Molecular hydrogen absorption systems in Sloan Digital Sky Survey
Abstract
We present a systematic search for molecular hydrogen absorption systems at high redshift in quasar spectra from the Sloan Digital Sky Survey (SDSS) II Data Release 7 and SDSS-III Data Release 9. We have selected candidates using a modified profile fitting technique taking into account that the Lyα forest can effectively mimic H2 absorption systems at the resolution of SDSS data. To estimate the confidence level of the detections, we use two methods: a Monte-Carlo sampling and an analysis of control samples. The analysis of control samples allows us to define regions of the spectral quality parameter space where H2 absorption systems can be confidently identified. We find that H2 absorption systems with column densities NH2 > 19 can be detected in only less than 3% of SDSS quasar spectra. We estimate the upper limit on the detection rate of saturated H2 absorption systems ( NH2 > 19) in Damped Ly-α (DLA) systems to be about 7%. We provide a sample of 23 confident H2 absorption system candidates that would be interesting to follow up with high resolution spectrographs. There is a 1σ r-i color excess and non-significant A V extinction excess in quasar spectra with an H2 candidate compared to standard DLA-bearing quasar spectra. The equivalent widths (EWs) of C II, Si II and Al III (but not Fe II) absorptions associated with H2 candidate DLAs are larger compared to standard DLAs. This is probably related to a larger spread in velocity of the absorption lines in the H2 bearing sample.