Physical Properties and Baryonic Content of Low-Redshift Intergalactic Lyα and O VI Absorption Line Systems: The PG 1116+215 Sight Line

Abstract

We present Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer observations of the intergalactic absorption toward QSO PG 1116+215 in the 900-3000 Å spectral region. We detect 25 Lyα absorbers along the sight line at rest-frame equivalent widths W_r>30 mÅ, yielding (dN/dz)_Lyα=166+/-20 over an unblocked redshift path Δz_Lyα=0.150. Two additional weak Lyα absorbers with W_r~15-20 mÅ are also present. Eight of the Lyα absorbers have large line widths (b>~40 km s^-1). The detection of narrow O VI absorption in the broad Lyα absorber at z=0.06244 supports the idea that the Lyα profile is thermally broadened in gas with T>10⁵ K. We find dN/dz~53 for broad Lyα absorbers with W_r>~30 mÅ and b>=40 km s^-1. This number drops to dN/dz~40 if the line widths are restricted to 40<=b<=100 km s^-1. If the broad Lyα lines are dominated by thermal broadening in hot gas, the amount of baryonic material in these absorbers is enormous, perhaps as much as half the baryonic mass in the low-redshift universe. We detect O VI absorption in several of the Lyα clouds along the sight line. Two detections at z=0.13847 and z=0.16548 are confirmed by the presence of other ions at these redshifts (e.g., C II-III, N II-III, N V, O I, O VI, and Si II-IV), while the detections at z=0.04125, 0.05895, 0.05928, and 0.06244 are based upon the Lyα and O VI detections alone. We find (dN/dz)_OVI~18 for O VI absorbers with W_r>50 mÅ toward PG 1116+215. The information available for 13 low-redshift O VI absorbers with W_r>=50 mÅ along six sight lines yields (dN/dz)_OVI~13 and Ω_b(OVI)>~0.0022h^-1₇₅, assuming a metallicity of 0.1 solar and an O VI ionization fraction f_OVI<=0.2. The properties and prevalence of low-redshift O VI absorbers suggest that they too may be a substantial baryon repository, perhaps containing as much mass as stars and gas inside galaxies. The redshifts of the O VI absorbers are highly correlated with the redshifts of galaxies along the PG 1116+215 sight line, though few of the absorbers lie closer than ~600 h^-1₇₅ to any single galaxy. We analyze the kinematics and ionization of the metal-line systems along this sight line and discuss the implications of these observations for understanding the physical conditions and baryonic content of intergalactic matter in the low-redshift universe.

Based on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

Based on observations obtained with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.