NGC 4936: abundant multi-phase medium in the central weak X-ray cool core and optically dark H I clouds in the group

arXiv:2610.10721 · astro-ph.GA · Submitted 2026-10-07 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "NGC 4936: abundant multi-phase medium in the central weak X-ray cool core and optically dark H I clouds in the group".

Jocelyn: The gist NGC 4936 provides an example of cold-gas accumulation in a group-central galaxy where atmospheric cooling and environmental perturbations may act together> How it works The study presents a…

Vera: First, who's behind it and why it matters.

Title and authors: Vera: So, to start, let's talk about the title itself—"NGC four thousand nine hundred thirty-six: abundant multi-phase medium in the central weak X-ray cool core and optically dark H I clouds in the group" <ref:2610.10721#pg1,NGC 4936: abundant multi-phase medium in the central weak X-ray>. It basically sets up what they found.

Jocelyn: They’re looking at how this one galaxy, NGC four thousand nine hundred thirty-six has a lot of gas that exists in different states—cold molecular gas, warm ionized gas, and some cold atomic hydrogen stuff <ref:2610.10721#pg1>.

Subrahmanyan: The implication here is that we need a better way to understand how this kind of cold material gets stored and maintained in groups where the hot X-ray environment isn't quite as aggressive as we might think.

The paper's summary: Vera: Okay, so the core finding is that they combined observations from MeerKAT, SOAR imaging, MUSE spectroscopy, Chandra X-ray data, and HST optical images to map out the gas.

Jocelyn: They focus on three main things: first characterizing where the molecular gas is and how it moves; second assessing if there's a connection between the cold gas and the ionized gas and dust; and third exploring if this cold material formed right there or came from outside.

Subrahmanyan: The summary points out that despite this system having an X-ray cooling time ratio of tcool/tff greater than twenty-five in its center, it hosts one of the most massive cold molecular gas reservoirs they've seen in a galaxy group setting <ref:2610.10721#pg2,hosts one of the most massive cold molecular gas reservoirs>.

The paper's improvements: Vera: Now, the authors suggest a few ways to think about this system differently. They seem to be testing the standard precipitation framework against their findings on NGC four thousand nine hundred thirty-six <ref:2610.10721#pg1>.

Jocelyn: What they are suggesting is that this system provides an interesting test of how we usually model gas condensation, especially when you have these high tcool/tff ratios like the one they found at twenty-five <ref:2610.10721#pg2,provides an interesting test of>.

Subrahmanyan: The paper also explores how to predict the relationship between different gas phases by tracing a common origin and reservoir between the ionised and molecular gas they observed.

Conclusion: Vera: So, to wrap up, this paper shows that in NGC four thousand nine hundred thirty-six you can have a lot of cold material—molecular gas extending up to about two kpc—even when the hot environment suggests it shouldn't be there as much <ref:2610.10721#pg1>.

Jocelyn: It’s important because it suggests that the current state we see in this system is a combination of atmospheric cooling processes happening alongside external environmental interactions.

Subrahmanyan: For us, this means that models for cold gas accumulation in galaxy groups might need to account for these combined effects of internal cooling and external perturbations acting at different times.

Vera: So, the full title "NGC four thousand nine hundred thirty-six: abundant multi-phase medium in the central weak X-ray cool core and optically dark H I clouds in the group" tells us a lot about this complex environment <ref:2610.10721#pg1,NGC 4936: abundant multi-phase medium in the central weak X-ray>.

Jocelyn: It really paints a picture of a system where multiple gas components are interacting on different scales.

Subrahmanyan: The next step for researchers will be to use these detailed kinematic and morphological findings to figure out which of those historical mechanisms—like external acquisition or enhanced cooling—is actually dominant in this specific galaxy group.

Prathamesh Tamhane, Ming Sun, Hao Chen, Thomas Connor, Megan Donahue, Mark Voit, Mpati Ramatsoku, Kokoro Hosogi, Tim Edge

Department of Physics and Astronomy, University of Alabama in Huntsville · Research Center for Astronomical Computing, Zhejiang Laboratory Research Center for Astronomical Computing

astro-ph.GA

Submitted: 2026-10-07

Updated: 2026-10-07

Comments: Revised and resubmitted to PASA. 14 figures, 3 tables

Code: https://github.com/astropy/photutils-datasets

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 65/100

The gist: The gist NGC 4936 provides an example of cold-gas accumulation in a group-central galaxy where atmospheric cooling and environmental perturbations may act together> How it works The study presents a

Key concepts

Molecular Gas Reservoir: This refers to the large amount of cold, dense material in NGC 4936, concentrated in a rotating disk of about one kiloparsec radius. It is primarily traced by CO emission and is crucial for understanding how cold gas accumulates in galaxy groups.
Ionised Gas (H$\alpha$): This gas exists at higher temperatures than molecular gas and is detected by H$\alpha$ emission. It extends out to about 15 kpc with complex filamentary structures, showing kinematics that follow the molecular gas but also indicating shocks or multiple velocity components.
tcool/tff Ratio: This ratio compares the cooling time of the hot intragroup medium (X-ray gas) to its free-fall time. A value near 25 in NGC 4936 suggests it is near a threshold where atmospheric condensation, or multiphase gas formation, is likely occurring.
Atmospheric Condensation: This process describes how hot gas in the center of a galaxy can cool and condense into colder phases (like molecular gas). The study favors this as the main source for NGC 4936's cold gas, though external acquisition might also play a role.

Terminology

AGN — 21× in this paper · explained in 1 episode
Core — 15× in this paper · explained in 2 episodes
Abundance — 12× in this paper · explained in 1 episode
Velocity Dispersion — 12× in this paper · explained in 2 episodes
Metallicity — 11× in this paper · explained in 3 episodes
Aperture — 10× in this paper · explained in 1 episode
Trace — 8× in this paper · explained in 2 episodes
Calibration — 6× in this paper · explained in 7 episodes
Consistency — 2× in this paper · explained in 3 episodes
Hubble Space Telescope — 2× in this paper
AGN Feedback — 1× in this paper · explained in 1 episode
Cooperation — 1× in this paper · explained in 2 episodes
Multi-Unit Spectroscopic Explorer — 1× in this paper
Signal-to-noise ratio (SNR) — 1× in this paper · explained in 2 episodes
Star Formation Rate (SFR) — 1× in this paper · explained in 5 episodes

Summary

The gist NGC 4936 provides an example of cold-gas accumulation in a group-central galaxy where atmospheric cooling and environmental perturbations may act together>

How it works

The study presents a multiwavelength analysis of NGC 4936, which is the central galaxy of a weak cool-core galaxy group hosting an unusually large cold/warm gas reservoir> The research combines observations from MeerKAT, SOAR narrow-band imaging data, MUSE integral-field spectroscopy, Chandra data, and optical imaging from HST and DEC to investigate the morphology and kinematics of the gas>

The analysis focuses on three main objectives:

Characterise the distribution and dynamics of molecular gas
Assess the connection between cold gas, ionised gas, and dust on galaxy-group scales
Explore whether the system represents in-situ cooling or externally acquired cold material

X-ray Properties of the Group

The hot intragroup medium around NGC 4936 exhibits thermodynamic properties that place it at the edge of the regime commonly associated with multiphase gas condensation, as its tcool/tff ratio reaches ∼ 25 in the innermost region The system has a central cooling time of tcool/tff ∼ 41 (25) at 10 kpc, and an aperture-averaged cooling time of tc,10kpc = 1.68+0.08−0.09 Gyr The derived bolometric X-ray luminosity is LXray = (9.5 ± 0.3) × 1042 erg s−1, which is ∼ 10 times lower than the luminosity of NGC 5044 in a slightly larger aperture in the same energy band

Molecular Gas Distribution and Kinematics

The molecular gas detected in NGC 4936 is concentrated within the central ∼ 2 kpc surrounding the nucleus and is distributed asymmetrically, with CO(1–0) emission peaking at the position of the central continuum source The velocity field is broadly consistent with rotational motion about the nucleus, with line-of-sight velocities ranging from −200 to 200 km s−1, although an extended redshifted component is present west of the nucleus reaching velocities of up to ∼ 300 km s−1 Rotating disk modelling using 3D-BAROLO suggests a molecular disk extending to a radius of ∼ 1.1 kpc with a rotational velocity of ∼ 200 km s−1, and the inclination is found to be 61.8 ± 1.6◦

Ionised Gas Distribution and Kinematics

The ionised gas traced by Hα extends to at least ∼ 15 kpc with a complex filamentary morphology that broadly follows molecular gas kinematics while extending well beyond the CO disc The velocity structure in the inner region is similar to that of the molecular gas, with rotational velocities ranging from −200 to 200 km s−1 The eastern filamentary extension shows blueshifted velocities ranging from −200 to −100 km s−1, while the western filamentary emission is redshifted with velocities between 200 and 300 km s−1 The FWHM values are generally elevated indicating multiple velocity components, unresolved velocity structure or shocks

Dust and Star Formation

Optical imaging reveals dust filaments extending to ∼ 14 kpc including structures that appear to wrap around a companion galaxy located only ∼ 2.5 kpc away in projection The Spitzer IR SED of NGC 4936 is fitted with a modified blackbody model, yielding a dust temperature of Tdust = 29.2+3.5−2.9 K and a total infrared luminosity of LTIR = 8.08+0.53−0.46 × 10 42 erg s−1 The star formation rate is estimated to be 0.29 M⊙ yr−1, corresponding to a molecular gas depletion time of 6.74 × 108 yr

Central AGN and Radio Continuum

The nuclear X-ray and millimetre continuum measurements provide evidence of variability on timescales of days The MeerKAT image shows a nuclear continuum source with a flux density of 39.76 ± 0.01 mJy, which corresponds to a spectral power of 1.02×10 22 W Hz−1 at 1.28 GHz The radio spectral index of the continuum source is α = 0.57 ± 0.09, consistent with the optically thin synchrotron emission from a low-luminosity AGN

H I in NGC 4936 and the Group Environment

The MeerKAT observations do not reveal any detection of H I in either absorption or emission, leading to a 3σ upper limit on the atomic gas mass of MHI < 1.13 × 10 7 M⊙ In contrast, the blind search of the MeerKAT cube found 20 H I structures across the wider environment of NGC 4936, with masses ranging from ∼ 4 × 10 7 to 6 × 10 8 M⊙ The extended, asymmetric, and partially optically dark H I distribution suggests that at least some of the atomic gas has been redistributed by tidal interactions and/or stripping processes

Implications for Cooling Thresholds in Galaxy Groups

NGC 4936 demonstrates that substantial cold gas reservoirs can exist in galaxy groups with relatively high tcool/tff ratios, suggesting that the present thermodynamic state does not reflect the conditions under which the cold gas originally formed The combination of a cool X-ray core, a dynamically active environment, and a substantial multiphase gas reservoir in NGC 4936 may represent a mode of cold gas accumulation that is more common in groups but has been under-characterised The results suggest that the present-day multiphase medium in NGC 4936 reflects the combined effects of atmospheric cooling and galaxy interactions operating over different evolutionary stages

Summary

The available shallow X-ray data does not show any evidence of AGN activity in the form of central point source or cavities and shocks, but the thermodynamic profiles indicate a weak cool-core with central tcool ∼ 25 The study detected a molecular gas reservoir of MH2 = (1.93 ± 0.05) × 10 8 M⊙ distributed in a rotating disk of size ∼ 1 kpc, with asymmetries and extended filamentary structure The ionised gas traced by Hα extends to ∼ 16 kpc and shows complex kinematics broadly consistent with rotation observed in the molecular gas in the inner ∼ 1 kpc regions but with complex filamentary emission at larger radii The central cold gas reservoir is strongly dominated by molecular gas, as a strong 3σ upper limit of MHI < 1.13 × 10 7 M⊙ was found in the centre of NGC 4936 The companion galaxy may be undergoing a minor merger with a velocity offset of ∼360 km s−1 and projected separation of 2.5 kpc, but shows no clear evidence of hosting a large cold gas reservoir that could explain the molecular gas in NGC 4936 via direct stripping The results suggest that the present-day multiphase medium in NGC 4936 reflects the combined effects of atmospheric cooling and galaxy interactions operating over different evolutionary stages<ref:2610.

Improvements for AI systems

textbfWeighting of Thermodynamic History Models by Environmental Contexts (NGC 4936 as Case Study): This allows AI to move beyond universal condensation thresholds by learning that "the observed molecular gas may reflect the cumulative thermodynamic history of the system, including atmospheric condensation during an earlier epoch, locally enhanced cooling driven by environmental perturbations, external acquisition during past interactions or additional effects such as angular momentum in the hot atmosphere that modify the conditions for condensation" (Section 4.1). The AI can then predict which of these historical mechanisms is most likely dominant based on observable kinematic and morphological features.
textbfMulti-Phase Gas Phase Coupling Prediction: The system's ability to trace a common origin and reservoir of cold gas between ionised and molecular gas suggests an improved model for predicting the relationship between phases. The AI can be trained to correlate specific velocity structures, such as the extended filamentary emission with complex kinematics broadly consistent with rotation observed in the molecular gas, thereby identifying regions where atmospheric cooling is actively driving both phases simultaneously.
textbfPredictive Modeling of H I Reservoir Origin: The AI can differentiate between internal and external gas sources by analyzing the properties of detected H I structures. Specifically, it can be trained to distinguish pre-existing gas-rich dwarf galaxies or gas removed from interacting galaxies (Section 4.3) from stripping or tidal origin based on whether the H I cloud exhibits a specific velocity dispersion profile compared to the main galaxy's rotation.
textbfModel Calibration for Cooling Time Discrepancies: The AI can be calibrated using the discrepancies found in prior studies, such as the finding that our use of a common cooling function ensures consistency between the recalculated profiles, but does not remove this model dependence (Appendix A). This allows the AI to automatically apply necessary systematic corrections when comparing published results from different analyses (B18a vs. L18 vs. D26) based on metallicity and abundance table assumptions.
textbfCharacterization of Interaction Signatures via Dust Morphology: The system can be used to train a model that identifies a diffuse low surface brightness plume extending to the N–NW (Appendix D) as evidence for tidal interaction, rather than a simple subtraction artifact. This enables the AI to flag potential merger or stripping events in other galaxy groups by searching for these specific, robust morphological features in multiwavelength residual maps.

Abstract

We present a multiwavelength analysis of NGC 4936, the central galaxy of a weak cool-core group hosting an unusually large cold/warm gas reservoir. NGC 4936 contains about2 times10 8 M of molecular gas in a rotating disk of radius about1 kpc, but no detected H I, with a 3σ upper limit of 1.13 times10 7 M within about2.5 kpc. The resulting H 2 /H I mass ratio >17 is consistent with a predominantly molecular medium in the high-pressure environments of central galaxies. SOAR imaging and MUSE spectroscopy reveal ionised gas extending to about15 kpc in a complex filamentary morphology that broadly follows the molecular gas kinematics while extending well beyond the CO disk. Dust filaments similarly extend to about14 kpc and appear to interact with a companion galaxy located only about2.5 kpc away in projection. NGC 4936 has a star formation rate of 0.29 M yr-1, and its AGN shows sub-mm and X-ray variability. We favour atmospheric condensation as the primary origin of the molecular gas, although externally acquired gas may contribute. Our recalculated thermodynamic profiles reach t cool/t ff about25 in the innermost region, placing the system near the threshold for multiphase condensation. The ongoing interaction may perturb the atmosphere and promote condensation while shaping the morphology and kinematics of the multiphase medium, rather than directly supplying most of the central cold gas. We additionally detect 20 H I structures in the surrounding group environment, with masses of about10 7 -- 10 8 M, including several with no obvious optical counterparts.