Cold and warm molecular gas in the outflow of 4C 12.50

We present deep observations of the (CO)-C-12(1-0) and (3-2) lines in the ultra-luminous infrared and radio galaxy 4C 12.50, carried out with the 30m telescope of the Institut de Radioastronomie Millimetrique. Our observations reveal the cold molecular gas component of a warm molecular gas outflow that was previously known from Spitzer Space Telescope data. The (CO)-C-12(3-2) profile indicates the presence of absorption at -950 km s(-1) from systemic velocity with a central optical depth of 0.22. Its profile is similar to that of the Hi absorption that was seen in radio data of this source. A potential detection of the 0 -> 1 absorption enabled us to place an upper limit of 0.03 on its central optical depth, and to constrain the excitation temperature of the outflowing CO gas to >= 65K assuming that the gas is thermalized. If the molecular clouds fully obscure the background millimeter continuum that is emitted by the radio core, the H-2 column density is >= 1.8 x 10(22) cm(-2). The outflow then carries an estimated cold H-2 mass of at least 4.2 x 10(3) M-circle dot along the nuclear line of sight. This mass will be even higher when integrated over several lines of sight, but if it were to exceed 3 x 10(9) M-circle dot, the outflow would most likely be seen in emission. Since the ambient cold gas reservoir of 4C 12.50 is 1.0 x 10(10) M-circle dot, the outflowing-to-ambient mass ratio of the warm gas (37%) could be elevated with respect to that of the cold gas.