Outflow of hot and cold molecular gas from the obscured secondary
  nucleus of NGC3256: closing in on feedback physics
  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 AGN 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-IR integral-field
spectroscopy obtained with VLT/SINFONI, we detect a kpc-scale structure of
high-velocity molecular hydrogen (H2) gas associated with the deeply buried
secondary nucleus of the IR-luminous merger NGC3256. We show that this
structure is likely the hot component of a molecular outflow, which is detected
also in the cold molecular gas by Sakamoto et al. This outflow, with a
molecular gas mass of M(H2)~2x10^7 Msun, is among the first to be spatially
resolved in both the hot H2 gas with VLT/SINFONI and the cold 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 ~6x10^-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 deg and gas
spread across a FWZI~1200 km/s. 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, with an average mass outflow rate of at least
~20 Msun/yr. By modeling the line-ratios of various near-IR H2 transitions, we
show that the H2 gas in the outflow is heated through shocks or X-rays to a
temperature of ~1900K. The energy needed to drive the outflow is likely
provided by a hidden Compton-thick AGN or by the nuclear starburst. We show
that the global kinematics of the molecular outflow in NGC3256 mimic those of
CO-outflows that have been observed at low spatial resolution in starburst- and
active galaxies.
