A search for molecular gas in a high - redshift radio galaxyR

There is reason to suspect that 4C 41.17, a radio galaxy at z = 3:800 0:003, contains a large quantity of enriched molecular gas. We describe a search for CO in 4C 41.17, taking advantage of the fact that the J = 1 ! 0 rotational transition is redshifted into the radio-frequency band centred at 23 GHz for 3:80 < z < 4:25. There is no sign of CO line emission in our resulting spectrum, which covers 2000kms ?1 (or 3:785 < z < 3:821), nor in the 78-kms ?1-wide channel maps. Our upper limit for the integrated line intensity is S l < 0:17Jy kms ?1 , for a line width of 500 kms ?1. Naively assuming a Galactic conversion factor, this places a limit of around 4 10 11 M on the mass of molecular hydrogen, which is similar to the stellar mass of a present-day giant galaxy, and consistent with gas-to-dust ratios as high as 4400.