ALMA observations of HCN and HCO+ outflows in the merging galaxy NGC 3256

TL;DR

This study uses ALMA to observe molecular outflows in the merging galaxy NGC 3256, revealing a two-phase outflow likely driven by jet-ISM interactions and differing between nuclei with and without AGN activity.

Contribution

First detailed ALMA observations of multiple molecular lines in NGC 3256's outflows, identifying a two-phase outflow structure and its possible origin.

Findings

High-density outflow co-located with southern nucleus AGN activity.

Line ratios indicate a two-phase (diffuse and clumpy) outflow.

Dense gas fraction increases with velocity offset.

Abstract

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.