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Novel insights on the Coma Cluster kinematics with DESI. I. Linking mass profile, orbital anisotropy and galaxy populations

Using a large spectroscopic sample from DESI and the MG-MAMPOSSt code, this study provides the tightest kinematic mass profile constraint for the Coma Cluster to date while revealing how galaxy orbital anisotropy and virial mass estimates vary across different galaxy populations, thereby linking dynamical properties to environmental evolution.

Original authors: S. Pedratti (Università di Milano-Bicocca, Milano, Italy), L. Pizzuti (Università di Milano-Bicocca, Milano, Italy, INAF - Osservatorio Astronomico di Trieste, Trieste, Italy), M. Fossati (Università

Published 2026-03-18

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CC BY 4.0

Original authors: S. Pedratti (Università di Milano-Bicocca, Milano, Italy), L. Pizzuti (Università di Milano-Bicocca, Milano, Italy, INAF - Osservatorio Astronomico di Trieste, Trieste, Italy), M. Fossati (Università di Milano-Bicocca, Milano, Italy, INAF, Osservatorio Astronomico di Brera, Milano, Italy), A. Biviano (INAF - Osservatorio Astronomico di Trieste, Trieste, Italy, IFPU, Institute for Fundamental Physics of the Universe, Trieste, Italy), A. Boselli (Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France), A. Ragagnin (INAF - Osservatorio Astronomico di Trieste, Trieste, Italy), A. Carlin (Università di Milano-Bicocca, Milano, Italy)

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 Coma Cluster as a massive, bustling cosmic city. It's not made of buildings and people, but of thousands of galaxies, swirling around a central hub of invisible "dark matter" that holds everything together with its gravity.

For decades, astronomers have tried to weigh this city and understand how its residents (the galaxies) move. But it's a tricky job. It's like trying to figure out the weight of a spinning carousel just by watching the horses run, without being able to see the central pole or the motor.

This paper is like a new, high-tech report card for the Coma Cluster, written by a team using data from DESI (the Dark Energy Spectroscopic Instrument). Think of DESI as a super-powerful camera that can take the "speed and direction" of over 2,000 galaxies at once, giving us a much clearer picture than ever before.

Here is the breakdown of what they found, using some everyday analogies:

1. Weighing the Invisible City

The main goal was to figure out the total mass of the cluster. Since most of the mass is Dark Matter (an invisible substance that doesn't emit light), they couldn't just put it on a scale. Instead, they used the Jeans Equation.

The Analogy: Imagine you are in a dark room with a bunch of people running around. You can't see them, but you can hear them. If they are running in tight circles, the room must be small and the walls (gravity) must be strong. If they are running in long, straight lines across the room, the room is huge.
The Result: By listening to the "speed" of the galaxies, the team calculated the cluster's weight. They found it weighs about 1.08 quadrillion times the mass of our Sun. This is the most precise "kinematic" (movement-based) weight ever calculated for this specific cluster.

2. The Three Types of Galaxy Residents

The researchers didn't just look at all the galaxies as a single group. They sorted them into three neighborhoods based on their color, which tells us their age and activity:

Red Sequence (The Retirees): These are old, red galaxies that have stopped making new stars. They are like the quiet, settled residents of the city who have lived there for generations.
Green Valley (The Transitioners): These are galaxies in the middle of changing. They are turning from blue (young) to red (old). They are like the "commuters" moving from the suburbs into the city center.
Blue Cloud (The Newcomers): These are young, blue, star-making galaxies. They are the energetic new arrivals, likely just arriving from the "countryside" (the space between galaxy clusters).

3. The Commute: How They Move

This is where the study got really interesting. They looked at the "orbits" of these different groups.

The Red Galaxies: They move in a fairly balanced, circular way, like cars driving in a steady traffic circle. They are settled and "relaxed."
The Blue Galaxies: These guys are wild! They are on very elongated, radial orbits.
- The Analogy: Imagine a roller coaster. The Red galaxies are on the flat, circular part of the track. The Blue galaxies are on the steep drop—they are plunging straight toward the center of the cluster at high speed, coming from far away.
The Green Galaxies: They are in the middle. As they fall toward the center, they are getting "stripped" of their gas (like wind blowing the leaves off a tree) and turning red.

4. The "Ram Pressure" Stripper

The paper explains why the blue galaxies are turning red. As they fall into the cluster at high speeds, they crash into a thick fog of hot gas that fills the space between galaxies.

The Analogy: Imagine sticking your hand out of a car window while driving 100 mph. The wind pushes your hand back. Now imagine that wind is so strong it rips the leaves off a tree. That is Ram Pressure Stripping. The hot gas in the cluster rips the fuel (gas) out of the falling blue galaxies, stopping them from making new stars and turning them into red, "dead" galaxies.

5. Why This Matters

The team found that if you only look at the "Blue Cloud" galaxies, you might overestimate how heavy the cluster is. Why? Because they are falling in so fast from the outside that they make the whole city seem bigger and more massive than it actually is.

By combining all the data and using a sophisticated computer code called MG-MAMPOSSt (which is like a super-advanced calculator for gravity), they managed to separate these effects.

The Big Takeaway:
This study is a perfect example of how the environment shapes a galaxy's life. It shows that galaxies aren't just floating randomly; they are on a journey. They fall in from the cosmic web, get stripped of their fuel by the cluster's atmosphere, and eventually settle down as the quiet, red galaxies we see in the center.

It's like watching a movie of a galaxy's life story, from its energetic youth in the blue clouds to its quiet retirement in the red sequence, all dictated by the gravity of the massive city it calls home.

1. Problem Statement

Galaxy clusters serve as critical laboratories for cosmology and astrophysics, yet determining their total mass distribution and understanding the interplay between galaxy evolution and their environment remains challenging. Previous studies of the Coma Cluster (Abell 1656) have yielded varying mass estimates ( $M_{200}$ ) ranging from $\sim 5 \times 10^{14}$ to $2.9 \times 10^{15} M_\odot$ using different techniques (weak lensing, X-ray, caustics, machine learning). Furthermore, the dynamical state of the cluster and how different galaxy populations (Red Sequence, Green Valley, Blue Cloud) trace the gravitational potential and orbital anisotropy are not fully resolved. There is a need for a robust, high-statistics kinematic analysis to constrain the mass profile, orbital anisotropy, and the dynamical state of the Coma Cluster using modern spectroscopic data.

2. Methodology

The authors utilized a new, large spectroscopic sample of Coma Cluster member galaxies from the Dark Energy Spectroscopic Instrument (DESI) Data Release 1 (DR1).

Dataset:
- Source: DESI SV (Science Verification) program targeting the Coma Cluster.
- Sample: 1,758 galaxies initially, reduced to 1,651 after removing interlopers using the Clean algorithm (Mamon et al. 2013) and excluding non-nuclear sources.
- Completeness: Estimated at 91% for galaxies brighter than $m_r < 19.7$ mag.
- Classification: Galaxies were classified into three populations based on the $(m_g - m_r)$ $(m_{g} - m_{r})$ vs. stellar mass ( $M_*$ $M_{*}$ ) diagram:
  - Red Sequence (RS): 1,216 galaxies (quiescent).
  - Green Valley (GV): 167 galaxies (transitioning).
  - Blue Cloud (BC): 249 galaxies (star-forming).
- Kinematic Space: The analysis used the projected phase-space (p.p.s.) defined by projected radius ( $R$ ) and line-of-sight (LoS) rest-frame velocity ( $v_{RF}$ ).
Analysis Tool:
- MG-MAMPOSSt: A code based on the Jeans equation that jointly reconstructs the total mass profile ( $M_{dyn}$ ) and orbital anisotropy profile ( $\beta$ ) from the projected phase-space distribution.
- Mass Models: Tested NFW, generalized NFW (gNFW), Hernquist, and Burkert profiles.
- Anisotropy Models: Tested generalized Tiret (gT) and generalized Osipkov-Merritt (gOM) profiles.
- Statistical Approach: Bayesian inference using MCMC (Metropolis-Hastings) to explore parameter space. Model selection was performed using Bayesian Information Criterion (BIC) and Akaike Information Criterion (AIC).
Robustness Checks:
- Variations in cluster center (NGC 4874, NGC 4889, X-ray peak).
- Variations in velocity rest frame (biweight mean, RS, GV, BC means).
- Variations in radial range (inner cutoff 50–200 kpc; outer cutoff 1.8–2.9 Mpc).
- Separate analysis of subsamples (RS, GV, BC) and a joint likelihood combination.

3. Key Contributions

High-Precision Kinematic Mass: Provided the tightest robust kinematic mass profile constraint for the Coma Cluster to date, utilizing a significantly larger spectroscopic sample than previous kinematic studies.
Multi-Component Mass Decomposition: Separated the total mass into dark matter, hot gas, and stellar mass components to derive a pure dark matter mass profile.
Population-Specific Dynamics: Investigated how different galaxy populations (RS, GV, BC) trace the gravitational potential and orbital anisotropy, revealing distinct dynamical behaviors linked to their evolutionary state.
Systematic Error Quantification: Rigorously tested the impact of center choice, velocity frame, and radial range on mass and anisotropy estimates, demonstrating the robustness of the results.

4. Key Results

Total Mass and Radius:
- Assuming an NFW mass profile and gOM anisotropy model (favored by BIC/AIC), the joint analysis yields:
  - Virial Mass: $M_{200} = 1.08^{+0.08}_{-0.09} (\text{stat}) \pm 0.09 (\text{syst}) \times 10^{15} M_\odot$ .
  - Virial Radius: $r_{200} = 2.12 \pm 0.06 (\text{stat}) \pm 0.07 (\text{syst}) \text{ Mpc}$ .
  - Scale Radius: $r_s = 0.48^{+0.27}_{-0.13} \text{ Mpc}$ .
- The results are consistent with, but generally higher than, weak lensing and X-ray+SZ estimates, while overlapping within $2\sigma$ with most literature values.
Dark Matter and Baryon Fraction:
- By subtracting the gas mass (from X-ray) and stellar mass (from photometry), the dark matter virial mass is estimated as $M_{200}^{DM} = 8.6^{+1.2}_{-0.8} \times 10^{14} M_\odot$ .
- The baryon fraction at $r_{200}$ is $f_b = 0.15 \pm 0.02$ , consistent with the cosmological value $\Omega_b/\Omega_m$ .
Orbital Anisotropy ( $\beta$ ):
- Global: The cluster shows a tendency toward radial orbits in both the central and outer regions, indicating a population of recently accreted galaxies.
- Population Differences:
  - RS Galaxies: Follow the global trend (radial orbits).
  - GV Galaxies: Show a strong increase in radial anisotropy in the central region, suggesting they are transitioning as they fall in.
  - BC Galaxies: Exhibit even more radial orbits in the outskirts. Their velocity distribution is significantly non-Gaussian (skewed to high velocities), suggesting they are infalling from large-scale filaments along the line of sight.
- Mass Bias: BC galaxies alone predict a significantly higher virial mass ( $M_{200} \approx 2.55 \times 10^{15} M_\odot$ ), likely due to their non-equilibrium infall from filaments, whereas RS galaxies provide a mass estimate consistent with the mixed sample.
Dynamical State:
- The Anderson-Darling test on LoS velocities shows RS galaxies are virialized ( $A^2 \sim 0.29$ ), while BC galaxies show strong deviations from Gaussianity ( $A^2 \sim 1.56$ ), confirming the cluster is not in full dynamical equilibrium due to ongoing infall.

5. Significance

This study establishes a new benchmark for kinematic mass measurements in galaxy clusters using DESI data. It confirms that while the Coma Cluster is largely relaxed, it retains signatures of recent accretion, particularly from star-forming (Blue Cloud) galaxies. The finding that different galaxy populations trace different dynamical states and mass estimates highlights the importance of accounting for galaxy selection effects in mass modeling. The derived mass and baryon fraction are consistent with $\Lambda$ CDM cosmology. These results provide a foundation for future work (Paper II) to investigate substructures and merging events in Coma, ultimately refining our understanding of how environment drives galaxy evolution (e.g., via ram-pressure stripping).