Outflow of hot and cold molecular gas from the obscured secondary nucleus of NGC?3256: closing in on feedback physics

摘要

The nuclei of merging galaxies are often deeply buried in dense layers of gas and dust. In these regions, gas outflows driven by starburst and active galactic nuclear activity are believed to play a crucial role in the evolution of these galaxies. However, to fully understand this process it is essential to resolve the morphology and kinematics of such outflows. Using near-infrared integral-field spectroscopy obtained with SINFONI on the Very Large Telescope, we detect a kpc-scale structure of high-velocity molecular hydrogen (H2) gas associated with the deeply buried secondary nucleus of the infrared-luminous merger-galaxy NGC?3256. We show that this structure is most likely the hot component of a molecular outflow, which was recently also detected in the cold molecular gas through CO emission. This outflow, with a total molecular gas mass of MH2 ~ 2 × 107M⊙, is among the first to be spatially resolved in both the hot molecular H2 gas with VLT/SINFONI and the cold molecular CO emitting gas with ALMA. The hot and cold components share a similar morphology and kinematics, with a hot-to-cold molecular gas mass ratio of ~ 6 × 10-5. The high (~100 pc) resolution at which we map the geometry and velocity structure of the hot outflow reveals a biconical morphology with opening angle ~40° and gas spread across a FWZI?~?1200 km?s-1. Because this collimated outflow is oriented close to the plane of the sky, the molecular gas may reach maximum intrinsic outflow velocities of ~1800 km?s-1, with an average mass outflow rate of at least ?outfl ~ 20 M⊙?yr-1. By modeling the line-ratios of various near-infrared H2 transitions, we show that the H2-emitting gas in the outflow is heated through shocks or X-rays to a temperature of T ~ 1900 ± 300?K. The energy needed to drive the collimated outflow is most likely provided by a hidden Compton-thick AGN or by the nuclear starburst. We show that the global kinematics of the molecular outflow that we detect in NGC?3256 mimic those of CO-outflows that have been observed at much lower spatial resolution in starburst- and active galaxies.