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

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 where galaxies traversing a dense intracluster medium (ICM) lose their gas reservoirs, leading to the quenching of star formation. However, in the early Universe ($z > 2$), protoclusters are dynamically young with immature hot atmospheres. Consequently, environmental effects in these regions have traditionally been assumed to be dominated by gravitational interactions (e.g., mergers, tidal stripping) 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 is sufficiently vigorous at $z > 4$ to remove the bulk of cold gas from massive galaxies and drive environmental quenching.

Methodology
The study focuses on SPT2349−56-C26 (hereafter C26), a massive galaxy within the SPT2349−56 protocluster at $z = 4.30$. The authors utilize a multi-wavelength approach combining high-resolution imaging and spectroscopy:

• Observations: Data were obtained from the James Webb Space Telescope (JWST) using NIRCam (F200W, F444W) and MIRI (F1000W, F1800W), and the Hubble Space Telescope (HST) using WFC3/IR (F110W, F160W). Complementary observations were conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) targeting the [C II] 158 $\mu$m, [N II] 205 $\mu$m, CO(2–1), CO(4–3) lines, and dust continuum at 850 $\mu$m and 3.2 mm.
• Analysis:
  • Morphology and Photometry: JWST and HST images were processed to create spectral energy distributions (SEDs) for the galaxy's "head" (compact stellar body), "tail" (elongated structure), and a downstream "knot." SED fitting was performed using MAGPHYS to derive stellar masses, star-formation rates (SFR), and specific SFRs.
  • Gas Tracers: ALMA data cubes were cleaned and analyzed to map the spatial distribution and kinematics of neutral gas ([C II]), ionized gas ([N II]), and molecular gas (CO). The authors calculated gas masses using standard conversion factors ($\alpha_{[CII]}$ and $\alpha_{CO}$), accounting for Cosmic Microwave Background (CMB) suppression effects at high redshift.
  • Kinematics: Position-velocity (PV) diagrams were constructed to determine if the gas and stars are kinematically connected or detached.
  • Physical Modeling: The authors compared the observed morphology and gas excitation against models of tidal interactions versus hydrodynamic stripping. They also estimated the minimum external pressure required to strip the galaxy's gas and compared this to the expected ICM density in the protocluster.

Key Results

1. Extreme Spatial Decoupling: C26 exhibits a "cometary" morphology with a compact stellar head and an elongated tail extending $\sim 20$ kpc. Crucially, the peak of the [C II] emission (tracing cold gas) is offset by approximately 6 kpc from the stellar head. The tail contains at least 55% of the total [C II] luminosity, indicating that the majority of the cold gas reservoir has been displaced from the stellar body.
2. Gas Mass and Excitation: The total molecular gas mass derived from [C II] is $M_{mol} \approx 3.5 \times 10^{10} M_{\odot}$, with the tail holding $\sim 2.0 \times 10^{10} M_{\odot}$. CO(2–1) detection confirms a similar gas mass ($2.8 \times 10^{10} M_{\odot}$), while CO(4–3) is undetected, implying a very low excitation ratio ($r_{42} < 0.13$). This suggests the stripped gas is diffuse and sub-thermally excited, consistent with low-redshift "jellyfish" galaxies, rather than dense, warm clouds typical of mergers.
3. Phase-Dependent Stripping: The dust continuum (850 $\mu$m) peak is offset from the stellar head by only 3.8 kpc, significantly less than the 6 kpc offset of the [C II] peak. This indicates that the more diffuse gas is stripped more efficiently than the denser dust-traced material.
4. Ionized Gas Interface: [N II] emission is detected on both flanks of the tail, suggesting the formation of ionized interface layers where the displaced cold gas interacts with the surrounding hot medium.
5. Exclusion of Merger Scenarios: The downstream knot contains less than 15% of the head's stellar mass, making a major merger unlikely to drive the observed large-scale displacement. Furthermore, the lack of enhanced star formation and high gas excitation argues against a tidal interaction origin.
6. ICM Conditions: The presence of a hot, dense ICM in SPT2349−56, detected via the thermal Sunyaev-Zeldovich effect, provides the necessary conditions for extreme RPS. The authors estimate that the ambient density at $z=4.3$ is sufficiently high to overcome the galaxy's restoring force, particularly given the likely turbulent and disturbed nature of the protocluster core.

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 study demonstrates that:

• RPS can remove the majority of a massive galaxy's cold gas reservoir even before the host cluster has fully assembled or relaxed.
• This mechanism offers a direct hydrodynamic pathway for environmental quenching at $z > 4$, challenging the assumption that gravitational interactions are the sole driver of galaxy evolution in early protoclusters.
• C26 represents an intermediate evolutionary stage where the gas reservoir is largely removed, but the stellar head has not yet fully quenched, linking early stripping events to the emergence of high-redshift quiescent galaxies.
• If such events are common, RPS may already be responsible for the formation of gas-poor populations in protocluster cores and the redistribution of baryons into the intracluster medium at very early cosmic epochs.