TY - JOUR
T1 - A high-resolution survey of low-redshift QSO absorption lines- Statistics and physical conditions of O VI absorbers
AU - Tripp, Todd M.
AU - Sembach, Kenneth R.
AU - Bowen, David V.
AU - Savage, Blair D.
AU - Jenkins, Edward B.
AU - Lehner, Nicolas
AU - Richter, Philipp
PY - 2008/7
Y1 - 2008/7
N2 - 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 (zabs ≪ 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 W r > 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.
AB - 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 (zabs ≪ 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 W r > 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.
KW - Cosmology: observations
KW - Intergalactic medium
KW - Quasars: absorption lines
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U2 - 10.1086/587486
DO - 10.1086/587486
M3 - Article
AN - SCOPUS:48249108439
SN - 0067-0049
VL - 177
SP - 39
EP - 102
JO - Astrophysical Journal, Supplement Series
JF - Astrophysical Journal, Supplement Series
IS - 1
ER -