Spatially resolved cold molecular outflows in ULIRGs

We present new CO(2–1) observations of three low-z (d ∼ 350 Mpc) ultra-luminous infrared galaxy (ULIRG) systems (six nuclei) observed with the Atacama large millimeter/submillimeter array (ALMA) at high spatial resolution (∼500 pc). We detect massive cold molecular gas outflows in five out of six nuclei (Mout ∼ (0.3−5) ×108 M). These outflows are spatially resolved with deprojected effective radii between 250 pc and 1 kpc although high-velocity molecular gas is detected up to Rmax ∼ 0.5−1.8 kpc (1−6 kpc deprojected). The mass outflow rates are 12−400 M yr−1 and the inclination corrected average velocity of the outflowing gas is 350−550 km s−1 (vmax = 500−900 km s−1). The origin of these outflows can be explained by the strong nuclear starbursts although the contribution of an obscured active galactic nucleus cannot be completely ruled out. The position angle (PA) of the outflowing gas along the kinematic minor axis of the nuclear molecular disk suggests that the outflow axis is perpendicular to the disk for three of these outflows. Only in one case is the outflow PA clearly not along the kinematic minor axis, which might indicate a different outflow geometry. The outflow depletion times are 15−80 Myr. These are comparable to, although slightly shorter than, the star-formation (SF) depletion times (30−80 Myr). However, we estimate that only 15−30% of the outflowing molecular gas will escape the gravitational potential of the nucleus. The majority of the outflowing gas will return to the disk after 5−10 Myr and become available to form new stars. Therefore, these outflows will not likely completely quench the nuclear starbursts. These star-forming powered molecular outflows would be consistent with being driven by radiation pressure from young stars (i.e., momentum-driven) only if the coupling between radiation and dust increases with increasing SF rates. This can be achieved if the dust optical depth is higher in objects with higher SF. This is the case in at least one of the studied objects. Alternatively, if the outflows are mainly driven by supernovae (SNe), the coupling efficiency between the interstellar medium and SNe must increase with increasing SF levels. The relatively small sizes (
L.C., S.G.B., and A.L. acknowledge financial support by the Spanish MEC under grants ESP2015-68964 and AYA2016-76682-C3-2-P.