CO line emission in the halo of a radio galaxy at z=2.6

We report the detection of luminous CO(3-2) line emission in the halo of the z = 2.6 radio galaxy (HzRG) TXS0828+193, which has no detected counterpart at optical to mid-infrared wavelengths implying a stellar mass less than or similar to few x 10(9) M(circle dot) and relatively low star formation rates. With the IRAM Plateau de Bure Interferometer (PdBI), we find two CO emission-line components at the same position at similar to 80 kpc distance from the HzRG along the axis of the radio jet, with different blueshifts of few 100 km s(-1) relative to the HzRG and a total luminosity of similar to 2 x 10(10) K km s(-1) pc(2) detected at a total significance of similar to 8 sigma. HzRGs have significant galaxy overdensities and extended haloes of metal-enriched gas often with embedded clouds or filaments of denser material, and likely trace very massive dark matter haloes. The CO emission may be associated with a gas-rich, low-mass satellite galaxy with very little ongoing star formation, in contrast to all previous CO detections of galaxies at similar redshifts. Alternatively, the CO may be related to a gas cloud or filament and perhaps jet-induced gas cooling in the outer halo, somewhat in analogy with extended CO emission found in low-redshift galaxy clusters.