Molecular hydrogen absorption systems in Sloan Digital Sky Survey

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 Ly alpha forest can effectively mimic H-2 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 H-2 absorption systems can be confidently identified. We find that H-2 absorption systems with column densities log N-H2 > 19 can be detected in only less than 3 per cent of SDSS quasar spectra. We estimate the upper limit on the detection rate of saturated H-2 absorption systems (N-H2 > 19) in damped Ly alpha (DLA) systems to be about 7 per cent. We provide a sample of 23 confident H-2 absorption system candidates that would be interesting to follow up with high-resolution spectrographs. There is a 1 Sigma r - i colour excess and non-significant A(V) extinction excess in quasar spectra with an H-2 candidate compared to standard DLA-bearing quasar spectra. The equivalent widths of C ii, Si ii and Al iii (but not Fe ii) absorptions associated with H-2 candidate DLAs are larger compared to standard DLAs. This is probably related to a larger spread in velocity of the absorption lines in the H-2-bearing sample.