The Low‐zIntergalactic Medium. I. OviBaryon Census

Abstract

Intergalacticabsorbersalonglinesofsighttodistantquasarsareapowerfuldiagnosticfortheevolutionandcontent of the intergalactic medium (IGM). In this study, we use theFar Ultraviolet Spectroscopic Explorer(FUSE) satellite to search 129 known Lyabsorption systems at z < 0:15 toward 31 active galactic nuclei (AGNs) for corresponding absorption from higher Lyman lines and the important metal ions O vi and C iii. We detect O vi in 40 systems, over a smaller range of column density (logNO vi ¼ 13:0 14:35) than seen in H i (logNH i ¼13:0 16:0). The coexistence of OviandHisuggestsamultiphaseIGM withwarmphotoionizedandhotionizedcomponents. Withimproved Ovi detection statistics, we find a steep distribution in O vi column density, dNO vi/dNO vi / N � 2:2� 0:1 O vi , suggesting that numerous weak O vi absorbers contain baryonic mass comparable to the rare strong absorbers. Down to 30 m8 equivalent width (O vi k1032) we find an absorber frequency dNO vi/dz � 17 � 3. The total cosmological mass fraction in this hot gas is at least WHIM ¼ (0:0022 � 0:0003)½h70(ZO/0:1 Z� )( fO vi/0:2)� � 1 , where we have scaled to fiducial values of oxygen metallicity, O vi ionization fraction, and the Hubble constant. Gas in the warm-hot inter- galactic medium (WHIM) at 10 5 -10 6 K contributes at least 4:8% � 0:9% of the total baryonic mass at z < 0:15. We thencombineempiricalscalingrelationsfortheobserved''multiphaseratio,''NH i/NO vi / N 0:9� 0:1 H i ,andforhydrogen overdensity in cosmological simulations, NH i / � 0:7 H , with the H i photoionization correction to derive the mean oxygen metallicity, ZO � (0:09 Z� )( fO vi/0:2) � 1 in the low-z multiphase gas. Given the spread in the empirical relations and in fO vi, the baryon content in the O vi WHIM could be as large as 10%. Our survey is based on a large improvement in the number of O vi absorbers (40 vs. 10) and total redshift path length (z � 2:2 vs. z � 0:5) compared to earlier surveys. Subject headingg cosmological parameters — cosmology: observations — intergalactic medium — quasars: absorption lines