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An extreme ram-pressure stripping event in a protocluster at redshift 4.3

This paper reports ALMA and JWST observations of a massive galaxy in a protocluster at redshift 4.3 undergoing extreme ram-pressure stripping, demonstrating that hydrodynamic gas removal can effectively quench massive galaxies in dense environments as early as $z>4$ .

Original authors: Dazhi Zhou, Scott C. Chapman, Manuel Aravena, Roger Deane, Anthony H. Gonzalez, Ryley Hill, Nicholas LeVar, Matthew A. Malkan, Adam Muzzin, Nan Zhang, Kedar A. Phadke, Vismaya R. Pillai, Manuel Solima

Published 2026-06-17✓ Author reviewed ⓘ

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CC BY 4.0

Original authors: Dazhi Zhou, Scott C. Chapman, Manuel Aravena, Roger Deane, Anthony H. Gonzalez, Ryley Hill, Nicholas LeVar, Matthew A. Malkan, Adam Muzzin, Nan Zhang, Kedar A. Phadke, Vismaya R. Pillai, Manuel Solimano, Justin S. Spilker, Nikolaus Sulzenauer, Joaquin D. Vieira, David Vizgan, George C. P. Wang, Axel Weiss

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 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 houses, and the "gas" inside them is the fuel (like wood or coal) needed to keep the fire of star formation burning. Usually, when a house is in a quiet neighborhood, it keeps its fuel safe. But when a house moves into a crowded, chaotic downtown area, things get messy.

This paper tells the story of a specific "house" (a galaxy named C26) located in a very crowded, developing neighborhood called a protocluster (SPT2349−56). This neighborhood is so far away that we are seeing it as it was when the universe was very young (about 12 billion years ago).

Here is the simple breakdown of what the scientists found:

1. The "Hurricane" Effect (Ram-Pressure Stripping)

Usually, scientists thought that in these young, crowded neighborhoods, the only thing that could mess up a galaxy was a slow, gravitational tug-of-war (like two people pulling on a rope). They didn't think the "wind" of the neighborhood was strong enough to blow things away yet.

But C26 tells a different story. It is being hit by a massive, invisible "wind" made of super-hot gas that fills the space between galaxies. As C26 moves through this hot gas, the wind is so strong that it acts like a hurricane blowing through a house. It is physically ripping the fuel (cold gas) out of the galaxy and leaving it trailing behind.

2. The "Comet" Shape

Because the wind is blowing so hard, C26 looks like a comet.

The Head: The front part of the galaxy is still intact. It's the "head" of the comet, where the stars are still hanging on.
The Tail: Behind the head, there is a long, glowing tail. This tail is made of the gas that was just ripped out of the galaxy.

The most shocking part is the distance. The scientists found that the "head" of the galaxy and the "tail" of the gas are separated by a huge gap (about 20,000 light-years (6 kiloparsecs)). It's as if the wind blew the fuel tank off the car and left it far behind, while the car kept driving.

3. The "Empty Tank" Problem

This is a big deal because gas is the fuel for making new stars.

Before the storm: C26 was a massive, gas-rich galaxy, full of fuel, ready to make stars.
After the storm: The wind (ram pressure) has stolen more than half of the galaxy's total fuel supply and pushed it into the tail.
The Result: The main part of the galaxy (the head) is now running on fumes. Even though it still has some stars, it is being "starved." Without that fuel, it will eventually stop making new stars and go quiet (a process astronomers call "quenching").

4. Why This Changes the Rules

For a long time, scientists believed that this kind of violent wind stripping only happened in old, mature galaxy clusters (like the downtown of an old city). They thought young clusters (like this one) were too "soft" to have such strong winds.

This paper proves them wrong. It shows that even in the very early universe (when the cluster was just a baby), the wind can be strong enough to strip a massive galaxy of its fuel. It's like finding a hurricane in a nursery; it shouldn't be that strong, but it is.

The Big Picture

Think of C26 as a car driving through a strong headwind. The wind is so powerful it blew the gas tank off the back of the car. The car (the galaxy) is still moving, but it's about to run out of gas and stop.

This discovery suggests that the environment of a galaxy cluster can "kill" a galaxy's ability to make stars much earlier in the universe's history than we previously thought. The wind isn't just a gentle breeze; in these dense regions, it's a forceful bulldozer that clears the fuel away, turning bright, active galaxies into quiet, dead ones very quickly.

Technical Summary: An extreme ram-pressure stripping event in a protocluster at redshift 4.3

Problem and Context
In the nearby Universe, ram-pressure stripping (RPS) is a well-established mechanism for quenching star formation in dense cluster environments by removing cold gas reservoirs from galaxies. However, in the early Universe ( $z > 2$ ), protoclusters are dynamically young with immature hot atmospheres, leading to the prevailing assumption that environmental effects are dominated by gravitational interactions rather than hydrodynamic stripping. While recent observations have detected gas tails in forming clusters at $z \approx 2.5$ and $3.0$, it remains unclear whether RPS can be sufficiently vigorous to quench massive galaxies at $z > 4$ . This study addresses the question of whether hydrodynamic stripping can effectively remove the majority of cold gas from massive galaxies in dense protocluster cores at such early epochs.

Methodology
The authors present a multi-wavelength analysis of SPT2349−56-C26 (hereafter C26), a massive galaxy within the SPT2349−56 protocluster at $z = 4.30$ . The study combines:

JWST Imaging: High-resolution NIRCam (F200W, F444W) and MIRI (F1000W) data to resolve stellar morphology and perform spectral energy distribution (SED) fitting.
ALMA Observations: Multi-line and continuum data including [C II] 158 $\mu$ m, [N II] 205 $\mu$ m, CO(2–1), CO(4–3), and dust continuum (850 $\mu$ m and 3.2 mm) to trace neutral gas, ionized gas, molecular gas, and dust.
HST Imaging: WFC3/IR data (F110W, F160W) to supplement photometric coverage for SED modeling.

The analysis involves component photometry to separate the galaxy into a "head," a "tail," and a "downstream knot." SED fitting using MAGPHYS was employed to derive stellar mass ( $M_*$ ) and specific star-formation rates (sSFR). Gas masses were derived using standard conversion factors for [C II] ( $\alpha_{[CII]}$ ) and CO ( $\alpha_{CO}$ ), with corrections for the elevated Cosmic Microwave Background (CMB) temperature at high redshift affecting CO excitation. Kinematic analysis via position-velocity (PV) diagrams and morphological comparisons were used to distinguish between tidal interactions and hydrodynamic stripping.

Key Results

Morphological Decoupling: C26 exhibits a "cometary" stellar morphology with a compact head and an elongated tail. Crucially, the stellar and gas components are strongly decoupled. The peak of the [C II] emission is offset by $\sim 6$ kpc from the stellar head, and the tail extends $\sim 20$ kpc.
Gas Displacement: The displaced gas tail contains more than half of the system's total cold-gas reservoir. The [C II] luminosity indicates a total molecular gas mass of $M_{\text{mol}} \approx 3.5 \times 10^{10} M_\odot$ , with the tail alone holding $M_{\text{tail}} \approx 2.0 \times 10^{10} M_\odot$ (55% of the total).
Phase-Dependent Offsets: The dust continuum peak is offset from the stellar head by $\sim 3.8$ kpc, while the [C II] peak is offset by $\sim 6$ kpc. This suggests phase-dependent stripping where diffuse gas is displaced more efficiently than denser dust-traced material.
Gas Excitation and State: The molecular gas is in a low-excitation state, evidenced by a non-detection of CO(4–3) and a low excitation ratio ( $r_{42} < 0.13$ ). The gas appears diffuse and sub-thermally excited, inconsistent with the dense, warm clouds typical of merger-driven starbursts.
Kinematic Connectivity: The [C II] emission shows a continuous velocity gradient along the tail, and the PV diagram indicates the gas remains kinematically connected to the head, ruling out a detached component typical of major mergers.
Stellar Properties: The stellar head has a mass of $M_* \approx 2.2 \times 10^{10} M_\odot$ with a relatively low sSFR ( $1.33 \text{ Gyr}^{-1}$ ) compared to main-sequence galaxies at $z \sim 4.5$ . The tail and knot show bluer colors and younger stellar populations, consistent with in-situ star formation in stripped material.

Significance and Claims
The paper claims that C26 provides direct evidence that ram-pressure stripping can dominate the quenching of massive star-forming galaxies as early as $z = 4.3$ . The authors argue that:

Early Environmental Quenching: Contrary to the expectation that environmental effects are weak in high-redshift protoclusters, the hot intracluster medium (ICM) in SPT2349−56 is dense enough to strip the majority of a massive galaxy's cold gas reservoir before the cluster reaches a mature state.
Mechanism Validation: The combination of large spatial offsets, phase-dependent stripping, low gas excitation, and kinematic connectivity strongly favors RPS over tidal interactions or mergers as the primary driver of the observed morphology.
Evolutionary Pathway: C26 represents an intermediate stage where the cold gas reservoir has been largely removed, reducing the gravitational restoring force of the remaining stellar head. This process links early stripping to the emergence of high-redshift quiescent galaxies in dense environments.
Broader Implications: If C26 is not unique, RPS may already be transforming a significant population of galaxies in the SPT2349−56 core, contributing to the formation of gas-poor populations and the early buildup of the diffuse intracluster medium.

The study concludes that RPS is a viable and potent mechanism for environmental quenching at $z > 4$ , challenging previous assumptions about the timing and efficiency of hydrodynamic stripping in the early Universe.