GCs well beyond z = 0, leveraging their potential to extend cosmic clock measurements to earlier epochs. Specifically, we used six-band JWST/NIRCam high-precision photometry of candidate stellar clusters in the Sparkler galaxy, located at redshift z = 1.378 and strongly lensed by the galaxy cluster SMACS J0723.3-7327. By employing stellar population models within a Bayesian inference framework, we constrained the clusters’ ages, star formation histories, metallicities, and dust attenuation. The five compact sources previously identified as GCs, based on their red spectral energy distributions being consistent with the colours of old stellar systems, yield a formation age of $1.9 ± 0.4$ Gyr on average. This result implies a total age of the Universe that aligns well with the Λ cold dark matter (ΛCDM) model derived from Planck18 data, even though no cosmological prior was imposed on the age of these objects, which were allowed to span up to 15 Gyr. Recent space-based observations have uncovered a wealth of lensed GCs as well as globulars within the member galaxies of the clusters themselves. These findings suggest that the pool of objects available for cosmic clock studies is enormous. A systematic multi-band photometric survey of GCs in and behind galaxy clusters, using facilities like Euclid and the James Webb Space Telescope, would therefore be a powerful tool for estimating cluster ages across a large range of redshifts, allowing the Universe to be dated across an unprecedented range of epochs.