Outflows of hot molecular gas in ultra-luminous infrared galaxies mapped with VLT-SINFONI

We present the detection and morphological characterization of hot molecular gas outflows in nearby ultra-luminous infrared galaxies (ULIRGs), using the Spectrograph for Integral Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope (VLT). We detect outflows observed in the 2.12 μm H₂ 1-0 S(1) line for three out of four ULIRGs that we analyzed, namely IRAS 12112+0305, IRAS 14348-1447, and IRAS 22491-1808. The outflows are mapped on scales of 0.7-1.6 kpc, show typical outflow velocities of 300-500 km s^-1, and appear to originate from the nuclear region. The outflows comprise hot molecular gas masses of M_{H2 (hot)} 6-8 ×10³ M_⊙. Assuming a hot-to-cold molecular gas mass ratio of 6 × 10^-5, as found in nearby luminous infrared galaxies, the total (hot + cold) molecular gas mass in these outflows is expected to be M_{H2 (tot)} 1 × 10⁸ M_⊙. This translates into molecular mass outflow rates of Ṁ_{H2 (tot)} 30-85 M_⊙ yr^-1, which is a factor of a few lower than the star formation rate in these ULIRGs. In addition, most of the outflowing molecular gas does not reach the escape velocity of these merger systems, which implies that the bulk of the outflowing molecular gas is re-distributed within the system and thus remains available for future star formation. The fastest H₂ outflow is seen in the Compton-thick AGN of IRAS 14348-1447, reaching a maximum outflow velocity of 900 km s^-1. Another ULIRG, IRAS 17208-0014, shows asymmetric H₂ line profiles different from the outflows seen in the other three ULIRGs. We discuss several alternative explanations for the line asymmetries in this system, including a very gentle galactic wind, internal gas dynamics, low-velocity gas outside the disk, or two superposed gas disks. We do not detect the hot molecular counterpart to the outflow previously detected in CO(2-1) in IRAS 17208-0014, but we note that our SINFONI data are not sensitive enough to detect this outflow if it has a small hot-to-cold molecular gas mass ratio of ≲9 × 10^-6.