🔭 astrophysics

A Tale of Tails: Star Formation and Stripping in Jellyfish Galaxies in the Strong Lensing Cluster MACS J0138.0-2155

Using VLT/MUSE spectroscopy and Hubble imaging, this study investigates ram-pressure stripping in four galaxies within the MACS J0138.0-2155 cluster, revealing that three jellyfish galaxies exhibit post-starburst disks with star formation confined to their blue-shifted, kinematically distinct tails.

Original authors: Catherine C. Gibson, Jackson H. O'Donnell, Tesla E. Jeltema

Published 2026-04-09

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Catherine C. Gibson, Jackson H. O'Donnell, Tesla E. Jeltema

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 vast, cosmic ocean. In this ocean, there are massive whirlpools called galaxy clusters. Inside these whirlpools, the water isn't empty; it's filled with a super-hot, invisible gas called the Intra-Cluster Medium (ICM).

Now, imagine a galaxy as a boat sailing through this ocean. Usually, a boat is calm and steady. But if that boat sails too fast through a thick, hot fog, the wind and pressure from the fog can rip things off the boat. In the universe, this is called Ram-Pressure Stripping.

This paper is a detective story about four specific "boats" (galaxies) in a massive cosmic whirlpool called MACS J0138.0-2155. The astronomers, acting like cosmic detectives, used powerful telescopes to see how this "wind" is tearing these galaxies apart and changing how they create stars.

Here is the story of what they found, broken down simply:

1. The "Jellyfish" Galaxies

Three of the four galaxies they studied look exactly like jellyfish.

The Head: This is the main body of the galaxy, where most of the stars live.
The Tail: As the galaxy speeds through the hot gas of the cluster, the wind strips away its gas and dust, dragging it behind like a long, glowing tentacle.

The astronomers call these "Jellyfish Galaxies." It's a perfect name because, just like a jellyfish drifting in the ocean, these galaxies are being pushed and pulled by the environment around them.

2. The Two Stages of the Storm

The paper focuses on two main stages of this "storm":

The Late Stage (Galaxies J1 and J2):
These two are like jellyfish that have been swimming in the storm for a long time. Their "heads" are quiet and dead. They used to be busy factories making new stars, but the wind blew all their fuel (gas) away. Now, the heads are full of old stars and are "quenching" (stopping star formation).
- The Twist: Even though the heads are dead, the tails are alive! The gas that was ripped off didn't just disappear; it got squished together in the tail, igniting new, bright blue stars. It's like a campfire being blown out, but the sparks flying off the end are starting new fires in the air.
The Early Stage (Galaxy J3):
This galaxy is just entering the storm. It still has a mix of old stars and new gas. It hasn't been stripped as cleanly as J1 or J2, so it's harder to tell where the "head" ends and the "tail" begins. However, it is currently making stars at a very high rate, almost like a factory running at full speed before the wind finally shuts it down.

3. The "Ghost" Galaxy (J4)

The fourth galaxy, J4, is a bit of a mystery. It looks a bit like the others, but it's actually a "ghost" in the sense that it isn't even in the cluster yet. It's in the foreground, closer to us, just passing by the cluster. It's like a car driving past a busy intersection; it hasn't entered the traffic jam yet, so it hasn't been "stripped" by the cluster's wind.

4. How They Figured It Out

The astronomers didn't just take a picture; they took a "sound recording" of the light (spectroscopy).

The Speed Test: They measured how fast the gas in the tails was moving. They found that the gas in the tails was moving at different speeds than the main galaxy. This confirmed that the gas was indeed being ripped off and pushed away, just like leaves blowing off a tree in a hurricane.
The Star Count: They calculated how many new stars were being born in these tails. They found that while the main bodies of the galaxies were dying, the tails were surprisingly active nurseries for new stars.

The Big Picture: Why Does This Matter?

You might wonder, "Why do we care about jellyfish in space?"

This research helps us understand how galaxies grow up and grow old.

The "Red and Dead" Mystery: We see many old, red galaxies in clusters that have stopped making stars. We didn't know exactly how they stopped.
The Answer: This paper shows that the "wind" of the cluster (Ram-Pressure Stripping) is the culprit. It strips the fuel away, stops the galaxy from making new stars, and turns it into a "red and dead" galaxy.

In summary:
This paper tells the tale of galaxies getting a cosmic haircut. The wind of a giant galaxy cluster is blowing their hair (gas) off, creating beautiful, glowing tails. While the main body of the galaxy is being silenced and dying, the hair flying behind it is sparking with new life. It's a dramatic, violent, yet beautiful process that shapes the universe we live in.

1. Problem and Scientific Context

Galaxies in dense cluster environments often exhibit distinct morphological and evolutionary differences compared to those in isolated fields, typically characterized by a lack of recent star formation ("red and dead" galaxies). A primary mechanism driving this transformation is Ram-Pressure Stripping (RPS), where the intra-cluster medium (ICM) interacts with a galaxy's interstellar medium (ISM), stripping gas and dust from the galactic disk. This process creates the characteristic "jellyfish" morphology—galaxies with elongated tails of stripped gas.

While RPS is well-studied in the local universe ( $z < 0.1$ ) and at high redshifts ( $z \gtrsim 1$ ), there is a gap in detailed characterization at moderate redshifts ( $z \sim 0.3-0.4$ ). Furthermore, understanding the specific kinematics of the stripped gas, the spatial distribution of in-situ star formation within tails, and the transition to post-starburst phases requires high-resolution integral field spectroscopy (IFS). This study addresses these gaps by investigating four candidate jellyfish galaxies within the massive, strong-lensing cluster MACS J0138.0-2155 ( $z = 0.336$ ).

2. Methodology

The authors utilized a multi-wavelength approach combining optical/near-infrared imaging and integral field spectroscopy:

Data Sources:
- Spectroscopy: MUSE (Multi-Unit Spectrographic Explorer) data from the ESO Very Large Telescope (VLT), combining a 2019 exposure (49 min) and a 2023 exposure (2.9 hours). The data covers the wavelength range 3500–6900 Å.
- Imaging: Hubble Space Telescope (HST) data in filters F555W, F105W, and F160W to visualize galactic structure and tails.
Sample Selection: Four galaxies (labeled J1, J2, J3, J4) were identified within the cluster. J1, J2, and J3 are cluster members; J4 is a foreground object.
Data Reduction & Binning:
- Spatial Binning: The MUSE data cubes were processed using the VorBin algorithm to create adaptive 2D spatial bins based on signal-to-noise (S/N) ratios. Two distinct binning schemes were employed:
  1. Continuum Binning: Based on S/N in the 6000–6150 Å range (target S/N = 20) to resolve stellar kinematics.
  2. Emission Binning: Based on the H $\alpha$ +[N II] doublet (target S/N = 25) to resolve gas kinematics and star formation rates (SFR).
- Redshift Handling: For J1 and J2, distinct redshifts were used for the "head" (stellar continuum) and "tail" (gas emission) due to significant velocity offsets.
Spectral Analysis:
- Fitting: The pPXF (penalized Pixel-Fitting) code was used to fit spectra. It utilized XSL DR3 stellar templates and Gaussian templates for gas emission lines.
- Kinematics: The code extracted line-of-sight velocity offsets and velocity dispersions ( $\sigma$ ) for both stellar and gas components.
- Star Formation Rates (SFR): Calculated using H $\alpha$ luminosity, corrected for stellar absorption (via pPXF), foreground extinction (NED), and internal dust extinction (via the Balmer decrement, assuming an intrinsic H $\alpha$ /H $\beta$ ratio of 2.86).
- Ionization Diagnostics: A BPT diagram ([O III]/H $\beta$ vs. [N II]/H $\alpha$ ) was constructed to distinguish between star formation, AGN, and shock ionization.

3. Key Contributions

Kinematic Resolution of Jellyfish Tails: The study provides spatially resolved gas and stellar kinematics for jellyfish galaxies at $z \approx 0.336$ , a regime less explored than local or high-redshift samples.
Differentiation of Stripping Stages: By applying the morphological classification scheme of Poggianti et al. (2025), the authors distinguish between late-stage stripping (J1, J2) and earlier stages (J3).
Post-Starburst Identification: The detection of strong Balmer absorption lines in the heads of J1 and J2 confirms these galaxies are in a post-starburst (K+A) phase, linking RPS directly to the quenching of star formation.
Foreground/Background Separation: The analysis successfully distinguishes cluster members from foreground objects (J4) and lensed arcs, clarifying the dynamical state of the system.

4. Key Results

Galaxy J1 and J2 (Late-Stage Stripping)

Morphology: Both exhibit distinct "heads" (stellar disks) and elongated ionized tails (~21 kpc for J1, ~16 kpc for J2).
Spectral Features: The heads show strong Balmer absorption, indicating A-type stellar populations and recent quenching (within ~1 Gyr). The tails show strong H $\alpha$ emission.
Kinematics:
- Velocity Gradients: Both galaxies show significant velocity gradients across their tails (up to ~400 km s $^{-1}$ ), consistent with gas being stripped and trailing behind the galaxy on a radial orbit.
- Velocity Dispersion: Gas velocity dispersion increases with distance from the head, peaking around 100–150 km s $^{-1}$ in the star-forming knots.
Star Formation: Star formation is confined to the tails.
- J1: SFR peaks at ~0.11 M $_\odot$ yr $^{-1}$ kpc $^{-2}$ in a central knot.
- J2: SFR peaks at ~0.06 M $_\odot$ yr $^{-1}$ kpc $^{-2}$ at the tail tip.
Classification: Classified as Jtype=2 jellyfish (tail length $\ge$ stellar disk length), though they lack ionized gas in the head, suggesting advanced stripping.

Galaxy J3 (Early-Stage Stripping)

Morphology: Less distinct separation between head and tail; the tail covers an area similar to the head.
Kinematics: Shows a clear velocity gradient and is likely in a post-pericenter passage, moving away from the cluster center.
Star Formation: Exhibits the highest SFR of the sample, reaching 0.49 M $_\odot$ yr $^{-1}$ kpc $^{-2}$ . The star-forming knot appears more central than in J1/J2, suggesting the stripping process is ongoing but not yet fully separated.
Ionization: The BPT diagram places J3 in the "composite" and "LINER" regions. The authors attribute the LINER-like emission to shock excitation from RPS rather than an AGN, supported by the lack of radial concentration of LINER bins and high gas velocity dispersions (~100 km s $^{-1}$ ).

Galaxy J4 (Foreground Object)

Located in the foreground ( $z \approx 0.3087$ ) but projected near a lensed arc.
Shows weak star formation (~0.014 M $_\odot$ yr $^{-1}$ kpc $^{-2}$ ) and no clear jellyfish morphology, likely due to low signal-to-noise.

General Findings

Blue Shift: All three cluster members (J1, J2, J3) are blue-shifted relative to the cluster mean, suggesting they are part of an infalling group.
Quenching Mechanism: The combination of post-starburst signatures in the heads and active star formation in the tails strongly supports RPS as the driver for both the triggering of in-situ star formation and the subsequent quenching of the main disk.
BPT Analysis: Most bins fall in the star-forming or composite regions. While shocks contribute (especially in J3), star formation remains the dominant ionization source.

5. Significance

This paper bridges a critical gap in the understanding of environmental galaxy evolution at moderate redshifts ( $z \sim 0.35$ ). It demonstrates that:

RPS is efficient even at moderate clustercentric distances in massive clusters, capable of stripping gas and triggering star formation in tails while simultaneously quenching the host galaxy.
Kinematic signatures (velocity gradients and dispersion) are robust tracers of the stripping stage and orbital phase (post-pericenter passage).
Post-starburst phases in jellyfish galaxies are a direct consequence of RPS, providing a timeline for the transition from active star formation to quiescence.

The study validates the use of MUSE IFU data to resolve complex kinematic structures in distant clusters, offering a template for future studies of galaxy evolution in dense environments. Future work is proposed to incorporate X-ray data to better constrain the ICM density and the specific interaction dynamics.