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

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, NH, 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, NHI. 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 NHI regions where the molecular mass fraction is negligible, 40% of members are located behind intermediate NHI regions where the molecular gas fraction would reach 5% on average, and the remaining 20% of members are located behind high NHI 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 NHI regions.