Ubiquitous cold and massive filaments in cool core clusters

¹ LERMA, Observatoire de Paris, PSL Research Univ., CNRS, Sorbonne Univ., 75014 Paris, France
e-mail: valeria.olivares@obspm.fr
² Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB1 0HA, UK
⁴ Sorbonne Université, CNRS, UMR7095, Institut d’Astrophysique de Paris, 98 bis Bd Arago, 75014 Paris, France
⁵ Institut Universitaire de France, Ministère de l’Education Nationale, de l’Enseignement Supérieur et de la Recherche, 1 Rue Descartes, 75231 Paris Cedex 05, France
⁶ Ecole Normale Supérieure, PSL Research Univ., CNRS, 75005 Paris, France
⁷ Physics & Astronomy Department, Michigan State University, East Lansing, MI 48824-2320, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁹ Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), Stanford University, 452 Lomita Mall, Stanford, CA 94305-4085, USA
¹⁰ University of Manitoba, Department of Physics and Astronomy, Winnipeg, MB R3T 2N2, Canada
¹¹ Centre for Extragalactic Astronomy, Durham University, DH1 3LE Durham, UK
¹² Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506-0055, USA
¹³ Université Cote d’Azur, Observatoire de la Cote d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France

Received: 23 February 2019
Accepted: 22 July 2019

Abstract

Multi-phase filamentary structures around brightest cluster galaxies (BCG) are likely a key step of AGN-feedback. We observed molecular gas in three cool cluster cores, namely Centaurus, Abell S1101, and RXJ1539.5, and gathered ALMA (Atacama Large Millimeter/submillimeter Array) and MUSE (Multi Unit Spectroscopic Explorer) data for 12 other clusters. Those observations show clumpy, massive, and long (3−25 kpc) molecular filaments, preferentially located around the radio bubbles inflated by the AGN. Two objects show nuclear molecular disks. The optical nebula is certainly tracing the warm envelopes of cold molecular filaments. Surprisingly, the radial profile of the Hα/CO flux ratio is roughly constant for most of the objects, suggesting that (i) between 1.2 and 6 times more cold gas could be present and (ii) local processes must be responsible for the excitation. Projected velocities are between 100 and 400 km s⁻¹, with disturbed kinematics and sometimes coherent gradients. This is likely due to the mixing in projection of several thin (and as yet) unresolved filaments. The velocity fields may be stirred by turbulence induced by bubbles, jets, or merger-induced sloshing. Velocity and dispersions are low, below the escape velocity. Cold clouds should eventually fall back and fuel the AGN. We compare the radial extent of the filaments, r_fil, with the region where the X-ray gas can become thermally unstable. The filaments are always inside the low-entropy and short-cooling-time region, where t_cool/t_ff <  20 (9 of 13 sources). The range of t_cool/t_ff of 8−23 at r_fil, is likely due to (i) a more complex gravitational potential affecting the free-fall time t_ff (sloshing, mergers, etc.) and (ii) the presence of inhomogeneities or uplifted gas in the ICM, affecting the cooling time t_cool. For some of the sources, r_fil lies where the ratio of the cooling time to the eddy-turnover time, t_cool/t_eddy, is approximately unity.