Excitation mechanisms in the intracluster filaments surrounding brightest cluster galaxies

¹ LERMA, Observatoire de Paris, PSL research Université, CNRS, Sorbonne Université, 75104 Paris, France
² SOFIA Science Center, USRA, NASA Ames Research Center, M.S. N232-12, Moffett Field, CA 94035, USA
e-mail: fpolles@usra.edu
³ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France
⁴ 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
⁵ École normale supérieure, Université PSL, Sorbonne Université, CNRS, LERMA, 75005 Paris, France
⁶ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁷ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305-4085, USA
⁸ Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France
⁹ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB1 0HA, UK
¹¹ Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA

Received: 20 October 2020
Accepted: 27 February 2021

Abstract

Context. The excitation of the filamentary gas structures surrounding giant elliptical galaxies at the center of cool-core clusters, also known as brightest cluster galaxies (BCGs), is key to our understanding of active galactic nucleus (AGN) feedback, and of the impact of environmental and local effects on star formation.

Aims. We investigate the contribution of thermal radiation from the cooling flow surrounding BCGs to the excitation of the filaments. We explore the effects of small levels of extra heating (turbulence), and of metallicity, on the optical and infrared lines.

Methods. Using the CLOUDY code, we modeled the photoionization and photodissociation of a slab of gas of optical depth A_V ≤ 30 mag at constant pressure in order to calculate self-consistently all of the gas phases, from ionized gas to molecular gas. The ionizing source is the extreme ultraviolet (EUV) and soft X-ray radiation emitted by the cooling gas. We tested these models comparing their predictions to the rich multi-wavelength observations from optical to submillimeter, now achieved in cool core clusters.

Results. Such models of self-irradiated clouds, when reaching sufficiently large A_V, lead to a cloud structure with ionized, atomic, and molecular gas phases. These models reproduce most of the multi-wavelength spectra observed in the nebulae surrounding the BCGs, not only the low-ionization nuclear emission region like optical diagnostics, [O III]λ 5007 Å/Hβ, [N II]λ 6583 Å/Hα, and ([S II]λ 6716 Å+[S II]λ 6731 Å)/Hα, but also the infrared emission lines from the atomic gas. [O I]λ 6300 Å/Hα, instead, is overestimated across the full parameter space, except for very low A_V. The modeled ro-vibrational H₂ lines also match observations, which indicates that near- and mid-infrared H₂ lines are mostly excited by collisions between H₂ molecules and secondary electrons produced naturally inside the cloud by the interaction between the X-rays and the cold gas in the filament. However, there is still some tension between ionized and molecular line tracers (i.e., CO), which requires optimization of the cloud structure and the density of the molecular zone. The limited range of parameters over which predictions match observations allows us to constrain, in spite of degeneracies in the parameter space, the intensity of X-ray radiation bathing filaments, as well as some of their physical properties like A_V or the level of turbulent heating rate.

Conclusions. The reprocessing of the EUV and X-ray radiation from the plasma cooling is an important powering source of line emission from filaments surrounding BCGs. CLOUDY self-irradiated X-ray excitation models coupled with a small level of turbulent heating manage to simultaneously reproduce a large number of optical-to-infrared line ratios when all the gas phases (from ionized to molecular) are modeled self-consistently. Releasing some of the simplifications of our model, like the constant pressure, or adding the radiation fields from the AGN and stars, as well as a combination of matter- and radiation-bounded cloud distribution, should improve the predictions of line emission from the different gas phases.