Molecular lines studies at redshift greater than 1
  Observations of CO molecules in the millimetrer domain at high redshift
(larger than 1), have provided interesting informations about star formation
efficiency, and its evolution with redshift. Due to the difficulty of the
detections, selection effects are important. The detection if often due to
gravitational amplification. Objects selected by their (far)infrared flux, are
in general associated to ULIRGS, mergers with starburst in the nuclear regions.
Quasars have been selected as powerful optical sources, and have been found to
be associated to starbursts, rich in gas. The gas fraction appears to be much
higher at redshift greater than 1.
  Quasars allow to probe the end of the reionisation period, and the relation
between bulge and black hole mass. However these selection bias could have led
us to miss some gaseous galaxies, with low-efficiency of star formation, such
as the more quiescent objects selected by their BzK colors at z=1.5 or 2.
