Globular Clusters in the Galaxy Cluster MACS0416 at z = 0.397

TL;DR

This study analyzes deep JWST/NIRCam SW imaging of the massive cluster MACS0416 at $z=0.397$ to build a photometric catalog of ~3×10^3 unresolved globular cluster (GC) candidates. Using PSF-fitting photometry with DAOPHOT across $F090W$–$F200W$, the authors quantify completeness via artificial-star tests, assess background contamination, and examine GC color distributions with KMM/GMM analyses. They model the globular-cluster luminosity function (GCLF) as a log-normal distribution, apply $K$-corrections, and fit the turnover while acknowledging depth limits that prevent direct turnover measurement; they find a turnover magnitude around $M_{0( ext{Abs})} \\approx -8.93$ mag and a dispersion $\\sigma \\approx 1.63$. A $K$-corrected CMD with PARSEC single-stellar-population tracks suggests most candidates are intermediate-to-old GCs with masses between $10^7$ and $10^8\,M_\odot$, and a few bright outliers may be ultra-compact dwarfs (UCDs) or stripped nuclei. The results indicate MACS0416 hosts a mature GC system consistent with others at similar redshift, while future deeper and radial analyses will improve turnover constraints and clarify the intracluster GC component in this merging cluster environment.

Abstract

We present a photometric analysis of globular clusters (GCs) in the massive galaxy cluster MACS J0416.1-2403 (z = 0.397) using deep JWST/NIRCam imaging from the PEARLS program. PSF photometry was performed in the short wavelength filters F090W, F115W, F150W, and F200W, yielding a catalog of approximately 3 x 10^3 unresolved, point-like sources consistent with a GC population. Artificial star tests indicate 80% completeness at F200W = 30.36 mag. The color-magnitude diagrams show a narrow GC sequence well reproduced by PARSEC single-stellar-population models spanning ages of 5-9 Gyr and metallicities from [M/H] = -2.0 to +0.2, consistent with evolved GC systems at this redshift. The globular cluster luminosity function (GCLF) follows a log-normal form truncated by incompleteness at the faint end. The brightest sources extend slightly beyond the locus of classical GCs, suggesting a small number of UCD-like systems or stripped nuclei, while the bulk of the population exhibits the luminosities and colors expected for mature globular clusters at z ~ 0.4.

Figures (9)

• Figure 1: Left: The MACS0416 full mosaic field with north at the top and east to the left. The red box outlines the central cluster and the region used in this analysis. As described in the text, the outer region of the mosaic image with shorter total exposures and higher sky noise was not used, except for increasing our stellar PSF sample. Right: Pink dots show the locations of the 2,971 GC candidates for MACS0416 found in this study. Cyan contours show the projected mass density from the lensing model of diego2023jwst.
• Figure 2: Upper panel: DAOPHOTsharp parameter versus $F200W$ magnitude for all detected sources. The dashed horizontal lines mark the adopted selection range ($-0.3 < \textit{sharp}_{200} < 0.35$), outside of which objects are excluded as non-stellar or poorly fit by the PSF. Lower panel: Results from artificial star tests, showing that genuine point sources occupy the same sharpness range and thus motivate the adopted boundaries. A bright-end cut at $F200W < 26$ is also applied, consistent with the luminosity limits of confirmed GCs at $z\sim0.4$harris2023jwstharris2024jwst.
• Figure 3: Upper panel: Photometric recovery completeness measured from artificial star tests. The recovery fraction $f$ is plotted in 0.3-magnitude bins. The modified hyperbolic tangent function given in the text is shown as the solid line. Lower panel: Internal measurement uncertainty $\sigma$ versus magnitude, as determined from the artificial-star tests.
• Figure 4: Upper panel: Measured recovery magnitude in $F200W$ of the artificial stars vs. input magnitude and color-coded by their recovery probability. The solid line shows the 1:1 relation. Lower panel: Residuals (recovered -- input magnitude) as a function of input magnitude. At brighter magnitudes, stars are accurately recovered, while systematic deviations and increased scatter appear at fainter magnitudes near the detection limit.
• Figure 5: CMDs of unresolved point-like sources in MACS0416, showing intrinsic color versus apparent $F200W$ magnitude for four filter combinations. Each point is color-coded by the photometric recovery probability derived from artificial star tests, interpolated as a function of $F200W$ magnitude. Importantly, the completeness values shown here reflect the joint detection probability across all four NIRCam filters ($F090W$, $F115W$, $F150W$, and $F200W$). Because inclusion in the CMD requires successful detection in all bands, the joint completeness decreases more rapidly at faint magnitudes than in any individual filter. The color bar ranges from 0 to 1.0 and illustrates this combined recovery probability. Error bars on the left side of each panel show the median photometric uncertainty in color and magnitude, computed in integer bins of $F200W$ magnitude. These bars highlight the increase in color uncertainty toward the faint end. In the last panel, the horizontal dashed line represents the 5$\sigma$ completeness limit.