HUBBLE/COS OBSERVATIONS OF THE Ly alpha FOREST TOWARD THE BL Lac OBJECT 1ES 1553+113

We present new moderate-resolution, far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES 1553+113 covering the wavelength range 1135 angstrom < lambda < 1795 angstrom. The data show a smooth continuum with a wealth of narrow (b < 100 km s(-1)) absorption features arising in the interstellar medium and intergalactic medium. These features include 41 Ly alpha absorbers at 0 < z(abs) < 0.43, 14 of which are detected in multiple Lyman lines and 6 of which show absorption in one or more metal lines. We analyze a metal-rich triplet (Delta cz similar to 1000 km s(-1)) of Ly alpha absorbers at z(abs) approximate to 0.188 in which O VI, N V, and CIII absorption is detected. Silicon ions (Si III, Si IV) are not detected to fairly strong upper limits and we use the measured Si III/C III upper limit to derive an abundance limit (C/Si) >= 4 (C/Si)(circle dot) for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and gamma-ray sources, 1ES 1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES 1553+113 has no direct redshift determination. We use intervening Ly alpha absorbers to place a direct limit on the redshift: z(em) > 0.395 based on a confirmed Ly alpha+O VI absorber and z(em) > 0.433 based on a single-line detection of Ly alpha. The current COS data are only sensitive to Ly alpha absorbers at z < 0.47, but we present statistical arguments that z(em) less than or similar to 0.58 (at a 1 sigma confidence limit) based on the non-detection of any Ly beta absorbers at z > 0.4.