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

Abstract

We present deep observations of the 12CO (1−0) and (3−2) lines in the ultra-luminous infrared and radio galaxy 4C 12.50, carried out with the 30 m telescope of the Institut de Radioastronomie Millimétrique. Our observations reveal the cold molecular gas component of a warm molecular gas outflow that was previously known from Spitzer Space Telescope data. The 12CO(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 H i 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 ≥65 K assuming that the gas is thermalized. If the molecular clouds fully obscure the background millimeter continuum that is emitted by the radio core, the H2 column density is ≥1.8 × 1022 cm-2. The outflow then carries an estimated cold H2 mass of at least 4.2 × 103 M⊙ 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 × 109 M⊙, the outflow would most likely be seen in emission. Since the ambient cold gas reservoir of 4C 12.50 is 1.0 × 1010 M⊙, the outflowing-to-ambient mass ratio of the warm gas (37%) could be elevated with respect to that of the cold gas.