Constraining the molecular gas reservoir associated with the protogalaxy candidate MS1512-cB58

We report on sensitive searches for molecular CO(3-2) emission from the recently discovered protogalaxy candidate MS1512-cB58 (cB58) at a redshift of z = 2.72. The rest-frame UV light from cB58 indicates that a large burst of star formation (M_stars∼4 x 10¹⁰h^-2₇₅M⊙) has occurred within less than 10⁸ yr. The limit on the CO line intensity is S(CO)<0.38 Jy km s^-1 which corresponds to a molecular gas mass of M(H₂)<2.7 x 10¹⁰h^-2₇₅M⊙ (q₀ = 0.5), assuming the Galactic CO to H₂ conversion factor. Although it is still uncertain whether cB58 is significantly gravitationally lensed by a foreground cluster at z = 0.37, the ratio of the UV luminosity to the CO luminosity may be nearly independent of lensing since the UV light appears to be extended. Assuming similar lensing magnification factors for both the UV and CO emission, we find the L_λ(UV)/L′(CO) luminosity ratio for cB58 is at least 20 times larger than that for typical nearby starburst galaxies. The high L_λ(UV)/L′(CO) ratio for cB58 may be indicative of a low metallicity and an IMF biased toward massive star formation or an intrinsically high star-formation efficiency. We also find that the system has consumed a significant fraction of its gas (M_gas/M_total<0.8), and the gas which has not already formed stars may be ejected by supernova-driven winds.