Hot WHIM counterparts of FUV O VI absorbers: Evidence in the line-of-sight towards quasar 3C 273

Abstract

Aims: We explore the high spectral resolution X-ray data towards the quasar 3C 273 to search for signals of hot (∼10^6-7 K) X-ray-absorbing gas co-located with two established intergalactic far-ultraviolet (FUV) O VI absorbers.
Methods: We analyze the soft X-ray band grating data of all XMM-Newton and Chandra instruments to search for the hot phase absorption lines at the FUV predicted redshifts. The viability of potential line detections is examined by adopting the constraints of a physically justified absorption model. The WHIM hypothesis is investigated with a complementary 3D galaxy distribution analysis and by detailed comparison of the measurement results to the WHIM properties in the EAGLE cosmological, hydrodynamical simulation.
Results: At one of the examined FUV redshifts, z = 0.09017 ± 0.00003, we measured signals of two hot ion species, O VIII and Ne IX, with a 3.9σ combined significance level. While the absorption signal is only marginally detected in individual co-added spectra, considering the line features in all instruments collectively and assuming collisional equilibrium for absorbing gas, we were able to constrain the temperature (kT = 0.26 ± 0.03 keV) and the column density (N_H × Z_⊙/Z = 1.3_-0.5^+0.6 × 10¹⁹ cm^-2) of the absorber. Thermal analysis indicates that FUV and X-ray absorption relate to different phases, with estimated temperatures, T_FUV ≈ 3 × 10⁵, and, T_{X - ray} ≈ 3 × 10⁶ K. These temperatures match the EAGLE predictions for WHIM at the FUV/X-ray measured N_ion-ranges. We detected a large scale galactic filament crossing the sight-line at the redshift of the absorption, linking the absorption to this structure.
Conclusions: This study provides observational insights into co-existing warm and hot gas within a WHIM filament and estimates the ratio of the hot and warm phases. Because the hot phase is thermally distinct from the O VI gas, the estimated baryon content of the absorber is increased, conveying the promise of X-ray follow-up studies of FUV detected WHIM in refining the picture of the missing baryons.