OUP accepted manuscript

Monthly Notices of the Royal Astronomical Society

Abstract

Blazars are some of the brightest UV and X-ray sources in the sky and are valuable probes of the elusive warm-hot intergalactic medium (WHIM; $T{\simeq} 10^5-10^7$ K). However, many of the brightest blazars$-$called BL Lac objects such as 1ES1553+113$-$have quasi-featureless spectra and poorly constrained redshifts. Here, we significantly improve the precision of indirect redshift constraints for blazars based on the edge of the $\rm{H\,I}$ Ly$α$ forest observed in their UV spectra. We develop a robust technique to constrain the redshift of a $z<0.5$ AGN or blazar with a $1σ$ uncertainty of ${\approx}0.01$ using only the position of its highest-redshift Ly$α$ absorber with $\log N_{\rm{H\,I}}/{\rm cm^{-2}} > 12.6$. We use a large sample of 192 AGN/QSOs at $0.01\lesssim z\lesssim0.45$ that have high-quality COS FUV spectra to characterize the intrinsic scatter in the gap between the AGN redshift and the edge of their Ly$α$ forest. We present new COS NUV data for 1ES1553+113 and confirm its redshift of $z=0.433$ using our technique. We apply our Ly$α$-forest-based redshift estimation technique to nine additional blazars with archival ${\it HST}$ UV spectra, most of which are key targets for future X-ray missions. Our inferred redshift constraints improve estimates for two BL Lacs (1ES1118+424 and S50716+714) and are consistent with previous estimates for the rest. Our results emphasize the need to obtain further UV spectra of bright blazars, of which many have uncertain redshifts, in order to maximize the scientific value of future X-ray WHIM observations that will improve our understanding of galaxy evolution.

Authors 7

1. J. Dorigo Jones

   University of Colorado Boulder · University of Michigan

   Affiliation as printed

   Department of Astronomy, University of Michigan, 1085 South University Ave., Ann Arbor, MI 48109-1107, USA

   Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, 391 UCB, 2000 Colorado Ave., Boulder, CO 80309, USA

2. Sowgat Muzahid

   Leibniz Institute for Astrophysics Potsdam · Inter-University Centre for Astronomy and Astrophysics

   Affiliation as printed

   IUCAA, Post Bag-04, Ganeshkhind, Pune, India -411007

   Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

3. J. C. Charlton

   Pennsylvania State University

   Affiliation as printed

   Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA

4. Anand Narayanan

   Indian Institute of Space Science and Technology

   Affiliation as printed

   Indian Institute of Space Science and Technology, Thiruvananthapuram 695547, Kerala, India

5. Sameer Sameer
6. Joop Schaye Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands

7. Nastasha Wijers Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands