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

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 54ks exposure, we marginally detected an O VIII Kα absorption line at 650.9 +0.8 −1.9 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 7 ×10 16 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 6 ×10 −5 cm −3 and 9Mpc. These strongly suggest detection of a WHIM in a filament associated with the Virgo cluster.

为了探测与宇宙大尺度结构相关的温热星系际介质（WHIM），我们利用XMM-牛顿卫星对室女座星系团后方的一个类星体进行了观测。在54千秒的曝光时间下，我们在反射光栅光谱仪（RGS）光谱中勉强探测到一条位于650.9 +0.8 −1.9电子伏特的氧Ⅷ Kα吸收线，统计置信度为96.4%。观测到的谱线中心能量与M 87星系的红移相符，因此该吸收体与室女座星系团有关。根据生长曲线，估计氧Ⅷ的柱密度为7×10¹⁶ 厘米⁻² 。在欧洲光子成像相机（EPIC）光谱中，在评估了室女座星系团中的热星系际介质（ICM）及各种背景成分后，发现了额外的辐射。我们查看了漫射软X射线背景的ROSAT全天空巡天（RASS）地图，确认室女座星系团外侧北部和西部区域的强度与我们所使用的背景模型一致，而东侧的强度明显更高，增强部分与EPIC数据中发现的额外辐射相当。我们认为大部分额外辐射与室女座星系团有关，尽管不能排除来自北天极支的可能贡献。利用柱密度和发射量，并假设氧丰度为0.1，电离分数为0.4，我们估计温热气体的平均电子密度和视线距离分别为6×10⁻⁵ 厘米⁻³ 和9百万秒差距。这些有力地表明在与室女座星系团相关的丝状结构中探测到了温热星系际介质。