A deep search for molecular gas in two massive Lyman break galaxies at
  z=3 and 4: vanishing CO-emission due to low metallicity?
  We present deep IRAM Plateau de Bure Interferometer (PdBI) observations,
searching for CO-emission toward two massive, non-lensed Lyman break galaxies
(LBGs) at z=3.216 and 4.058. With one low significance CO detection (3.5 sigma)
and one sensitive upper limit, we find that the CO lines are >~ 3-4 times
weaker than expected based on the relation between IR and CO luminosities
followed by similarly, massive galaxies at z=0-2.5. This is consistent with a
scenario in which these galaxies have low metallicity, causing an increased
CO-to-H_2 conversion factor, i.e., weaker CO-emission for a given molecular
(H_2) mass. The required metallicities at z>3 are lower than predicted by the
fundamental metallicity relation (FMR) at these redshifts, consistent with
independent evidence. Unless our galaxies are atypical in this respect,
detecting molecular gas in normal galaxies at z>3 may thus remain challenging
even with ALMA.
