A Warm/Hot Intergalactic Medium toward 3C 120?

Abstract

We observed the Seyfert I active/broad-line radio galaxy 3C 120 with the Chandra high-energy transmission gratings and present an analysis of the soft X-ray spectrum. We identify the strongest absorption feature (detected at >99.9% confidence) with O VIII Lyα (FWHM = 1010 km s-1), blueshifted by -5500 ± 140 km s-1 from systemic velocity. The absorption may be due to missing baryons in a warm/hot intergalactic medium (WHIGM) along the line of sight to 3C 120 at z = 0.0147 ± 0.0005, or it could be intrinsic to the jet of 3C 120. Assuming metallicities of ~0.1 Z☉, we estimate an ionic column density of NO VIII > 3.4 × 1016 cm- 2 for the WHIGM and a filament depth of less than 19 h Mpc. We find a baryon overdensity greater than 56 relative to the critical density of a Λ-dominated cold dark matter universe, which is in reasonable agreement with WHIGM simulations. We detect, at marginal significance, absorption of O VIII Lyα at z ~ 0 due to a hot medium in the Local Group. We also detect an unidentified absorption feature at ~0.71 keV. Absorption features that might be expected along with O VIII Lyα were not significant statistically. Relative abundances of metals in the WHIGM and local absorbers may therefore be considerably different from solar.