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-rayabsorbing gas co-located with two established intergalactic 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 +0.6 −0.5 × 10 19 cm −2 ) of the absorber. Thermal analysis indicates that FUV and X-ray absorption relate to different phases, with estimated temperatures, T FUV ≈ 3 × 10 5 , and, T X−ray ≈ 3 × 10 6 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.

Detection of a Multiphase Intragroup Medium: Results from the COS-IGrM Survey

“…Heckman et al (2002) and Bordoloi et al (2017) (and references therein) show that O VI observed in the intergalactic medium, CGM of galaxies, and the Milky Way halo can be explained by radiative cooling models. These models agree with observations and simulations showing complex, multiphase structures at the interfaces between hot and cold gas (Oppenheimer & Davé 2009;Churchill et al 2012;Pachat et al 2016;Narayanan et al 2018;Ahoranta et al 2020).…”

The warm-hot circumgalactic medium around EAGLE-simulation galaxies and its detection prospects with X-ray and UV line absorption

“…Some extragalactic O vii, O viii, and Ne ix X-ray-line absorption has been found with current instruments, but with difficulty. Kovács et al (2019) found O vii absorption by stacking Xray observations centred on H i absorption systems near massive galaxies, though they targeted large-scale structure filaments rather than the CGM, while Ahoranta et al (2020) found O viii and Ne ix at the redshift of an O vi absorber. Bonamente et al (2016) found likely O viii absorption at the redshift of a broad Lyman α absorber.…”

“…Until such missions are launched, progress can be made with deep follow-up of FUV absorption lines with current X-ray instruments. The simulations can also help interpret the small number of absorbers found with current instruments (e.g., Nicastro et al 2018;Kovács et al 2019;Ahoranta et al 2020); e.g. Johnson et al (2019) used galaxy information to re-interpret the lines found by Nicastro et al (2018).…”

Detection capabilities of the Athena X-IFU for the warm-hot intergalactic medium using gamma-ray burst X-ray afterglows

“…However, a 4.5σ combined significance, or 3.2σ each, for 40 spectral elements searched in the spectrum results in a 0.08% chance of the fluctuations being random. A reduced blind line search region could be achieved with a low redshift GRB as the background source or with an a priori redshift from UV data or known galaxy locations, as in Kovács et al (2019) and Ahoranta et al (2020).…”

“…A study by Kovács et al (2019) quotes a 3.3σ detection of O vii towards H1821+643 from a stacked spectrum of Chandra observations with an 8 Ms total exposure. A&A 642, A24 (2020) Ahoranta et al (2020) performed spectral analysis of Chandra and XMM-Newtown observations at previously determined FUV redshifts, yielding two X-ray line candidates of Ne ix Heα and O viii Lyα at a combined significance of 3.9σ. In spite of the large observational efforts, only these few marginal detections have been achieved so far.…”