Hidden Cooling Flows in Galaxy Cores

• Hidden cooling flows are cooling processes in galaxy cores where intrinsic absorption by cold gas conceals expected soft X-ray emissions, leading to underestimated cooling rates.
• Detection relies on high-resolution X-ray spectroscopy and multi-layer absorption modeling, with suppressed Fe L-shell and O VII lines and absorption columns of ~10²¹–10²² cm⁻² as key diagnostics.
• Implications include revisions of mass budgets in galaxies, suggesting that hidden cooled gas may drive low-mass star formation and fuel black hole growth despite AGN feedback.

Hidden cooling flows are cooling flows in the centers of galaxy clusters, groups, ellipticals, and even spiral galaxies whose presence is concealed by local intrinsic absorption from cold gas commingled with the cooling hot phase. Rather than being directly observable through standard soft X-ray emission lines, much of the radiative signature from gas cooling below ∼1 keV is absorbed and can only be detected via detailed spectroscopic modeling—including an intrinsic multilayer absorption formalism—and through reprocessed emission at longer wavelengths such as the far infrared. Hidden cooling flows carry implications for the mass budget, energy cycle, fate of cooled gas, and the effectiveness of AGN feedback in massive halos.

1. Definition, Phenomenology, and Prevalence

Hidden cooling flows (HCFs) are cooling flows in which the expected soft X-ray emission from gas cooling to sub-keV temperatures is largely absorbed by cold, dusty gas internal to the flow. Standard spectral models, even when accounting for foreground Galactic absorption, greatly underestimate the true cooling rate unless the intrinsic absorption is properly modeled (Fabian et al., 20 Aug 2025, Ivey et al., 6 Nov 2024, Fabian et al., 2023, Fabian et al., 14 Oct 2024, Fabian et al., 2022, Fabian et al., 2022). The absorbed X-ray luminosity emerges energetically in the far-infrared, as verified by the agreement between hidden cooling rates inferred from X-ray spectra and FIR luminosities in Centaurus and elliptical galaxies (Ivey et al., 6 Nov 2024, Fabian et al., 2022).

HCFs have now been detected or statistically allowed in every cool core system subjected to high-resolution XMM-Newton RGS spectroscopy, including clusters (Centaurus, Perseus, A1835, SPT-CLJ2344-4243/Phoenix), groups (e.g., NGC5044, several radio galaxy-hosting systems), ellipticals (M49, M84, NGC 1316, NGC 1332, NGC 1404), and even prominent nearby spiral galaxies (Sombrero, M51, NGC 253) (Fabian et al., 20 Aug 2025, Ivey et al., 6 Nov 2024, Fabian et al., 2023). The hidden mass cooling rates range from 0.3–1.1 M⊙ yr⁻¹ in spirals, up to 1–8 M⊙ yr⁻¹ in ellipticals, 2–40 M⊙ yr⁻¹ in groups and clusters, and exceeding 1000 M⊙ yr⁻¹ in the most extreme clusters (Phoenix, MACS1931) (Fabian et al., 2022).

2. Detection and Modeling Methodology

High-resolution X-ray spectroscopy with the XMM-Newton RGS is central to detecting HCFs. The key methodology involves:

• Fitting the overall spectrum with a combination of a hot (APEC) component for the ambient plasma and a cooling flow model (e.g., mkcflow) for the gas cooling to low temperatures.
• Modifying the mkcflow emission with a multi-layer photoelectric absorption model, typically realized as:

$$F_{\text{transmitted}} = \frac{F_e \left(1 - e^{-\sigma N_{\rm H}}\right)}{\sigma N_{\rm H}}$$

where $F_e$ is the emitted flux, $\sigma$ is the energy-dependent cross section, $N_{\rm H}$ is the intrinsic absorbing column. This formalism is implemented in XSPEC as mlayer or via custom partial covering models.

• Quantifying both an absorbed (hidden) and an unabsorbed (visible) mass cooling rate, representing the division between emission that is reprocessed by cold clouds and that which escapes the system (Fabian et al., 2022, Ivey et al., 6 Nov 2024).

Diagnostic features are primarily the suppression of Fe L-shell and O VII lines (or their absence), and the requirement for significant "intrinsic" absorbing columns ($N_{\rm H} \sim 10^{21}$–$10^{22}$ cm⁻²) with near-unity covering fractions to reconcile the observed and model spectra.

3. Multiwavelength Energy Budget and Reprocessing

Energetically, the soft X-ray luminosity absorbed by cold, dusty gas must be re-emitted at longer wavelengths. In all systems analyzed, the absorbed cooling flow luminosity calculated over soft X-ray bands (e.g., 0.1–3 keV) is found to be in agreement—within a factor of a few—with the observed FIR luminosity from IR observations (Herschel, Spitzer), thus preserving energy conservation (Fabian et al., 2022, Ivey et al., 6 Nov 2024). In practice, this means that:

• The majority of soft X-ray energy from cooling gas is reradiated as thermal emission by cold dust grains.
• The total mass of cold material implied by cumulative hidden cooling over Gyr timescales substantially exceeds that seen in molecular line or CO surveys, suggesting either further cooling to undetectably low temperatures, fragmentation into low-mass stars or brown dwarfs, or dispersal by dynamical processes (Fabian et al., 14 Oct 2024, Fabian et al., 2023, Ivey et al., 6 Nov 2024).

4. Physical Origin: Microphysics and Environmental Factors

The physical origin of hidden cooling flows is rooted in the spatial coexistence of cold molecular/dusty clouds with the cooling hot phase:

• The intrinsic absorption arises from dense, cold clouds embedded (or formed) within the cooling radius. These clouds are sufficiently opaque to soft X-rays due to the energy dependence of the photoelectric cross-section.
• The coupling between the multi-phase ICM/CGM and the embedded cold phase is reinforced by spatial alignment of cold gas (traced by CO, Hα, [C II], and dust emission) with the regions of lowest entropy hot gas and sites of young star formation (McDonald et al., 2014, Ivey et al., 6 Nov 2024).
• In clusters and groups, AGN feedback reduces overall cooling rates by offsetting heating, but is largely ineffective at fully quenching central cooling or removing embedded cold material. Feedback typically deposits its energy in the outer core, leaving the inner regions susceptible to ongoing cooling (Fabian et al., 20 Aug 2025, Fabian et al., 14 Oct 2024). In ellipticals and groups with mass cooling rates below ~15 M⊙ yr⁻¹, AGN feedback is insufficient to suppress central cooling entirely.

5. Implications for Galaxy and Black Hole Evolution

HCFs fundamentally alter the baryon cycle in massive galaxies and their central black holes:

• The hidden cooling mass flow rates are significantly higher than measured star formation rates (e.g., by factors of ∼20 in elliptical galaxies). This suggests the cooled gas does not form massive stars with a normal IMF, but rather fragments into low-mass stars, substellar objects, or possibly forms an undetected cold baryonic reservoir (Ivey et al., 6 Nov 2024, Fabian et al., 14 Oct 2024).
• Theoretical analysis indicates that in high-pressure central regions, the Jeans mass drops below $0.1\,M_{\odot}$, favoring the formation of low-mass stars and a bottom-heavy IMF.
• Some fraction of cooled, low-mass objects or very cold gas may eventually accrete onto central supermassive black holes without luminous AGN signatures, providing a “hidden” accretion channel (Fabian et al., 2023).
• In massive clusters with extreme HCFs (e.g., Phoenix), short-lived episodes of rapid cooling—directly imaged via [Ne VI] 7.65 μm with JWST—can drive brief, intense bursts of star formation, linking cooling and black hole feedback in an episodic cycle (Reefe et al., 12 Feb 2025).

6. Universality and Context Across Environments

HCFs are not limited to clusters but are now demonstrated across the mass spectrum:

Environment	Typical HCF Ṁ [M⊙ yr⁻¹]	Core N_H [10²² cm⁻²]	AGN Feedback Efficacy
Spiral galaxies	0.3–1.1	≲1	Ineffective (inner kpc)
Elliptical galaxies	0.5–8	0.5–2	Ineffective (inner kpc)
Galaxy groups	2–20	1–2	Modest, outer core
Regular clusters	10–100	1–5	Outer core, mostly
Extreme clusters	1000+	2–6	Outer core, mostly

HCFs are detected even in spiral galaxies, where the circumgalactic medium reveals soft X-ray spectra consistent with low-level cooling flow activity modulated by intrinsic absorption. This finding suggests that radiative cooling and the associated hiding of gas are likely universal processes within all dense galactic environments (Fabian et al., 20 Aug 2025).

7. Future Prospects and Observational Tests

Quantitative predictions for JWST and other next-generation facilities forecast decisive tests of the HCF paradigm:

• Mid-IR forbidden lines, especially [Ne VI] 7.65 μm (tracing gas at $T\simeq10^{5.5}$ K), are proposed as robust diagnostics of large-scale cooling, being absent of depletion effects and directly scaling with the true cooling flow rate (Fabian et al., 14 Oct 2024, Reefe et al., 12 Feb 2025).
• JWST, with high spatial and spectral resolution, is capable of mapping extended [Ne VI] to directly image the cooling flow, as realized in the Phoenix cluster, where the [Ne VI] emission is cospatial with X-ray entropy minima, cold gas, and star formation (Reefe et al., 12 Feb 2025).
• Forthcoming X-ray spectrometers (XRISM Resolve, Athena X-IFU) will resolve temperature-resolved cooling rates and map the velocity structure of the hidden flow, placing further constraints on mass deposition and kinematics (Fabian et al., 2022).

References to Key Equations, Concepts, and Metrics

• Intrinsic absorption model (multilayer): $F_t = \frac{F_e (1 - e^{-\sigma N_H})}{\sigma N_H}$
• Classical cooling rate: $\dot{M}_{\rm cool} = \frac{2 L\,\mu m_p}{5 k T}$
• Energy conservation: Absorbed X-ray luminosity ≈ FIR luminosity
• Mass cooling rate discrepancies: HCF Ṁ ≫ “visible” SFR
• Jeans mass (in high P) favors bottom-heavy IMF

Conclusion

Hidden cooling flows have revised the canonical understanding of gas cooling in cluster and group cores, elliptical and spiral galaxies. By directly modeling intrinsic absorption, multi-phase structure, and energy reprocessing, they unify several lines of observational evidence—high FIR/X-ray ratios, disparate star formation and cooling rates, and the existence of undetected cold baryonic reservoirs—while reasserting the limited efficacy of AGN feedback in the innermost cooling zones. The next decade will provide definitive empirical tests of mass deposition rates, fate of cooled material, and the dynamic interplay between radiative cooling, star formation, and black hole growth across the galaxy mass spectrum.