Probing Warm-Hot Intergalactic Medium Associated with the Virgo Cluster Using an Oxygen Absorption Line

Abstract

Abstract To detect a warm-hot intergalactic medium (WHIM) associated with the large-scale structure of the universe, we observed a quasar behind the Virgo cluster with XMM-Newton. With 54 ks exposure, we marginally detected an O VIII $\mathrm{K}\alpha$ absorption line at $650.9^{+0.8}_{-1.9} \,\mathrm{eV}$ in the RGS spectra, with a statistical confidence of 96.4%. The observed line center energy is consistent with the redshift of M 87, and hence the absorber is associated with the Virgo cluster. From the curve of growth, the O VIII column density was estimated to be $\gtrsim 7 \times 10^{16} \,\mathrm{cm}^{-2}$. In the EPIC spectra, excess emission was found after evaluating the hot ICM in the Virgo cluster and various background components. We inspected the RASS map of the diffuse soft X-ray background, and confirmed that the level of the north and west regions just outside of the Virgo cluster is consistent with the background model that we used, while that of the east side is significantly higher and the enhancement is comparable with the excess emission found in the EPIC data. We consider a significant portion of the excess emission to be associated with the Virgo cluster, although a possible contribution from the North Polar Spur cannot be excluded. Using the column density and the emission measure and assuming an oxygen abundance of 0.1 and an ionization fraction of 0.4, we estimate that the mean electron density and the line-of-sight distance of the warm-hot gas are $\lesssim 6 \times 10^{-5} \,\mathrm{cm}^{-3}$ and $\gtrsim 9 \,\mathrm{Mpc}$. These strongly suggest detection of a WHIM in a filament associated with the Virgo cluster.