ALMA observations of HCN and HCO+ outflows in the merging galaxy NGC
  3256
  We report ~2" resolution Atacama Large Millimeter/submillimeter Array
observations of the HCN(1-0), HCO+(1-0), CO(1-0), CO(2-1), and CO(3-2) lines
towards the nearby merging double-nucleus galaxy NGC 3256. We find that the
high density gas outflow traced in HCN(1-0) and HCO+(1-0) emission is
co-located with the diffuse molecular outflow emanating from the southern
nucleus, where a low-luminosity active galactic nucleus (AGN) is believed to be
the dominant source of the far-infrared luminosity. On the other hand, the same
lines were undetected in the outflow region associated with the northern
nucleus, whose primary heating source is likely related to starburst activity
without obvious signs of AGN. Both HCO+(1-0)/CO(1-0) line ratio (i.e. dense gas
fraction) and the CO(3-2)/CO(1-0) line ratio are larger in the southern outflow
(0.20$\pm$0.04 and 1.3$\pm$0.2, respectively) than in the southern nucleus
(0.08$\pm$0.01, 0.7$\pm$0.1, respectively). By investigating these line ratios
for each velocity component in the southern outflow, we find that the dense gas
fraction increases and the CO(3-2)/CO(1-0) line ratio decreases towards the
largest velocity offset. This suggests the existence of a two-phase (diffuse
and clumpy) outflow. One possible scenario to produce such a two-phase outflow
is an interaction between the jet and the interstellar medium, which possibly
triggers shocks and/or star formation associated with the outflow.
