A Survey of Metal Lines at High-redshift (I) : SDSS Absorption Line Studies - The Methodology and First Search Results for OVI

Abstract

We report the results of a systematic search for signatures of metal lines in quasar spectra of the Sloan Digital Sky Survey (SDSS) Data Release 3(DR3), focusing on finding intervening absorbers via detection of their OVI doublet. Here we present the search algorithm, and criteria for distinguishing candidates from spurious Lyman $α${} forest lines. In addition, we compare our findings with simulations of the Lyman $α${} forest in order to estimate the detectability of OVI doublets over various redshift intervals. We have obtained a sample of 1756 OVI doublet candidates with rest-frame equivalent width > 0.05 Å in 855 AGN spectra (out of 3702 objects with redshifts in the accessible range for OVI detection). This sample is further subdivided into 3 groups according to the likelihood of being real and the potential for follow-up observation of the candidate. The group with the cleanest and most secure candidates is comprised of 145 candidates. 69 of these reside at a velocity separation > 5000 km/s from the QSO, and can therefore be classified tentatively as intervening absorbers. Most of these absorbers have not been picked up by earlier, automated QSO absorption line detection algorithms. This sample increases the number of known OVI absorbers at redshifts beyond z$_{abs} > 2.7 substantially.