Absorbed Cooling Flows in Galaxies & Clusters

Updated 22 August 2025

Absorbed cooling flows are radiatively driven inflows in high-density regions where intrinsic photoelectric absorption obscures soft X-ray emissions.
They lead to revised mass cooling rates and energy budget estimates, significantly impacting feedback processes in clusters, groups, and galaxies.
Their multiphase structure—encompassing hot, warm, and cold gas with dust—challenges classical models and informs studies of AGN feedback and galaxy evolution.

Absorbed cooling flows describe a class of radiatively driven inflows in galaxies, groups, and clusters in which part or most of the expected cooling signature—especially in soft X-rays—is "hidden" by intrinsic photoelectric absorption from cold gas embedded within the cooling sites. The cooling processes operate in high-density, low-entropy regions of hot atmospheres where the cooling time falls below ~100 Myr, but AGN feedback, multiphase structure, and the presence of dusty or molecular absorbers result in a complex observed and physical state. Absorbed ("hidden") cooling flows have been identified in clusters, groups, elliptical galaxies, and, more recently, in nearby spirals, yielding revised estimates of mass cooling rates, energetic balances, and significant implications for feedback processes, star formation, and galaxy evolution.

1. Spectroscopic Evidence and Intrinsic Absorption

High–resolution X-ray spectroscopy, especially using XMM–Newton’s Reflection Grating Spectrometer (RGS), has revealed absorbed cooling flows in a variety of systems, with emission modified by cold, intrinsic absorbers. The observed spectra require the inclusion of a multilayer or distributed absorption term:

Intrinsic absorption model (IAM): Soft X-ray emission from cooling flow components (e.g., modeled via mkcflow in XSPEC, with T_min ~ 0.1 keV) is subject to internal photoelectric absorption, typically with columns N_H' ~ 10²² cm^–2. Each layer of emitting gas is overlaid by absorbing material, leading to a transmission factor for n such layers:

$F_t = (F_e (1 – e^{-σ N_H})) / (σ N_H)$

where $F_e$ is the emitted flux, $σ$ is the cross-section, and $N_H$ is the total column (Ivey et al., 6 Nov 2024, Fabian et al., 20 Aug 2025).

Spectral signatures: The presence of absorbed cooling flows is inferred from the suppression of soft X-ray lines (e.g., OVII, Fe XVII), a steep drop-off in the detectable emission below 1 keV, and a requirement for intrinsic absorption to fit observed spectra in clusters, groups, and ellipticals.

This intrinsic absorption is interpreted as due to cold gas and dust intermixed with the cooling hot phase, resulting in a large fraction of the cooling flow escaping detection at X-ray wavelengths.

2. Mass Cooling Rate Estimates and Energy Budget

Absorbed cooling flows yield revised estimates of mass cooling (or deposition) rates:

Clusters: Imaging-based (unabsorbed) analyses often infer mass cooling rates $\lesssim 20-100$ M $_\odot$ yr $^{-1}$ in typical cool-core clusters. Spectral fits including intrinsic absorption reveal that "hidden" components can be much larger, often by factors of several, with some massive systems (e.g., Centaurus, RX J1504.1–0248) exceeding 1000 M $_\odot$ yr $^{-1}$ in the core (Russell et al., 5 Jan 2024, Fabian et al., 14 Oct 2024).
Groups/Ellipticals: Recent surveys report hidden cooling rates in the range 0.5–8 M $_\odot$ yr $^{-1}$ per galaxy (Ivey et al., 6 Nov 2024, Fabian et al., 20 Aug 2025), often an order of magnitude higher than classical unabsorbed rates.
Spirals: Absorbed soft X-ray spectra in the Sombrero, Whirlpool, and Sculptor galaxies indicate circumgalactic cooling flows of 0.3–1.1 M $_\odot$ yr $^{-1}$ (Fabian et al., 20 Aug 2025).

The energy absorbed in the X-ray regime is energetically sufficient to explain the observed far-infrared (FIR) luminosities in these systems, implying reradiation of the "hidden" power at longer wavelengths.

3. Physical Mechanisms and Thermodynamic Structure

Absorbed cooling flows are characterized by several physical mechanisms:

Multiphase structure: Gas cools radiatively from keV temperatures through intermediate "warm" and finally cold molecular/dusty phases, populating a multiphase medium. The intrinsic absorbers—cold clouds—are co-spatial with cooling gas and both absorb and eventually reradiate the lost energy (Ivey et al., 6 Nov 2024, Fabian et al., 20 Aug 2025).
Hydrodynamic flows: The underlying hydrodynamics often reconcile with the classical cooling flow equations:

$\dot{M} = 4\pi r^2 \rho v, \qquad v \frac{d\ln K}{dr} = -\frac{1}{t_{\text{cool}}}$

where $K$ is entropy (Sultan et al., 21 Oct 2024).

Density profiles: Cores exhibit steep density gradients as cooling proceeds (e.g., $n_e(r) \sim r^{-3.04}$ in H1821+643 (Russell et al., 5 Jan 2024)), consistent with hydrostatic compression as the gas loses pressure support.
AGN Feedback Suppression: While AGN outflows (shocks, bubbles, turbulence) dominate heating at cluster scales and outer regions, they are less effective at heating the densest inner, cooling gas in groups and ellipticals, so that absorbed cooling persists even in the presence of strong jet activity (Fabian et al., 14 Oct 2024, Fabian et al., 20 Aug 2025).

The dominant absorption and "hiding" of the soft X-ray signal results from the spatial interleaving of cooling and cold phases, not from simply a global decrement in net cooling.

4. Observational Diagnostics and Multiwavelength Constraints

Absorbed cooling flows are probed through a combination of X-ray, infrared, and, in some cases, millimeter studies:

X-ray spectroscopy: Intrinsic absorption is required by high N_H values and covers the full line emission complex below 1 keV. Imaging may underestimate the true cooling rate unless hidden components are modeled (Fabian et al., 14 Oct 2024, Ivey et al., 6 Nov 2024).
Infrared signatures: Absorbed X-ray emission reemerges at FIR wavelengths, with measured luminosities in agreement with those required by the energy deficit. Diagnostic mid-IR lines, such as [NeVI] 7.65 μm, are predicted to trace cooling gas between 1.5–6 × 10⁵ K and are targets for JWST and ALMA (Fabian et al., 14 Oct 2024).
Molecular gas: The presence of cold clouds is independently supported by molecular line (CO) emission and multiwavelength dust features observed in core galaxies (e.g., in H1821+643 (Russell et al., 5 Jan 2024)).
Star formation: Despite large cooling rates, star formation rates inferred from UV/optical/IR photometry are typically much lower, suggesting a significant proportion of cooled material forms very low-mass stars or accumulates in molecular gas (Ivey et al., 6 Nov 2024).

A comparison of cooling rates from RGS fits, imaging, and FIR emission generally supports the absorbed cooling flow framework.

5. The Role and Limits of AGN Feedback

AGN feedback operates by injecting mechanical energy (jets, bubbles) or, less efficiently, radiation into the ICM/ISM to arrest cooling. However:

In rich clusters, AGN feedback reduces total cooling rates, primarily in the outer regions, so that only a fraction ( $\gtrsim$ 50%, but rarely $>$ 90%) of the mass dropout is suppressed (Fabian et al., 14 Oct 2024).
For groups, ellipticals, and cluster cores, central cooling rates remain high ("cooling dominates") with AGN power distributed mostly outside the central dense region (Fabian et al., 20 Aug 2025).
AGN feedback is ineffective at fully quenching inner cooling even in the presence of strong radio jets or X-ray cavities; thus, absorbed (hidden) cooling flows persist.

These findings highlight the spatially inhomogeneous coupling of AGN feedback and the importance of considering both resolved and absorbed cooling components in mass and energy budget studies.

6. Broader Implications for Galaxy and CGM Evolution

The hidden cooling flow phenomenon has wide ramifications:

Galaxy groups and ellipticals: High-pressure, high-density cores with cooling times $<$ 100 Myr foster environments where the cooling gas is converted, not into normal stellar populations, but perhaps predominantly into low-mass stars or accumulates as molecular gas (Ivey et al., 6 Nov 2024).
Spiral galaxies: Similar absorbed cooling processes may regulate the circumgalactic medium (CGM), with the geometry likely more extended and possibly strongly influenced by disk rotation (Fabian et al., 20 Aug 2025).
Star formation and feedback regulation: The apparent suppression of "classical" star formation relative to cooling rates suggests a shift to a bottom-heavy IMF mode or that much of the cooled gas is otherwise "dark" (e.g., locked in brown dwarfs).
Consistency with theoretical models: Absorbed cooling flows establish an observational baseline for CGM and cluster evolution, with implications for entropy profiles and thermodynamic state in both clusters and halos (Sultan et al., 21 Oct 2024).

Absorbed cooling flows thus represent a robust, multiphase inflow channel that is critical to the mass, energy, and evolutionary cycles of galaxies and galaxy systems.

7. Future Directions and Observational Prospects

Several avenues are prioritized for ongoing and future research:

High-resolution X-ray spectroscopy (e.g., XRISM, Athena) will provide deeper insights into temperature distributions, ionization structure, and the spatial extent of intrinsic absorption.
Mid-IR and mm interferometry (JWST, ALMA) targeting cooling flow diagnostics (e.g., NeVI, molecular lines) will test whether the gas cools smoothly to low temperatures and track re-emission of absorbed X-ray power.
Simulations including radiative cooling, cold gas absorption, AGN feedback, and resolved multiphase structure are essential to quantify the mass flow, star formation, and feedback coupling in various galaxy morphologies.
Comparison between system types (elliptical versus spiral CGM) may elucidate how absorbed cooling flows shape mass assembly, angular momentum transport, and the baryon budget.

Understanding absorbed cooling flows is thus required for a comprehensive physical model of feedback-regulated galaxy and cluster evolution, the fate of cooling baryons, and the interpretation of observed star formation suppression in massive systems.