A first GLIMPSE into star clusters populations across cosmic time

TL;DR

This study delivers the first large census of high-redshift star clusters (222 objects; 145 from GLIMPSE) enabled by JWST/NIRCam in strongly lensed fields, revealing predominantly young clusters that mostly formed during cosmic noon ($1<z<4$) with a subset tracing formation into the reionization era. By combining homogeneous SED fitting with BPASS templates and rigorous lens-modeling (M1–M3), the authors measure cluster masses ($\sim 10^4$–$10^8$ M$_\odot$), compact sizes ($\lesssim 20$ pc), and extreme stellar densities ($\Sigma_{M_*}$ up to $10^6$ M$_\odot$/pc$^2$), and demonstrate that many high-$z$ clusters overlap with local NSCs and GC progenitors. The first direct star cluster mass function at $z>1$ is well described by a power-law with slope $\beta_{50\%} \approx -1.89$, with no statistically significant evidence for a high-mass truncation given current data. These findings imply that massive, dense clusters were common in the early Universe and may play a central role in GC formation and IMBH seeding, highlighting the transformative impact of deep JWST lensing surveys on our understanding of star cluster formation across cosmic time.

Abstract

We present the first sample of 222 high-redshift (z>0.5) star clusters, detected with JWST/NIRCam in 78 magnified galaxies from different galaxy cluster fields. The majority of the systems (~60%) is observed in the very deep NIRCam observations of the cluster AbellS1063 (GLIMPSE program), showing the power that deep observations, combined with lensing, has to reveal these primordial stellar structures. We perform simultaneous size-flux estimates in all available NIRCam filters and spectral energy distribution (SED) fitting analysis to recover star cluster physical properties. All star cluster candidates have very high magnification. Star clusters and clumps show similar ages and redshift distributions, although noticeable differences are seen in their masses, sizes and stellar surface densities inherent to the lack of resolution in the latter group. We reconstruct the formation redshift of star clusters and find that the large majority of the observed star clusters show young ages (<100 Myr) and seems to form at cosmic noon (CN,1<z<4). A small sample of CN star clusters is about 1 Gyr old, these potential globular clusters have formed well within cosmic reionization. Star clusters have stellar densities in the range 10^2 to 10^6 M/pc^2, with median values around 10^4 pc2. Their sizes and densities better overlap with those of nuclear star clusters in the local Universe. These intrinsic properties make high-z star clusters a viable channel to grow intermediate mass black holes. We use Bayesian inference to make first direct measurement of the star cluster mass function at z>1, based on a subsample of 60 star clusters younger than 100 Myr and with masses above 2e6 Msun. The star cluster mass function is well described by a power-law with slope beta = -1.89 suggesting that a power-law -2 function might already be in place in the distant Universe.

Figures (11)

• Figure 1: NIRCam composite color image of the 61 selected GLIMPSE galaxies. The image is a combination of all 7 broadband and 2 medium-band filters$^{1}$ . We show only one of the multiple images per system. The black circles indicate the position of the star cluster candidates ($\rm R_{eff}\leq 20$ pc). Redshift ($\rm z_p$ for photometric and $\rm z_s$ for spectroscopic redshift) is reported for each galaxy.
• Figure 2: Color-color diagram (F150W-F200W vs. F200W-F356W) of the GLIMPSE high-redshift stellar clumps (grey points) and identified intra-cluster GCs (dark orange stars) at $z=0.35$. The blue and green lines show the Yggdrasil stellar evolutionary tracks based instantaneous burst (IB) and constant star formation (over 10Myr) models, with metallicity fixed at $\rm 2\% \ Z_{\odot}$ or $\rm 40\% \ Z_{\odot}$ and with a nebular covering fraction of 0.5 at $z=0.35$. The tracks start at 1 Myr and end at 7 Gyrs. The orange frame highlights the color-color location of the intra-cluster GCs. Stellar clumps located within this box, have been excluded from the final sample.
• Figure 3: GLIMPSE stellar clumps absolute AB magnitude (corresponding to the V band, rest-frame) as a function of their effective radius. The points are color-coded in redshift. The black contours highlight the distribution of Abell2744 stellar clumps in the same plane (claeyssens2025, the three contours represent 10, 50 and 99% of the sample). The vertical black line shows the star cluster size threshold (fixed at 20 pc for this study). The symbols circled in black indicate systems that have only a size upperlimit. Thanks to very deed observations and strong lensing magnification, the GLIMPSE sample enables us to detect fainter and smaller clumps than in Abell2744 NIRCam observations.
• Figure 4: GLIMPSE galaxy properties, from left to right: stellar mass ($\rm M_{*}$), SFR (estimated over the last 10 Myr) and median magnifications ($\mu$) extracted from the reference lens model. The stellar mass and SFR are obtained from Bagpipes SED fitting using a non-parametric SFH (Leja2019) as described in the Section \ref{['sec:glimpse_sample']}. The grey distributions represent the total GLIMPSE galaxy sample hosting clumps, while the red ones highlight the galaxies which host star cluster candidates. Vertical lines shows the median values of each distribution.
• Figure 5: Properties of the JWST high-redshift stellar clumps (in grey, $\rm R_{eff}>20$ pc) and star clusters (in red, $\rm R_{eff}<20$ pc)). The distributions include the GLIMPSE sample as well as the 5 literature samples (see Section \ref{['sec:section3']} and Table 1). The top row shows, from left to right, the redshift, the stellar mass ($\rm M_{*}$), the stellar mass surface density ($\rm \Sigma_{M_{*}}$), SFR and the SFR surface density ($\rm \Sigma_{SFR}$). The bottom row shows, from left to right, the effective radius ($\rm R_{eff}$), mass-weighted age, lensing magnification ($\mu$) and V band absolute magnitude ($\rm M_{Vband}$). The parameters obtained from SED fitting for the star clusters are measured with two different SFHs with exponential decline with $\tau=1 \ \rm Myr$ (in dark red, dashed) and $\tau=10 \ \rm Myr$ (in red). The parameters obtained from SED fitting for the stellar clumps are only measured using a SFH with exponential decline profile and $\tau=10 \ \rm Myr$ (grey). The histograms are not corrected for completeness.