2012ApJ...751L..24W

We have used the Green Bank Telescope to carry out a deep search for redshifted CO J = 2-1 line emission from an extended (>17 kpc) Lyα blob (LAB), "Himiko," at z ∼ 6.595. Our non-detection of CO J = 2-1 emission places the strong 3σ upper limit of L'_CO< 1.8x10¹⁰x(ΔV/250)^1/2 K km/s.pc2 on the CO line luminosity. This is comparable to the best current limits on the CO line luminosity in LABs at z ∼ 3 and lower-luminosity Lyα emitters at z ≳ 6.5. High-z LABs appear to have lower CO line luminosities than the host galaxies of luminous quasars and submillimeter galaxies at similar redshifts, despite their high stellar mass. Although the CO-to-H₂ conversion factor is uncertain for galaxies in the early universe, we assume X_CO= 0.8 M_☉(K km/s.pc2)^–1 to obtain the limit M(H₂) <1.4x10¹⁰ M_☉ on Himiko's molecular gas mass; this is a factor of ≳ 2.5 lower than the stellar mass in the z ∼ 6.595 LAB.