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

¹ Centro de Astrobiología (INTA-CSIC), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: bjornemonts@gmail.com
² Istituto di Astrofisica e Planetologia Spaziali (INAF), Via Fosso del Cavaliere 100, 00133 Roma, Italy
³ Observatorio Astronómico Nacional (OAN), Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
⁴ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain

Received: 13 March 2014
Accepted: 29 August 2014

Abstract

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 M_H2 ~ 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 Ṁ_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.