Letter to the Editor

Formation of cold filaments in cooling flow clusters^*

¹ LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: yves.revaz@obspm.fr
² Institut of Radio Astronomy (IRAM), Domaine Universitaire, 300 rue de la piscine, 38400 Saint-Martin d'Hères, France

Received: 24 October 2007
Accepted: 20 November 2007

Abstract

Emission-lines in the form of filamentary structures is common in bright clusters characterized by short cooling times. In the Perseus cluster, cold molecular gas, tightly linked to the Hα filaments, has been recently revealed by CO observations. In order to understand the origin of these filamentary structures, we have investigated the evolution of the hot ICM gas perturbed by the AGN central activity in a Perseus like cluster. Using very-high resolution TreeSPH simulations combined with a multiphase model and a model of plasma bubbles, we have been able to follow the density and temperature evolution of the disturbed ICM gas around the bubbles. Our simulations show that a fraction of the gas present at the center of clusters is trapped and entrained by the rising buoyant bubble to higher radius where the AGN heating is less efficient. The radiative cooling makes it cool in a few tens of Myr below , forming cold filamentary structures in the wake and in the rim of the bubbles. The predicted cold gas formed outside the cluster center is in agreement with the total CO mass and density profile of the observed molecular gas as well as with the kinematics of the Hα filaments. This scenario explains the Hα and CO filaments observed in luminous clusters without contradicting the observed lack of gas. It also emphasizes that if the AGN feedback provides some heating (negative feedback) it also perturbs the ICM, increasing its cooling (positive feedback).