HUBBLE/COS OBSERVATIONS OF THE Lyα 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 Å < λ < 1795 Å. 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 Lyα 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 (Δ cz ∼ 1000 km s −1 ) of Lyα absorbers at z abs ≈ 0.188 in which O vi , N v , and C iii 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) ☉ 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 γ-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 Lyα absorbers to place a direct limit on the redshift: z em >0.395 based on a confirmed Lyα+O vi absorber and z em >0.433 based on a single-line detection of Lyα. The current COS data are only sensitive to Lyα absorbers at z < 0.47, but we present statistical arguments that z em ≲ 0.58 (at a 1σ confidence limit) based on the non-detection of any Lyβ absorbers at z >0.4.