An OVI Baryon Census of the Low-z Warm-Hot Intergalactic Medium

Abstract

Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium (IGM). In this study, we use the FUSE satellite to search 129 known lya absorption systems at z<0.15 toward 31 AGN for corresponding absorption from higher Lyman lines and the important metal ions OVI. We detect OVI in 40 systems over a smaller range of column density than seen in HI. The co-existence of OVI and HI suggests a multiphase IGM, with warm photoionized and hot ionized components. With improved OVI detection statistics, we find a steep distribution in OVI column density, dN(OVI)/dN_OVI N_OVI^(-2.2+-0.1), suggesting that numerous weak OVI absorbers contain baryonic mass comparable to the rare strong absorbers. The total cosmological mass fraction in this hot gas is at least Omega_WHIM=(0.0022+-0.0003. Gas in the WHIM at 10^5-6 K contributes at least 4.8+-0.9% of the total baryonic mass at z<0.15. We then combine empirical scaling relations for the observed "multiphase ratio", N_HI/N_OVI N_HI^(0.9+-0.1), and for hydrogen overdensity in cosmological simulations, N_HI delta_H^{0.7}, with the HI photoionization correction to derive the mean oxygen metallicity, Z_O (0.09 Z_sun)(f_OVI/0.2)^(-1) in the low-z multiphase gas. Given the spread in the empirical relations and in f_OVI, the baryon content in the OVI WHIM could be as large as 10%. Our survey is based on a large improvement in the number of OVI absorbers (40 vs. 10) and total redshift pathlength (dz 2.2 vs. dz 0.5) compared to earlier surveys.