Physical properties and baryonic content of low-redshift intergalactic Lyα and OVI absorption line systems:: The PG 1116+215 sight line

We present Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer observations of the intergalactic absorption toward QSO PG 1116+ 215 in the 900 - 3000 Angstrom spectral region. We detect 25 Lyalpha absorbers along the sight line at rest-frame equivalent widths W-r > 30 mAngstrom, yielding (dN/dz)(Lyalpha) = 166 +/- 20 over an unblocked redshift path Deltaz(Lyalpha) = 0.150. Two additional weak Lyalpha absorbers with W-r approximate to 15-20 mAngstrom are also present. Eight of the Lyalpha absorbers have large line widths (bgreater than or similar to40 km s(-1)). The detection of narrow O vi absorption in the broad Lyalpha absorber at z = 0.06244 supports the idea that the Lyalpha profile is thermally broadened in gas with T > 10(5) K. We find dN/dz approximate to 53 for broad Lyalpha absorbers with W-r greater than or similar to 30 mAngstrom and b greater than or equal to 40 km s(-1). This number drops to dN/dz approximate to 40 if the line widths are restricted to 40 less than or equal to b less than or equal to 100 km s(-1). If the broad Lyalpha 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 Lyalpha 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 Lyalpha and O VI detections alone. We find (dN/dz) O VI approximate to 18 for O VI absorbers with W-r greater than or equal to 50 mAngstrom toward PG 1116+215. The information available for 13 low-redshift O VI absorbers with W-r greater than or equal to 50 mAngstrom along six sight lines yields (dN/dz)(O VI) approximate to 13 and Omega(b)( O VI) greater than or similar to0.0022 h(75)(-1) assuming a metallicity of 0.1 solar and an O VI ionization fraction f(O VI) less than or equal to 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 similar to 600 h(75)(-1) 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.