A deep search for CO J=2-1 emission from a Lyman-Alpha blob at z~6.595

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) Ly-Alpha blob (LAB), "Himiko", at z~6.595. Our non-detection of CO J=2-1 emission places the strong 3-sigma upper limit of L'_CO < 1.8x10^10 x sqrt(dV/250) K km/s pc^2 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 Lyman-Alpha emitters (LAEs) at z>~6.5. High-z LABs appear to have lower CO line luminosities than the host galaxies of luminous quasars and sub-mm 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 Msun (K km/s pc^2)^-1 to obtain the limit M(H_2) < 1.4 x 10^10 Msun on Himiko's molecular gas mass; this is a factor of >2.5 lower than the stellar mass in the z~6.595 LAB.