Early results from GLASS-JWST. VII: evidence for lensed, gravitationally bound proto-globular clusters at z=4 in the Hubble Frontier Field A2744

Understand

We investigate the blue and optical rest-frame sizes (lambda~2300A-4000A) of three compact star-forming regions in a galaxy at z=4 strongly lensed (x30, x45, x100) by the Hubble Frontier Field galaxy cluster A2744 using GLASS-ERS JWST/NIRISS imaging at 1.15um, 1.50mu and 2.0mu with PSF < 0.1&#34;.

• In particular, the Balmer break is probed in detail for all multiply-imaged sources of the system.
• With ages of a few tens of Myr, stellar masses in the range (0.7-4.0) x 10^6 Msun and optical/ultraviolet effective radii spanning the interval 3 < R_eff < 20 pc, such objects are currently the highest redshift (spectroscopically-confirmed) gravitationally-bound young massive star clusters (YMCs), with stellar mass surface densities resembling those of local globular clusters.
• Optical (4000A, JWST-based) and ultraviolet (1600A, HST-based) sizes are fully compatible.