Warm molecular gas in the primeval galaxy IRAS 10214 + 4724

We observed CO(6 - 5) and CO(3 - 2) emission at z = 2.286 from the extremely luminous infrared galaxy IRAS 10214+4724. We show that the CO(3 - 2) line is always comparable in strength to the 1-0 line, in galaxies at high redshift, due to the warmer microwave background. The CO(6 - 5) line traces warm molecular gas, which is found in the Milky Way only in regions of high-mass star formation. The molecular gas in 10214+4724 is warm and dense with n(H_2_) ~ 5000 cm^-3^ if T_kin_ = 50 K. The total mass of molecular hydrogen is M(H_2_) = 1 x 10^11^ h^-2^ M_sun_. Although this is about 200 times less mass than initially estimated by Brown & VandenBout (1991), it is still 3-5 times larger than the molecular mass of any other galaxy yet observed and 60 h^-2^ more than the H_2_ mass of the Milky Way. A starburst can power 10214+4724 only if predominantly high-mass stars are produced. This extraordinary primeval galaxy has most of its mass in molecular gas, has already collapsed, and is undergoing an extreme starburst that is generating metals in abundances close to solar.