The Molecular Gas Content Of z > 6.5 Lyman-α Emitters

Abstract

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 < 6.1 × 109 × (ΔV/300)1/2 K km s−1 pc2 and $M_{\rm H_2} < 4.9 \times 10^{9} \times (\Delta V/300)^{1/2} \times (X_{\rm CO}/0.8) \: M_\odot$. These are the strongest limits obtained so far for a z gsim 6 galaxy. For IOK-1, the constraints are somewhat less sensitive, L'CO < 2.3 × 1010 × (ΔV/300)1/2 K km s−1 pc2 and $M_{\rm H_2} < 1.9 \times 10^{10} \times (\Delta V/300)^{1/2} \times (X_{\rm CO}/0.8) \: M_\odot$. 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 gsim 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.