Hubble/cos observations of the Lya forest toward the BL Lac object 1ES 1553+113

Abstract

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 < zabs < 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 Lya absorbers at z abs ≈ 0.188 in which Ovi, Nv, 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)⊙ 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: zem > 0.395 based on a confirmed Lyα+Ovi absorber and zem > 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 zem ≲ 0.58 (at a 1σ confidence limit) based on the non-detection of any Lyβ absorbers at z > 0.4. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.