Active galactic nuclei ( AGN ) release a huge amount of energy into the intracluster medium ( ICM ) with the consequence of offsetting cooling and star formation ( AGN feedback ) in the centers of cool core clusters .
The Phoenix cluster is among the most massive clusters of galaxies known in the Universe .
It hosts a powerful starburst of several hundreds of Solar masses per year and a large amount of molecular gas in the center .
In this work we use the high-resolution Reflection Grating Spectrometer ( RGS ) on board XMM- Newton to study the X-ray emitting cool gas in the Phoenix cluster and heating-cooling balance .
We detect for the first time evidence of O VIII and Fe XXI-XXII emission lines , the latter demonstrating the presence of gas below 2 keV .
We find a cooling rate of 350 \pm _ { 200 } ^ { 250 } M _ { \odot } { yr } ^ { -1 } below 2 keV ( at the 90 % confidence level ) , which is consistent with the star formation rate in this object .
This cooling rate is high enough to produce the molecular gas found in the filaments via instabilities during the buoyant rising time .
The line broadening indicates that the turbulence ( \sim 300 km s ^ { -1 } or less ) is below the level required to produce and propagate the heat throughout the cool core .
This provides a natural explanation for the coexistence of large amounts of cool gas , star formation and a powerful AGN in the core .
The AGN activity may be either at a young stage or in a different feedback mode , due to a high accretion rate .