2021A&A...656A.107A

Aims. The observational constraints on the baryon content of the warm-hot intergalactic medium (WHIM) rely almost entirely on far ultraviolet (FUV) measurements. However, cosmological, hydrodynamical simulations predict strong correlations between the spatial distributions of FUV and X-ray absorbing WHIM. We investigate this prediction by analyzing XMM-Newton X-ray counterparts of FUV-detected intergalactic OVI absorbers known from FUSE and HST/STIS data, thereby aiming to gain understanding on the properties of the hot component of FUV absorbers and to compare this information to the predictions of simulations.
Methods. We study the X-ray absorption at the redshift of the only significantly detected OVI absorber in the Ton S 180 sightline's FUV spectrum, found at z_OVI = 0.04579 ± 0.00001. We characterize the spectral properties of the OVI-OVIII absorbers and explore the ionization processes behind the measured absorption. The observational results are compared to the predicted warm-hot gas properties in the EAGLE simulation to infer the physical conditions of the absorber.
Results. We detect both OVI and OVII absorption at a 5σ confidence level, whereas OVIII absorption is not significantly detected. Collisional ionization equilibrium (CIE) modeling constrains the X-ray absorbing gas temperature to log T_CIE (K) = 6.22 ± 0.05 with a total hydrogen column density N_H = 5.8_–2.2^+3.0 x Z_☉/Z_abs x 10¹⁹ cm^–2. This model predicts an OVI column density consistent with that measured in the FUV, but our limits on the OVI line width indicate > 90% likelihood that the FUV-detected OVI arises from a different, cooler phase. We find that the observed absorber lies about a factor of two further away from the detected galaxies than is the case for similar systems in EAGLE.
Conclusions. The analysis suggests that the detected OVI and OVII trace two different - warm and hot - gas phases of the absorbing structure at z ≃ 0.046, of which the hot component is likely in collisional ionization equilibrium. As the baryon content information of the studied absorber is primarily imprinted in the X-ray band, understanding the abundance of similar systems helps to define the landscape for WHIM searches with future X-ray telescopes. Our results highlight the crucial role of line widths for the interpretation and detectability of WHIM absorbers.