The origin of the near-IR line emission from molecular, low and high ionization gas in the inner kiloparsec of NGC6240

TL;DR

This study uses K-band integral field spectroscopy to analyze the origin of molecular and ionized gas emission in NGC6240, revealing thermal processes, shocks, and AGN radiation as key excitation mechanisms in different regions.

Contribution

It provides detailed spatial mapping of emission lines in NGC6240, clarifying the roles of shocks, X-ray heating, and AGN radiation in gas excitation within a luminous infrared galaxy.

Findings

H2 emission mainly from thermal processes, possibly X-ray heating by AGN.

Shocks likely dominate between the nuclei, indicated by high line ratios.

[FeII] and H2 emissions share similar origins, with [CaVIII] linked to AGN ionization.

Abstract

The understating of the origin of the H2 line emission from the central regions of galaxies represent an important key to improve our knowledge about the excitation and ionization conditions of the gas in these locations. Usually these lines can be produced by Starburts, shocks and/or radiation from an active galactic nucleus (AGN). Luminous Infrared Galaxies (LIRG) represent ideal and challenging objects to investigate the origin of the H2 emission, as all processes above can be observed in a single object. In this work, we use K-band integral field spectroscopy to map the emission line flux distributions and kinematics and investigate the origin of the molecular and ionized gas line emission from inner 1.4x2.4 kpc2 of the LIRG NGC6240, known to be the galaxy with strongest H2 line emission. The emission lines show complex profiles at locations between both nuclei and surrounding the northern nucleus, while at locations near the southern nucleus and at 1" west of the northern nucleus, they can be reproduced by a single gaussian component. We found that the H2 emission is originated mainly by thermal processes, possible being dominated by heating of the gas by X-rays from the AGN at locations near both nuclei. For the region between the northern and southern nuclei shocks due to the interacting process may be the main excitation mechanism, as indicated by the high values of the H2l2.12um/Brg line ratio. A contribution of fluorescent excitation may also be important at locations near 1" west of the northern nucleus, which show the lowest line ratios. The [FeII]l2.072um/Brg ratio show a similar trend as observed for H2l2.12um/Brg, suggesting that [FeII] and H2 line emission have similar origins. Finally, the [CaVIII]l2.32um coronal line emission is observed mainly in regions next to the nuclei, suggesting it is originated gas ionized by the radiation from the AGN.