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 (Δcz ~ 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)_sun 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.