The Distribution of Quenched Galaxies in the Massive z = 0.87 Galaxy Cluster El Gordo

TL;DR

This study uses JWST/NIRCam imaging from the PEARLS program to probe environmental quenching in the massive $z=0.87$ cluster El Gordo, which has two mass components. Cluster members are identified via a hybrid color–color and photometric redshift approach, yielding a robust sample for quenching analysis. Quenched galaxies (defined as >1 dex below the star-formation main sequence) constitute ~63% of $M_*>10^9 ext{ M}_ ext{sun}$ members, with a clear mass-dependent radial trend: low-mass galaxies quench more strongly near the mass centers, while high-mass galaxies show little radial dependence, pointing to different quenching pathways. Comparisons with field and other clusters indicate elevated quenched fractions in El Gordo and support a combination of environmental processes (e.g., ram-pressure and strangulation) and internal mechanisms, with deeper data necessary to extend the analysis to dwarfs and refine quenching timescales at $z\sim1$.

Abstract

El Gordo (ACT-CL J0102$-$4915) is a massive galaxy cluster with two major mass components at redshift $z=0.87$. Using SED fitting results from JWST/NIRCam photometry, the fraction of quenched galaxies in this cluster was measured in two bins of stellar mass: $9<\log{({M_*}/\mathrm{M}_{\odot})}<10$ and $10\leq\log{({M_*}/\mathrm{M}_{\odot})}<12$. While there is no correlation between the quenched fraction and angular separation from the cluster's overall center of mass, there is a correlation between the quenched fraction and angular separation from the center of the nearest of the two mass components for the less-massive galaxies. This suggests that environmental quenching processes are in place at $z\sim1$, and that dwarf galaxies are more affected by those processes than massive galaxies.

Figures (7)

• Figure 1: Color--color diagrams with (F356W$-$F444W) versus (F090W$-$F200W). The blue points represent objects without spectroscopic redshifts, and the orange triangles represent the 206 objects with spectroscopic redshifts 2012ApJ...748....7M2023AA...678A...3C. Redshift tracks for two simple stellar populations 2003MNRAS.344.1000B are plotted, one with age 1 Gyr track and one with age 3 Gyr. Redshifts along each track correspond to the color bar at right, and each track is highlighted green for $0.77<z<0.97$. The lime green triangles show the 80 objects with $0.862<z_\mathrm{spec}<0.890$2023ApJ...952...81F. The dark green triangles show the 38 objects outside this range but within $0.80<z_\mathrm{spec}<1.0$. The top panel shows a wide color range with the black dashed box marking the cluster-member selection region. The bottom panel shows a zoomed-in view of the cluster selection box.
• Figure 2: A comparison of the $z_{\rm spec}$ and $z_{\rm phot}$, colored by stellar mass. The inset box shows the area $0.77<z<0.97$ to highlight the sources within $0.1$ of the cluster redshift, $z=0.87$. The dashed line shows $\hbox{$z_{\rm spec}$} = \hbox{$z_{\rm phot}$}$, and the dotted lines inside the smaller box show the exact cluster redshift range, $0.862<\hbox{$z_{\rm spec}$}<0.890$.
• Figure 3: The parameters $\log(\mathrm{SFR})$ vs. $\log(M_*)$ derived by SED fits of JWST/NIRCam photometry of El Gordo cluster members. The dashed black line shows the star-formation main sequence best fit from 2014ApJS..214...15S for $z=0.87$. The solid black line is 1 dex below the SFMS, representing the quenched threshold as described by 2021MNRAS.500.4004D and 2023ApJ...954...98P. The color represents the age/$\tau$ value for each galaxy according to the color bar on the right---as expected, galaxies with lower SFRs have older ages parameterized this way. The shaded region represents the stellar mass range in which we are incomplete, and the dotted grey lines represent the two mass bins we split the data into.
• Figure 4: Positions of cluster galaxies. Quenched objects are shown as triangles with colors indicating specific SFR as shown on the color bar to the right of the plot. The grey circles mark the other (i.e., not quenched) cluster galaxies. The cyan square and green pentagon mark the southeast and northwest mass centers, respectively, and the pink and yellow triangles mark the luminosity-weighted mass centers (LWMCs) within 1.7 and 2.5 Mpc 2023ApJ...952...81F. The red star marks the center of mass as computed using the Bagpipes stellar masses of the cluster objects in Module B. The regions separated by black lines represent the three regions where quenched fraction was measured. The circles are centered on the mass centers and have radii 26$^{\prime\prime}$, 55$^{\prime\prime}$, and 96$^{\prime\prime}$.
• Figure 5: The quenched fraction of galaxies in El Gordo as a function of distance to the nearest mass center. Colored points show the quenched fractions within 3 angular separation bins from the nearest mass center. Angular separation from the nearest mass center is given in arcseconds on the bottom axis, and proper distance from the nearest mass center is given in kiloparsecs on the top axis. The sample is separated into $9 < \log{(M_*/\mathrm{M}_{\odot})} < 10$ (green circles), and $10 \leq \log{(M_*/\mathrm{M}_{\odot})} < 12$ (blue triangles). Shading indicates the boundaries between different regions.