2009ApJ...697L..33W

We present results from a sensitive search for CO J = 1-0 line emission in two z > 6.5 Lyman-α emitters (LAEs) with the Green Bank Telescope. CO J = 1-0 emission was not detected from either object. For HCM 6A, at z ∼ 6.56, the lensing magnification factor of ∼4.5 implies that the CO nondetection yields stringent constraints on the CO J = 1-0 line luminosity and molecular gas mass of the LAE, L'_CO< 6.1x10⁹x(ΔV/300)^1/2 K km/s pc2 and. These are the strongest limits obtained so far for a z ≳ 6 galaxy. For IOK-1, the constraints are somewhat less sensitive, L'_CO< 2.3x10¹⁰ x(ΔV/300)^1/2 K km/s pc2 and. The nondetection of CO J = 1-0 emission in HCM 6A, whose high estimated star formation rate, dust extinction, and lensing magnification make it one of the best high-z LAEs for such a search, implies that typical z ≳ 6 LAEs are likely to have significantly lower CO J = 1-0 line luminosities than massive submillimeter galaxies and hyperluminous infrared quasars at similar redshifts, due to either a significantly lower molecular gas content or a higher CO-to-H₂ conversion factor.