Star formation at the smallest scales: a JWST study of the clump populations in SMACS0723

Abstract

We present the clump populations detected in 18 lensed galaxies at redshifts 1 to 8.5 within the lensing cluster field SMACS0723. The recent JWST Early Release Observations of this poorly known region of the sky have revealed numerous point-like sources within and surrounding their host galaxies, undetected in the shallower HST images. We use JWST multiband photometry and the lensing model of this galaxy cluster to estimate the intrinsic sizes and magnitudes of the stellar clumps. We derive optical restframe effective radii from <10 to hundreds pc and masses ranging from ∼105 to 109 M⊙, overlapping with massive star clusters in the local universe. Clump ages range from 1 Myr to 1 Gyr. We compare the crossing time to the age of the clumps and determine that between 45 and 60 per cent of the detected clumps are consistent with being gravitationally bound. On average, the dearth of Gyr old clumps suggests that the dissolution time scales are shorter than 1 Gyr. We see a significant increase in the luminosity (mass) surface density of the clumps with redshift. Clumps in reionisation era galaxies have stellar densities higher than star clusters in the local universe. We zoom in into single galaxies at redshift <6 and find for two galaxies, the Sparkler and the Firework, that their star clusters/clumps show distinctive colour distributions and location surrounding their host galaxy that are compatible with being accredited or formed during merger events. The ages of some of the compact clusters are between 1 and 4 Gyr, e.g. globular cluster precursors formed around 9-12 Gyr ago. Our study, conducted on a small sample of galaxies, shows the potential of JWST observations for understanding the conditions under which star clusters form in rapidly evolving galaxies.

JWST/NIRCam Probes Young Star Clusters in the Reionization Era Sunrise Arc

“…The total flux of the source is obtained by extrapolating to infinite the best-fit 2D Gaussian shape. The final photometric errors include in quadrature the Poissonian error, as well as the uncertainties in the measured parameters of the shape (for more details, see Claeyssens et al 2023). The latter error is significantly larger in the nebular knots (see Figure 4), which are not well detected in the continuum bands.…”

“…The fluxes and sizes of each star cluster candidate/knot are obtained by adopting the methodology presented in Claeyssens et al (2023), following Messa et al (2022).…”

“…The SED fitting analysis has been performed following the methodology described in Claeyssens et al (2023).…”

“…Recently, exquisite JWST/NIRCam and NIRISS imaging of lensed fields at an unprecedented space-based angular resolution of 40-140 mas spanning 1-5 μm have revealed compact sources with physical properties resembling dense stellar clusters observed locally by, e.g., Adamo et al (2020a). In particular, such instrumental advances led to the discovery of z ; 1.37 star clusters with evolved ages of 0.1-4 Gyr (Mowla et al 2022;Claeyssens et al 2023) in a galaxy dubbed the "Sparkler," which is lensed by cluster SMACS 0723, observed during the JWST Early Release Observations (Pontoppidan et al 2022). Relatively young protoglobular clusters at z ; 4 with estimated ages of <30 Myr have also been identified in highly magnified galaxies by the A2744 galaxy cluster (Vanzella et al 2022b) observed during the ERS-GLASS campaign (JWST program 1342, PI: T. Treu;Treu et al 2022).…”

Precision Modeling of JWST's First Cluster Lens SMACS J0723.3–7327*

“…In addition, we separately model the cluster-member galaxy at (R.A. = 110.8402908 decl. = −73.4559518) which, being closest to the lensed image dubbed "The Sparkler" (image 2.2), has a disproportionate influence on the lens model (see Claeyssens et al 2023 andMowla et al 2022, and references therein for a more detailed discussion of the Sparkler). Altogether, we thus jointly optimize 146 galaxies using our constant mass-luminosity relation.…”