Abstract
Abstract Cluster cool cores possess networks of line-emitting filaments. These filaments are thought to originate via uplift of cold gas from cluster centers by buoyant active galactic nuclei (AGNs) bubbles, or via local thermal instability in the hot intracluster medium (ICM). Therefore, the filaments are either the signatures of AGN feedback or feeding of supermassive black holes. Despite being characterized by very short cooling times, the filaments are significant Hα emitters, which suggests that some process continuously powers these structures. Many cool cores host diffuse radio mini halos and AGN injecting radio plasma, suggesting that cosmic rays (CRs) and magnetic fields are present in the ICM. We argue that the excitation of Alfvén waves by CR streaming, and the replenishment of CR energy via accretion onto the filaments of high-plasma-β ICM characterized by low CR pressure support, can provide the adequate amount of heating to power and sustain the emission from these filaments. This mechanism does not require the CRs to penetrate the filaments, even if the filaments are magnetically isolated from the ambient ICM, and it may operate irrespectively of whether the filaments are dredged up from the center or form in situ in the ICM. This picture is qualitatively consistent with non-thermal line ratios seen in the cold filaments. Future X-ray observations of the iron line complex with XARM, Lynx, or Athena could help to test this model by providing constraints on the amount of CRs in the hot plasma that is cooling and accreting onto the filaments.
Highlights
Hα filaments are ubiquitous in cool core galaxy clusters and giant elliptical galaxies (e.g., McDonald et al (2010); Werner et al (2014))
Cluster cool cores possess networks of line-emitting filaments. These filaments are thought to originate via uplift of cold gas from cluster centers by buoyant active galactic nuclei (AGN) bubbles, or via local thermal instability in the hot intracluster medium (ICM)
We presented a model for powering of Hα filaments by cosmic rays (CRs)
Summary
Hα filaments are ubiquitous in cool core galaxy clusters and giant elliptical galaxies (e.g., McDonald et al (2010); Werner et al (2014)) These filaments are known to contain multiphase gas emitting CO lines (e.g., Salome et al (2006)), far-infrared lines (Werner et al 2013), near-infrared emission lines (e.g., Donahue et al (2000)), optical lines (e.g., Canning et al (2011)), and soft X-rays (Fabian et al 2003). Dunn & Fabian (2004) suggest that the magnetic pressure and the pressure of the radio-emitting CR electrons inside the bubbles is small compared to the ambient ICM pressure This suggests that the bubbles are filled with either non-radiating CR particles or ultra-hot thermal gas.
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