The z = 9.625 Cosmic Gems Galaxy was a "Compact Blue Monster" Propelled by Massive Star Clusters

TL;DR

The paper investigates how an extremely compact, high-redshift galaxy can form multiple massive bound star clusters within a very small region. By combining JWST photometry, lensing magnification, and a parameterized star cluster mass function, the authors show that to account for the five ∼10^6 M⊙ clusters in the Cosmic Gems arc, the stellar mass formed in clusters must be a large majority of the burst mass, requiring a top-heavy SCMF (β > −2) and/or a high cluster-formation efficiency Γ approaching unity. The delensed host mass and size imply a very high stellar mass surface density, and the inferred recent sSFR likely drove a brief UV-bright phase possibly analogous to a "Blue Monster" episode, with implications for reionization and globular cluster formation. These results suggest extreme clustered star formation in early galaxies, with magnified JWST observations enabling direct study of proto-globular clusters, and motivate future ELT/JWST surveys to build statistics on cluster formation at cosmic dawn.

Abstract

The recent discovery of five massive stellar clusters at z=9.625 in the Cosmic Gems has raised the question about the formation mechanism of star clusters in the first half Gyr after the Big-Bang. We infer the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM ~ (n$_0$) $M^β$), assuming three different slopes $β$ = -1.5, -2.0 and -2.5 and different lower-mass limits between $10^2$ and $10^5$ Msun. The total integrated cluster stellar mass is compared to the stellar mass inferred from the counter-image of the Cosmic Gems, which provides the best, modestly magnified ($μ$ = 1.84$\pm$0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy is estimated as 3.5$_{-1.8}^{+3.3}$ x$10^7$ Msun, with an effective radius of Reff = 103$_{-15}^{+13}$ parsec and a stellar surface mass density of $Σ$mass = 520$_{-225}^{+340}$ Msun pc$^{-2}$. Accounting for normalization uncertainties - including different lensing magnification scenarios for the arc - a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter-image, without exceeding the galaxy's stellar mass. By extrapolating the physical properties at the peak of the burst we find that in its recent past (<~ 30 Myr) the Cosmic Gems galaxy has likely experienced a specific star formation rate (sSFR) exceeding 25 Gyr$^{-1}$ and luminosity approaching the ``blue monster'' regime (M$_{UV}$ < -20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and shed light into the formation of bound star clusters that might survive to z = 0 as globular clusters, older than 13 Gyr.

Figures (8)

• Figure 1: JWST/ NIRCam color image of the portion of the galaxy cluster SPT0615 field including the CG arc and the CI, (white circles), along with the critical lines at the $z=9.625$ for Lenstooljullo2007 and glaficoguri2010oguri2021 lens models, marked with red and green lines, respectively. The yellow shaded square marks the field of view of the JWST/NIRSpec IFU observations (see Messa25_CG).
• Figure 2: JWST/ NIRCam imaging and Galfit fitting of the Cosmic Gems arc and its counter-image. In the left panels, the sharp dropout of the arc and counter-image in the Hubble (F435W+F606+F814W, blue channel) and JWST/NIRCam (F090W+F115W, green channel) RGB rendering, with the detection in the redder JWST/NIRCam bands (red channel of the RGB rendering, as indicated in the figure). In the color image showing the counter-image, the predicted position from the new Glafic (yellow star) and LENSTOOL (green star) models are reported (from MM25). The regions outlined with yellow squares are zoomed in on the right panels in the NIRCam F150W band, along with the Galfit modeling and residuals (in counts units, rightmost panels).
• Figure 3: The corner plot (right) and the SED fitting results of the counter-image (left). We fix $z=9.625$ throughout and adopt an age not younger than 10 Myr (see text for more details). Red/blue lines on the left indicate fits excluding/including the F150W data point. The red line shows the fiducial SED fit solution (see Figure \ref{['noF150W']} for a comparison of corner plots with and without the F150W band data point). Horizontal bars indicate the bandwidth. On the right panel the inset shows the stacked short-wavelength bands (F150W + F200W) image of the CI where the 2$\sigma$ contour is outlined. The blue and red stars in the corner-plot mark the best and median solutions. The same is indicated with the vertical blue line (best solution) and dotted/dashed red lines (median and 16-84% percentiles). The mass-weighted age and the current stellar mass are reported.
• Figure 4: Monte Carlo realizations of the fraction of the stellar mass of the CG galaxy residing in the population of bound star clusters is shown (uncertainties on the normalization of the SCMF and the stellar mass of the host galaxy are included, see text for details). From left to right the SCMF is evaluated for slopes $\beta$ of $-1.5$, $-2.0$, and $-2.5$, adopting three different low mass limits as indicated in the legend of each panel. Calculations have been performed assuming the fiducial magnification values (while the behavior with varying magnification is shown in Figure \ref{['multi_hist']}). The vertical line bar marks the case where the total stellar mass of the star clusters equals that of the host galaxy (fraction equal to 100%).
• Figure 5: Statistical sampling of the SCMF. The color-coded probability of having 5 or more massive star clusters (with masses exceeding $10^6$ M_⊙ M$_{\odot}$) is shown as a function of the slope of the stellar cluster mass function (SCMF, $\beta$) and the low mass limit used to integrate the SCMF. The top panel represents the scenario where the entire mass of the CG galaxy is composed of stellar clusters ($\Gamma = 100\%$), while the bottom panel illustrates the case with $\Gamma=50\%$.