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Initial Investigations of the Outskirts of XLSSC 122

This study utilizes Hubble Space Telescope observations to characterize the galaxy cluster XLSSC 122 at redshift 1.98, revealing a rapid increase in red galaxies toward the center indicative of fast quenching mechanisms like ram pressure stripping, while finding no evidence of infalling groups and a luminosity function evolution rate consistent with lower-redshift clusters.

Original authors: Eleanore. B. Todd, Jon. P. Willis, Rebecca. E. A. Canning, Ophélie. K. Leste, Rahma. Alfarsy, Steven W. Allen, Gabriel Brammer, Joseph. N. Burchett, Adam. B. Mantz, Spencer. A. Stanford

Published 2026-06-08

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Original authors: Eleanore. B. Todd, Jon. P. Willis, Rebecca. E. A. Canning, Ophélie. K. Leste, Rahma. Alfarsy, Steven W. Allen, Gabriel Brammer, Joseph. N. Burchett, Adam. B. Mantz, Spencer. A. Stanford

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, bustling city. In this city, galaxies are like individual houses or neighborhoods. Some neighborhoods are quiet, filled with older, retired residents (red galaxies), while others are lively hubs of construction and new life (blue, star-forming galaxies).

This paper is a detailed investigation into a specific, massive "city" in the early universe called XLSSC 122. This city is a galaxy cluster located so far away that we are seeing it as it was when the universe was only about 3 billion years old (a time known as redshift 1.98). The researchers used the Hubble Space Telescope to take a "panoramic photo" of this city, looking not just at the busy downtown core, but all the way out to the suburbs and even the countryside, covering a distance ten times larger than the city's main boundary.

Here is what they found, broken down into simple concepts:

1. The "City" is Older Than It Looks

Usually, when we look at young cities in the early universe, we expect them to be chaotic and full of new construction. However, XLSSC 122 is surprisingly mature.

The Finding: The researchers found that the "downtown" (the center of the cluster) is packed with "retired" red galaxies. As they moved further out toward the "suburbs," they found more "young" blue galaxies.
The Analogy: Think of it like walking from a quiet, old neighborhood in the city center out to the bustling construction sites on the edge. The transition happens very quickly. As soon as a galaxy enters the city limits, it stops building new stars and turns "red." This suggests the city has a very efficient, fast-acting mechanism that shuts down new construction immediately upon arrival.

2. The "Wind" That Stops the Construction

Why do these galaxies stop making stars so fast? The paper suggests a process called Ram Pressure Stripping.

The Analogy: Imagine driving a convertible car (the galaxy) with the top down, full of fuel (gas for star formation). If you drive at high speed into a strong headwind (the hot gas that fills the space between galaxies in the cluster), the wind blows all your fuel out of the tank.
The Result: Once the fuel is gone, the car can't move or build anything new. The researchers found that this "wind" is strong enough in XLSSC 122 to strip the gas from incoming galaxies, turning them from blue (active) to red (quiet) very quickly.

3. No "Commuter Trains" or "Feeder Roads"

In the local universe (our current time), we often see groups of galaxies falling into big clusters like trains arriving at a station, or streams of galaxies flowing along "cosmic filaments" (like feeder roads).

The Finding: The researchers looked very carefully at the outskirts of XLSSC 122 to see if they could spot these incoming groups or streams. They found nothing.
The Analogy: It's as if they stood at the edge of the city and looked for a line of cars arriving, but the road was empty. The galaxies are just scattered out there, not arriving in clumps.
The Implication: This suggests XLSSC 122 is a very "relaxed" and settled city. It has likely already absorbed most of its neighbors and isn't currently in the middle of a chaotic merger event.

4. A Second, Smaller City Nearby

While studying the main cluster, the team also spotted a smaller, separate cluster of galaxies nearby called CL J021741.7-034546.

The Finding: This second cluster is similar to the first but has fewer "retired" red galaxies and more "young" blue ones. It's like a smaller, younger town next to the big, mature city.

5. The "Blueprint" of the City

The researchers also counted how many galaxies there are at different brightness levels (a "luminosity function").

The Finding: They found that the distribution of galaxy sizes in this ancient city is very similar to cities we see closer to us in time, just slightly dimmer.
The Analogy: It's like looking at a blueprint of a house built 10,000 years ago and realizing the floor plan is almost identical to a house built today, just with slightly older furniture. This tells us that the "rules" of how galaxies grow and evolve haven't changed drastically over the last 10 billion years.

Summary

In short, this paper is a census of a distant, ancient galaxy city. The main takeaways are:

Fast Quenching: Galaxies entering this city have their star formation shut off almost immediately, likely due to the "wind" of hot gas stripping away their fuel.
Mature Structure: The city looks settled and relaxed, with no signs of incoming traffic or construction groups on the horizon.
Consistent Evolution: Despite being in the early universe, the way these galaxies are organized and how they evolve looks surprisingly similar to the galaxy clusters we see in our own neighborhood today.

The researchers have also released a new, expanded list (catalogue) of 74 confirmed members of this city, providing a better map for future astronomers to study how these cosmic cities grow.

Technical Summary: Initial Investigations of the Outskirts of XLSSC 122

Problem and Motivation
Galaxy clusters serve as critical laboratories for understanding galaxy evolution across cosmic time. While the local universe exhibits a clear bimodal distribution of galaxy colors (red sequence vs. blue cloud) driven by environmental quenching mechanisms like ram pressure stripping, the dominant quenching mechanisms in the early universe ( $z > 1.5$ ) remain less constrained. Previous studies suggest a shift from mass-dominated quenching at high redshift to environment-dominated quenching at lower redshift, yet data on clusters at $z \approx 2$ is scarce. Furthermore, the role of infalling groups and cosmic filaments in pre-processing galaxies before they reach the cluster core is not fully understood at these epochs. This study addresses these gaps by investigating the galaxy cluster XLSSC 122 at $z=1.98$ , extending observations from the cluster core out to 10 virial radii ($3$ Mpc) to trace the radial evolution of galaxy properties and quenching mechanisms.

Methodology
The authors utilized a combination of Hubble Space Telescope (HST) data from Cycle 25 (core observations) and Cycle 30 (outskirts observations).

Imaging and Photometry: Observations were conducted using WFC3/IR (F140W, F105W) and ACS (F814W) filters. Images were processed using the calwf3 pipeline and Grizli software wrapper for reduction, bias subtraction, and cosmic ray rejection. Contiguous mosaics were created using Astrodrizzle and aligned to a reference source catalog. Photometry was extracted using SExtractor with fixed and variable elliptical apertures.
Spectroscopy: Slitless spectroscopy was obtained using the G141 grism. Spectral extraction and classification were performed using Grizli. Redshifts were determined by fitting three template sets (continuum, continuum + line complexes, and continuum + individual lines) over a grid of $z=0-4.0$ .
Membership Classification: Galaxies were classified into Gold, Silver, and Bronze categories based on the integral of the redshift probability density function ( $p_{memb}$ ) over the range $1.96 < z < 2.00$ , alongside signal-to-noise (S/N) ratios. A similar process was applied to identify a secondary structure at $z=1.93$ (CL J021741.7-034546) to avoid contamination.
Analysis: The study computed radial number density profiles, color-magnitude diagrams (CMDs), red fractions, and luminosity functions. The density profile was fitted with a Navarro-Frenk-White (NFW) model. Ram pressure stripping conditions were modeled using ICM density profiles and galaxy mass estimates to test quenching scenarios.

Key Contributions

Expanded Catalogue: The paper presents an updated photometric and spectroscopic catalogue for XLSSC 122, increasing the number of spectroscopically confirmed member galaxies to 74 (Gold and Silver categories), with 35 new members identified in the outer regions.
Extended Radial Coverage: This is the first study to map the properties of XLSSC 122 members out to $10 \times R_{500}$ ($3$ Mpc), providing a comprehensive view from the cluster core to the field-like outskirts.
Secondary Cluster Identification: The analysis identifies and characterizes a physically distinct galaxy cluster, CL J021741.7-034546, at $z=1.93$ located near the edge of the Cycle 25 data.

Results

Galaxy Density and Structure: The radial galaxy number density profile follows an NFW distribution with a scale radius of $R_s = 110^{+63}_{-65}$ kpc. The study finds no clear evidence of infalling groups or cosmic filaments within the observed volume, suggesting XLSSC 122 may be a mature, relaxed system.
Color Bimodality and Red Fraction: A clear bimodal color distribution is observed. The red fraction ( $f_R$ ) increases significantly from the outskirts to the core, rising from $0.44 \pm 0.17$ in the outskirts to $0.78 \pm 0.14$ in the core. This rapid increase occurs within the virial radius, contrasting with the lower quenched fraction ($0.20$) observed in the CANDELS field at similar redshifts.
Spectral Properties: Median spectra of red and blue populations show distinct differences, with the blue population exhibiting stronger [OIII]5007 emission and a higher continuum in the 3800–4000 Å region, indicative of younger stellar populations.
Luminosity Function: The Schechter function fit for the red sequence yields a fainter characteristic magnitude ( $M^* \approx 23.18$ ) compared to clusters at $z \approx 1$ with similar temperatures, despite a similar low-mass slope ( $\alpha \approx 0.03$ ). This implies a similar galaxy evolution rate to lower-redshift clusters but with a fainter scale magnitude, potentially due to mass differences or K-correction variations.
Quenching Mechanisms: The rapid rise in the red fraction near the virial radius is consistent with fast quenching mechanisms, specifically ram pressure stripping. Modeling suggests that while ram pressure can explain weak stripping for massive galaxies, it may lead to more rapid quenching for lower-mass galaxies. The lack of H $\beta$ absorption features in the core suggests these galaxies have been quenched for at least 0.5 Gyr.

Significance
The paper claims that XLSSC 122 represents a mature galaxy cluster at $z=1.98$ that exhibits quenching processes and a galaxy luminosity distribution similar to lower-redshift clusters, shifted by approximately 1 magnitude. The observed rapid increase in the red fraction upon infall supports the presence of fast quenching mechanisms, such as ram pressure stripping, operating in the early universe. The lack of detected infalling substructures suggests that the dominant quenching mechanism shift observed in the literature may be intrinsic to the evolution of the cluster environment itself rather than solely driven by external pre-processing in filaments. These findings provide necessary constraints on the dominant quenching methods at $z > 1.5$ , bridging the gap between high-redshift mass-dominated quenching and local environmental quenching.