A Survey of Metal Lines at High-redshift (II) : SDSS Absorption Line Studies - OVI line density, space density and gas metallicity at z_abs ~ 3.0

Abstract

We have analysed a large data set of OVI absorber candidates found in the spectra of 3702 SDSS quasars, focusing on a subsample of 387 AGN sightlines with an average S/N>5.0, allowing for detection of absorbers above rest-frame equivalents widths W_r>0.19 A for the OVI 1032 A component. Accounting for random interlopers mimicking an OVI doublet, we derive for the first time a secure lower limit for the redshift number density $\Delta N/\Delta z$ for redshifts z_abs>2.8. With extensive Monte Carlo simulations we quantify the losses of absorbers due to blending with the ubiquitous Lyman forest lines, and estimate the success rate of retrieving each individual candidate as a function of its redshift, the emission redshift of the quasar, the strength of the absorber and the measured S/N of the spectrum by modelling typical Ly forest spectra. These correction factors allow us to derive the 'incompleteness and S/N corrected' redshift number densities of OVI absorbers :$\Delta N_{OVI,c}/\Delta z_c(2.8<z<3.2)=4.6+-0.3,at3.2<z<3.6=6.7+-0.8,andat3.6<z<4.0=8.4+-\mathrm{2.9.}WecanplaceasecurelowerlimitforthecontributionofOVItotheclosuremassdensityattheredshiftsprobedhere:$Ω_{OVI} (2.8 < z < 3.2) >1.9x10^{-8} h^-1. We show that the strong lines we probe account for over 65\% of the mass in the OVI absorbers; the weak absorbers, while dominant in line number density, do not contribute significantly to the mass density. Making a conservative assumption about the ionisation fraction, and adopting the Anders (1989) solar abundance values, we derive the mean metallicty of the gas probed in our search : $ζ(2.8 < z < 3.2) > 3.6 x 10^-4 h, in good agreement with other studies. These results demonstrate that large spectroscopic datasets such as SDSS can play an important role in QSO absorption line studies, in spite of the relatively low resolution.

24 pages, 4 figures, 3 tables, accepted by AJ, follow-up paper to arXiv:0707.1700 with statistical analysis of absorber sample. Note the addition of M. Pieri as author. Substantially revised from earlier version

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• What Shapes the Galaxy Mass Function? Exploring the Roles of Supernova-Driven Winds and AGN
• Star Formation Feedback and Metal Enrichment History Of The Intergalactic Medium
• The HI Content of the Universe over the Past 10 Gyrs
• Precious Metals in SDSS Quasar Spectra I: Tracking the Evolution of Strong, 1.5 < z < 4.5 CIV Absorbers with Thousands of Systems
• A high resolution study of intergalactic O VI absorbers at z~2.3
• The Surprisingly Constant Strength of O VI Absorbers over Cosmic Time
• Tracing the 10${}^7$K Warm-Hot Intergalactic Medium with UV absorption lines