Searching for molecular gas in a radio galaxy at redshift 3.8

There is reason to suspect that 4C 41.17, a radio galaxy at z = 3.8, contains a large quantity of enriched molecular gas. We have searched for CO in 4C 41.17, taking advantage of the fact that the J = 1-0 transition is redshifted into the K radio band for 3.80 < z < 4.25. There is no sign of CO line emission in our resulting spectrum (which covers 2000 km s-1, or 3.785 < z < 3.821), nor in the channel maps. Our limit for the integrated line intensity is SCOΔv < 0.17 Jy km s-1, for a line width of 500km s-1. Naively assuming a Galactic conversion factor gives M(H2) < 4 × 1011 M⊙, similar to the stellar mass of a present-day giant galaxy, and consistent with gas-to-dust ratios as high as 4400