Limits on the molecular gas content of z ~ 5 LBGs

We present limits on the molecular gas content of Lyman break galaxies (LBGs) at z ~ 5 from observations targeting redshifted CO(1-0) and CO(2-1) line emission. We observed a single field containing eight spectroscopically confirmed z ~ 5 LBGs, seven of which are contained within a narrow (z = 4.95 ± 0.08) redshift range and the eighth is at z = 5.2. No source was individually detected. Assuming the CO to H2 conversion factor for vigorous starbursts, we place upper limits on the molecular gas content of individual z ~ 5 LBGs of M(H₂) ≲ 10¹⁰ M_⊙. From a stacking analysis combining all of the non-detections, the typical z ~ 5 LBG has an H₂ mass limit comparable to their stellar mass, <3.1 × 10⁹ M_⊙. This limit implies that, given the star formation rates of these systems (measured from their UV emission), star formation could be sustained for at most ~100Myr, similar to the typical ages of their stellar populations. The lack of a substantially larger reservoir of cold gas argues against the LBGs being UV-luminous superstarbursts embedded in much larger UV-dark systems and as a result increases the likelihood that at least those LBGs with multiple components are starbursts triggered by mergers. The sources responsible for re-ionization are expected to be starbursts similar to these systems, but with lower luminosities, masses and consequently with star formation time-scales far shorter than the recombination time-scale. If so, the ionized bubbles expected in the IGM during the re-ionization era will only infrequently have UV-luminous sources at their centres.