CHEX-MATE: X-ray absorption and molecular content of the interstellar medium toward galaxy clusters

¹ Università degli studi di Roma ‘Tor Vergata’, Via della Ricerca Scientifica, 1, 00133 Roma, Italy
e-mail: herve.bourdin@roma2.infn.it
² INFN, Sezione di Roma 2, Università degli Studi di Roma Tor Vergata, Via della Ricerca Scientifica, 1, Roma, Italy
³ INAF, IASF-Milano, Via A. Corti 12, 20133 Milano, Italy
⁴ Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, MI 48824, USA
⁵ Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁶ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Via Pietro Gobetti 93/3, 40129 Bologna, Italy
⁷ INFN, Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
⁸ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ HH Wills Physics Laboratory, University of Bristol, Tyndall Ave, Bristol BS8 1TL, UK
¹¹ IRAP, Université de Toulouse, CNRS, CNES, UT3-UPS, Toulouse, France
¹² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

Received: 9 June 2023
Accepted: 28 July 2023

Abstract

The X-ray spectrum of extragalactic sources, such as galaxy clusters, is affected by the photo-absorption of various components of the Galactic interstellar medium (ISM). The resulting spectral distortion contributes to the systematics of cluster temperature measurements. It essentially depends on the neutral (atomic+molecular) Galactic hydrogen density column, N_H, which remains challenging to map across the sky in the lack of a straightforward tracer of the molecular gas phase in the ISM. Combining data from the HI4PI and Planck HFI sky surveys, we investigate the mass fraction of molecular gas across the line of sight of CHEX-MATE galaxy clusters by searching for thermal dust emission excess with respects to the neutral atomic hydrogen density column, N_HI. Consistent with earlier studies of the ISM based on IRAS and Planck data, we detect dust emission excess along the line of sight of some members of the CHEX-MATE cluster catalogue that are mostly localised behind dense ISM regions. We find that the CHEX-MATE cluster catalogue can be divided into three categories: 40% of members are located behind low N_HI regions where the molecular mass fraction is negligible, 40% of members are located behind intermediate N_HI regions where the molecular gas fraction would reach 5% on average, and the remaining 20% of members are located behind high N_HI regions that locally exhibit even higher molecular gas fractions. The apparent cluster temperature shifts associated with the molecular content of the ISM are about 1% or less for most CHEX-MATE clusters, but can exceed 5% in the highest N_HI regions.