We present observations of redshifted CO ( 1-0 ) and CO ( 2-1 ) in a field containing an overdensity of Lyman break galaxies ( LBGs ) at z = 5.12 .
Our Australia Telescope Compact Array observations were centered between two spectroscopically-confirmed z = 5.12 galaxies .
We place upper limits on the molecular gas masses in these two galaxies of M ( H _ { 2 } ) < 1.7 \times 10 ^ { 10 } M _ { \odot } and < 2.9 \times 10 ^ { 9 } M _ { \odot } ( 2 \sigma ) , comparable to their stellar masses .
We detect an optically-faint line emitter situated between the two LBGs which we identify as warm molecular gas at z = 5.1245 \pm 0.0001 .
This source , detected in the CO ( 2-1 ) transition but undetected in CO ( 1-0 ) , has an integrated line flux of 0.106 \pm 0.012 Jy km s ^ { -1 } , yielding an inferred gas mass M ( H _ { 2 } ) = ( 1.9 \pm 0.2 ) \times 10 ^ { 10 } M _ { \odot } .
Molecular line emitters without detectable counterparts at optical and infrared wavelengths may be crucial tracers of structure and mass at high redshift .