H$α$-X-ray Surface Brightness Correlation for Filaments in Cooling Flow Clusters (2501.01902v2)

Abstract: Massive galaxies in cooling flow clusters display clear evidence of feedback from Active Galactic Nuclei (AGN). Joint X-ray and radio observations have shown that AGN radio jets push aside the surrounding hot gas and form cavities in the hot intracluster medium (ICM). These systems host complex, kiloparsec-scale, multiphase filamentary structures, from warm ionized (10,000 K) to cold molecular ($<$100 K). These striking clumpy filaments are believed to be a natural outcome of thermally unstable cooling from the hot ICM, likely triggered by feedback processes while contributing to feeding the AGN via Chaotic Cold Accretion (CCA). However, the detailed constraints on the formation mechanism of the filaments are still uncertain, and the connection between the different gas phases has to be fully unveiled. By leveraging a sample of seven X-ray bright cooling-flow clusters, we have discovered a tight positive correlation between the X-ray surface brightness and the H$\alpha$ surface brightness of the filaments over two orders of magnitude, as also found in stripped tails.

• The paper quantifies a nearly constant Hα/X-ray surface brightness ratio of 4.1, linking the hot intracluster medium with warm filamentary structures.
• It employs novel imaging techniques like GMCA and pGMCA applied to deep Chandra observations of seven X-ray bright cooling flow clusters.
• The findings support AGN feedback models by demonstrating how chaotic cold accretion drives multiphase gas dynamics in cluster filaments.

H$\alpha$-X-ray Surface Brightness Correlation for Filaments in Cooling Flow Clusters

Cooling flow clusters provide a critical environment for understanding the interplay between Active Galactic Nuclei (AGN) feedback and intracluster medium (ICM) dynamics. The paper on H$\alpha$-X-ray surface brightness correlation revisits the complexity of filamentary structures in cooling flow clusters. These structures span multiphase layers from warm ionized to cold molecular states, often associated with AGN activity.

The paper concentrates on a sample of seven X-ray bright cooling-flow clusters, where significant findings include a strong positive correlation between X-ray and H$\alpha$ surface brightness across two orders of magnitude. This correlation emphasizes the intricate relationship between the hot ICM and warm filamentary structures, suggesting a substantial contribution of chaotic cold accretion (CCA) triggered by AGN feedback. Notably, previous observations recorded similar spatial correlations, but this work marks the first attempt at quantifying the X-ray to H$\alpha$ surface brightness ratio within the cluster filaments.

The authors employ novel imaging techniques using the General Morphological Component Analysis (GMCA) and its Poissonian variant (pGMCA) to isolate X-ray filamentary structures from the underlying X-ray halo. These imaging analyses are vital for yielding cleaner observations of the filament structures against the bright X-ray core backgrounds. The analysis further leverages deep Chandra observations, ensuring detailed spectral and structural imaging of clusters such as Perseus, M87, Centaurus, Hydra-A, Abell 1795, Abell 2597, and PKS 0745-191.

In terms of numerical results, the research identifies a nearly constant X-ray/H$\alpha$ surface brightness ratio of 4.1 with a scatter of approximately 10-30%, signaling a local process contributing to gas excitation. This correlation aligns with recent findings in stripped tails of jellyfish galaxies indicating potentially similar underlying physical processes like energetic particle interactions, turbulent mixing layers, or EUV/X-ray reprocessing.

The paper further assesses the electron pressure equilibrium of the filament phases, concluding that X-ray and H$\alpha$ filaments are not in pressure equilibrium. The pressure differences between X-ray halos and filaments might involve additional non-thermal pressure components, likely due to magnetic fields or turbulent processes. The presence of strong magnetic fields within filament structures is consistent with prior high-resolution observations and theoretical models pointing towards magnetic support against gravitational collapse.

This research supports the theoretical model predictions of CCA and precipitation, where thermodynamic instabilities induce condensation of X-ray overdensities, tightly linking hot and cool gas phases. These findings have significant implications for AGN feeding mechanisms and feedback cycles, as well as for the understanding of filament properties and evolution in galaxy clusters.

Future directions suggested include the in-depth correlation analysis between cold molecular (CO) phase filaments and ongoing ALMA observations for broader comprehension of multiphase gas dynamics. Understanding these correlations could reveal further insights into the influence of AGN on the ICM and galaxy evolution, providing new avenues for exploring thermal instabilities and their role in AGN feedback loops.

This paper rigorously defines a cornerstone in unraveling the complexities of multiphase filaments in cooling flow clusters, providing clarity on AGN feedback and its relationship with the intercluster environment. It also opens prospects for future studies to explore additional parameters and refine theoretical models of multiphase cosmic structures.