HST/COS Observations of the Ly alpha Forest toward the BL Lac Object 1ES1553+113

Abstract: We present new far-ultraviolet spectra from the Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES1553+113 covering the wavelength range 1135-1795 A. The data show a smooth continuum with a wealth of narrow absorption features arising in the ISM and IGM. These features include 41 Lya absorbers at 0<z\<0.43, fourteen of which are detected in multiple Lyman lines and six in one or more metal lines. We analyze a metal-rich triplet of Lya absorbers at z=0.188 in which OVI, NV, and CIII absorption is detected. Silicon ions (SiIII/IV) are not detected to fairly strong upper limits, and we use the measured SiIII/CIII upper limit to derive an abundance limit [C/Si]\>0.6 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, 1ES1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES1553+113 has no direct redshift determination. We use intervening Lya absorbers to place a direct limit on the redshift: z_em>0.395 based on a confirmed Lya+OVI absorber and z_em>0.433 based on a single-line detection of Lya. COS/FUV data are only sensitive to Lya absorbers at z<0.47, but we present statistical arguments that z_em<0.58 based on the non-detection of any Lyb absorbers at z>0.4.

Overview of Hubble/COS Observations of the Lyα Forest toward the BL Lac Object 1ES 1553+113

This paper presents detailed observations of the BL Lac object 1ES 1553+113 using the Hubble Space Telescope's Cosmic Origins Spectrograph (HST/COS). The research provides an extensive analysis of the Lyα forest and associated metal-line absorptions along the line of sight towards this object. The observations extend our understanding of the interstellar medium (ISM) and the intergalactic medium (IGM), focusing on the moderate-resolution, far-ultraviolet spectra within the wavelength range of 1135 Å to 1795 Å. The study identifies numerous absorption features, providing insights into their physical properties and redshift measurements.

Key Findings

1. Absorption Features and Redshift Determination:

   • The HST/COS data revealed a multitude of narrow absorption features, including 41 Lyα absorbers in the redshift range 0 < z_abs < 0.43.
   • The detection of metal-rich absorption lines such as OVI, NV, and CIII in a triplet of Lyα absorbers at z_abs ≈ 0.188 is notable, with measured abundance constraints indicating a high (C/Si) ratio.
   • For the first time, a direct limit on the redshift of 1ES 1553+113 is established as z_em > 0.395, based on the presence of intervening Lyα absorption with confirmed OVI lines, extending this to z_em > 0.433 with a single-line detection of Lyα.

2. Metallicity and Abundance Ratios:

   • The analysis of the triplet at z ≈ 0.188 indicates a high nitrogen-to-oxygen ratio, with N/NV suggesting an elevated abundance ratio compared to typical IGM absorbers.
   • Limits on silicon ion detection lead to the derivation of an abundance constraint (C/Si) ≥ 4 times the solar value for the strongest component of this complex.

3. Statistical Analysis of IGM Absorption:

   • The paper shows that the frequency of IGM absorbers aligns well with prior surveys using FUSE and HST/STIS data.
   • It also employs statistical arguments to estimate the redshift of 1ES 1553+113, constraining it to z_em ≲ 0.58 based on the non-detection of Lyβ absorbers at z > 0.4.

Implications and Future Research

The findings offer practical and theoretical implications for understanding the large-scale structure of the universe, particularly in exploring galaxy filaments and the gas dynamics within the IGM. This work highlights 1ES 1553+113 not only as a significant source for studying the Lyα forest but also as a key candidate for investigating the warm-hot intergalactic medium (WHIM).

Future observational campaigns, particularly with improvements in sensitivity and wavelength coverage, are necessary to test the abundance patterns further and refine the redshift constraints. Observations with a broader wavelength range could potentially reveal higher redshift features and confirm the presence of elusive broad-line absorption systems.

In conclusion, this paper exemplifies the utility of BL Lac objects as probes for the cosmic web, providing critical data for IGM studies and contributing to our understanding of the chemical abundance patterns in the low-redshift universe. The continued evolution of ultraviolet spectroscopic capabilities will further enhance our understanding of these structures and the role they play in cosmic evolution.