Sparkler

The Sparkler is a highly magnified, distant galaxy (z ≈ 1.4) about 9 billion light-years away. It is ringed by relatively bright, point-like sources — the sparkles — which earlier photometric studies suggested are ancient globular clusters, an unusually old population for a lensed galaxy at that epoch. Because those conclusions rested on photometry with large parameter uncertainties, alternatives — younger star clusters, or even ultra-compact satellites — remained possible.

To reduce those uncertainties, new spectroscopy of the most highly magnified image (magnification > 10–100) was obtained with JWST/NIRSpec IFU (PRISM + G140M, GO 2969). Combined with the galaxy’s extensive multi-wavelength coverage (JWST NIRCam, NIRISS and MIRI), the PRISM + G140M data help break the age–metallicity degeneracy.

The project set out to (a) determine whether the sparkles are genuinely old star clusters, (b) constrain their physical properties within globular-cluster scaling relations, and (c) characterise the host galaxy to understand what makes the Sparkler distinct.