Hidden in Plain Sight: Searching for Globular Clusters Within JWST Observations of the PLCK G165.7+67.0 Galaxy Cluster

Abstract

Although the James Webb Space Telescope (JWST) has received much attention for its ability to search deeper into the cosmos than ever before, it also enhances our capability to study objects closer to us in the Universe. We apply a methodology of subtracting intracluster light to the PLCK G165.7+67.0 (G165; $z$ = 0.35) cluster, revealing a population of unresolved point-like sources including globular clusters (GCs). By applying a fitting algorithm in color space used to select galaxy cluster members, we uncover over 900 globular cluster candidates from our point source sample. We also identify candidates by estimating the contribution of interlopers to the point source sample, yielding an estimate of 793$\pm$ 83 globular cluster candidates. We find the color-selected sources to be approximately correlated spatially with the intracluster light and lensing mass of the cluster. The observed luminosity function of the sources shows a turnover point fainter than the completeness limit, so we use fixed-parameter curve fitting models to predict a K-corrected turnover point between $-9.4 \leq M_{\rm F200W} \leq -10.7$ mag, although we predict the expected K-corrected turnover point should be closer to $-7.7 \leq M_{\rm F200W} \leq -8.4$ mag. We discuss the dynamical state of this disturbed galaxy cluster with a bimodal mass distribution using the spatial distribution of GC candidates and find that the radial profiles of our color-selected GC candidates are very consistent with the lensing-derived surface mass density at $>$50 kpc.

Figures (6)

• Figure 1: RGB image cutouts of the G165 cluster throughout the ICL subtraction process. All six filters are used with (R,G,B) = (F356W + F444W, F200W + F277W, F090W + F150W). Top Left Panel: Original image before any subtraction. Top Right Panel: Modeled light using ProPane. Bottom Panel: Final light-subtracted image with a single red contour showing the region where the lens model-derived convergence is $\kappa = 0.25$Kamieneski2024. The green squares are the 13 objects with well-fit photometric redshifts between $0.3 < z_{phot} < 0.4$ and the visible blue border in the image is due to lower exposures and dithering effects in the longer wavelength filters.
• Figure 2: Compactness selection of point sources using the photometric difference of aperture diameters $0\hbox{$.\!\!^{\prime\prime}$}15$ and $0\hbox{$.\!\!^{\prime\prime}$}3$ in the F200W filter as a function of F200W magnitude in $0\hbox{$.\!\!^{\prime\prime}$}15$ diameter aperture. Our selection in dark blue includes all point sources in the field, including GCs, interloper UCDs, and foreground stars. Within this selection, we include the 13 objects (green squares) with well-fit photometric redshifts between $0.3 < z_{phot} < 0.4$. The vertical black line at 29.73 mag indicates the 50$\%$ completeness limit for $0\hbox{$.\!\!^{\prime\prime}$}18$ aperture diameters, and any point sources fainter than this limit are still used, but highly uncertain. The gray points are removed objects that do not meet the compactness threshold (horizontal dashed black lines centered around the solid line).
• Figure 3: Example of SED fitting of object close to the cluster redshift, ran by EAZY BrammerEAZY. The title shows the redshift found at the minimum $\chi^2$ and the RA/DEC if the point source. The computed photometric redshift of this particular source is at $z$ = 0.348. Left Panel: SED fit using $F_{\nu}$ [$\mu$Jy] vs filter wavelength [Å]. Middle Panel: $\chi^2$ values at different redshifts, used to predict the best-fit redshift for the source. Right Panel: RGB image of object, shown within the crosshair at the center of the image.
• Figure 4: Color-color diagram using F277W-F356W vs F150W-F200W to select G165 cluster members (within the compactness criterion) at $z$ = 0.35. Gray points show the point sources that do not pass our color selection, red points within the black dashed box show the selected globular cluster candidates determined to be likely at the redshift of the galaxy cluster, green squares are point sources with photometric redshifts between 0.3 and 0.4, and blue diamonds are objects with spectroscopic redshifts between 0.3 and 0.4 (centered on the cluster redshift). BC03 stellar populations BruzualCharlot2003 at 1, 3, and 7 Gyr are shown by the rainbow-colored points, ranging from redshifts of 0 to 2 encoded by the colorbar.
• Figure 5: Recovery rate as a function of flux for injected artificial point sources. The blue points show the recovery rate of the artificial objects at different magnitudes. The solid black line shows the best-fit solution using Equation \ref{['Equation2']}, with parameters $\beta$ = 3.3 and $m_{1}$ = 30.06. We find that the completeness limit at 50 percent is $m_{\rm F200W}$ = 29.73 mag, indicated by the dashed line. Note, real sources mistaken for test sources leads to values $>$1.