A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers
Abstract
Using high-resolution ultraviolet spectra of 16 low-z QSOs obtained with the E140M echelle mode of the Space Telescope Imaging Spectrograph, we study the physical conditions and statistics of O VI absorption in the intergalactic medium (IGM) at z < 0.5. We identify 51 intervening (zabsll zQSO) O VI systems comprising 77 individual components, and we find 14 "proximate" systems (zabs ≈ zQSO) containing 34 components. For intervening systems (components) with rest-frame equivalent width Wr > 30 mÅ, the number of O VI absorbers per unit redshift dN/dz = 15.6-2.4+2.9 (21.0-2.8+3.2) , and this decreases to dN/dz = 0.9-0.5+1.0 (0.3-0.3+0.7) for Wr > 300 mÅ. The number per redshift increases steeply as zabs approaches zQSO; we find that dN/dz is ≈3-10 times higher within 2500 km s-1 of zQSO. The most striking difference between intervening and proximate systems is that some proximate absorbers have substantially lower H I/O VI ratios. The lower ratios in proximate systems could be partially due to ionization effects, but these proximate absorbers must also have significantly higher metallicities. We find that 37% of the intervening O VI absorbers have velocity centroids that are well aligned with corresponding H I absorption. If the O VI and the H I trace the same gas, the relatively small differences in line widths imply that the absorbers are cool, with T < 105 K. Most of these well-aligned absorbers have the characteristics of metal-enriched photoionized gas. However, the O VI in the apparently simple and cold systems could be associated with a hot phase with T ≈ 105.5 K if the metallicity is high enough to cause the associated broad Lyα absorption to be too weak to detect. We show that 53% of the intervening O VI systems are complex multiphase absorbers that can accommodate both lower metallicity collisionally ionized gas with T > 105 K and cold photoionzed gas.
Based on observations with (1) the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 05-26555, and (2) the NASA-CNES/ESA Far Ultraviolet Spectroscopic Explorer mission, operated by the Johns Hopkins University, supported by NASA contract NAS 05-32985.- Publication:
-
The Astrophysical Journal Supplement Series
- Pub Date:
- July 2008
- DOI:
- 10.1086/587486
- arXiv:
- arXiv:0706.1214
- Bibcode:
- 2008ApJS..177...39T
- Keywords:
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- cosmology: observations;
- intergalactic medium;
- quasars: absorption lines;
- Astrophysics
- E-Print:
- Accepted for publication in the Astrophysical Journal Supplements. Substantially revised and improved based on comments from the referee and other readers