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

TL;DR

This study presents early JWST observations of gravitationally lensed star-forming regions at z=4, revealing young, massive star clusters with properties similar to local globular clusters, indicating early cluster formation in the universe.

Contribution

First JWST-based detailed analysis of high-redshift, gravitationally bound young massive star clusters, demonstrating their sizes, masses, and densities comparable to local globular clusters.

Findings

Identified star clusters with stellar masses (0.7-4.0) x 10^6 Msun

Measured effective radii between 3 and 20 parsecs

Confirmed high star formation rate surface density in the galaxy

Abstract

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". 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. The contribution to the ultraviolet underlying continuum emission (1600A) is ~30%, which decreases by a factor of two in the optical for two of the YMCs (~4000A rest-frame), reflecting the young ages (<30 Myr) inferred from the SED fitting and supported by the presence of high-ionization lines secured with VLT/MUSE. Such bursty forming regions enhance the sSFR of the galaxy, which is ~10 Gyr^-1. This galaxy would be among the extreme analogs observed in the local Universe having high star formation rate surface density and high occurrence of massive stellar clusters in formation.