Chandra Detection of the First X-Ray Forest along the Line of Sight To Markarian 421
Abstract
We present the first >=3.5 (conservative) or >=5.8 σ (sum-of-lines significance) detection of two warm-hot intergalactic medium (WHIM) filaments at z>0, which we find along the line of sight to the blazar Mrk 421. These systems are detected through highly ionized resonant metal absorption in high-quality Chandra ACIS and HRC Low Energy Transmission Grating (LETG) spectra of Mrk 421, obtained following our two Target of Opportunity requests during two outburst phases (F0.5-2keV=40 and 60 mcrab; 1 crab=2×10-8 ergs s-1 cm-2). Columns of He-like oxygen and H-like nitrogen can be detected in the co-added LETG spectrum of Mrk 421 down to a sensitivity of NOVII>=8×1014 cm-2 and NNVII>=1015 cm-2, respectively, at a significance >=3 σ. The two intervening WHIM systems that we detect have O VII and N VII columns of NOVII=(1.0+/-0.3)×1015 cm-2, NN VII=(0.8+/-0.4)×1015 cm-2 and NOVII=(0.7+/-0.3)×1015 cm-2, NNVII=(1.4+/-0.5)×1015 cm-2, respectively. We identify the closest of these two systems with an intervening WHIM filament at cz=3300+/-300 km s-1. The second system, instead, at cz=8090+/-300 km s-1, is identified with an intervening WHIM filament located ~13 Mpc from the blazar. The filament at cz=3300+/-300 lies <~5 Mpc from a known H I Lyα system at cz=3046+/-12 km s-1 (Shull and coworkers) whose 3 σ maximal H I kinetic temperature, as derived from the observed line FWHM, is T<=1.2×105 K. This temperature is inconsistent with the temperature measured for the X-ray filament, so if the systems are related, a multiphase WHIM is required. Combining UV and far-ultraviolet (FUV) upper limits on the H I Lyα and the O VI2s-->2p transitions with our measurements in the X-rays, we show that, for both filaments, equilibrium collisional ionization (with residual photoionization by both the diffuse UV and X-ray background and the beamed emission of Mrk 421 along our line of sight) provides acceptable solutions. These solutions define ranges of temperatures, metallicity ratios, and equivalent H column densities, which are in good agreement with the predictions of hydrodynamical simulations for the formation of large-scale structures in the universe. From the detected number of WHIM filaments along this line of sight we can estimate the number of O VII filaments per unit redshift with columns larger than 7×1014 cm-2, dNOVII/dz(NOVII>=7×1014)=67+88-43, consistent, within the large 1 σ errors, with the hydrodynamical simulation predictions of dNOVII/dz(NOVII>=7×1014)=30. Finally, we measure a cosmological mass density of X-ray WHIM filaments Ωb=0.027+0.038-0.019×10[O/H]-1, consistent with both model predictions and the estimated number of ``missing'' baryons at low redshift.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2005
- DOI:
- 10.1086/431270
- arXiv:
- arXiv:astro-ph/0504187
- Bibcode:
- 2005ApJ...629..700N
- Keywords:
-
- Galaxies: BL Lacertae Objects: Individual: Name: Markarian 421;
- Galaxies: Intergalactic Medium;
- Galaxies: Quasars: Absorption Lines;
- Astrophysics
- E-Print:
- 51 pages, 18 figures. Accepted for publication in the ApJ