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 $\log {\rm N_{H_2}} > 19$
can be detected in only less than 3% of SDSS quasar spectra. We estimate the
upper limit on the detection rate of saturated H$_2$ absorption systems ($\log
{\rm N_{H_2}} > 19$) in Damped Ly-$\alpha$ (DLA) systems to be about 7%. 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$\sigma$ $r-i$ color excess and non-significant $A_{\rm V}$ extinction excess
in quasar spectra with an H$_2$ 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 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.
