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The origin of the near-IR line emission from molecular, low and high ionization gas in the inner kiloparsec of NGC 6240


The understating of the origin of the \(\mbox{H}_{2}\) line emission from the central regions of galaxies represents 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 Starburst, 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 \(\mbox{H}_{2}\) 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 \(1.4\times2.4~\mbox{kpc}^{2}\) of the LIRG NGC 6240, known to be the galaxy with strongest \(\mbox{H}_{2}\) 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 \(1^{\prime\prime}\) west of the northern nucleus, they can be reproduced by a single Gaussian component. We found that the \(\mbox{H}_{2}\) 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 \(\mbox{H}_{2}\,\lambda2.12~\upmu \mbox{m}/\mbox{Br}\gamma\) line ratio. A contribution of fluorescent excitation may also be important at locations near \(1^{\prime\prime}\) west of the northern nucleus, which show the lowest line ratios. The [Fe ii]\(\lambda2.072~\upmu \mbox{m}/\mbox{Br}\gamma\) ratio show a similar trend as observed for \(\mbox{H}_{2}\,\lambda2.12~\upmu \mbox{m}/\mbox{Br}\gamma\), suggesting that [Fe ii] and \(\mbox{H}_{2}\) line emission have similar origins. Finally, the [Ca viii]\(\lambda2.32~\upmu \mbox{m}\) coronal line emission is observed mainly in regions next to the nuclei, suggesting it is originated gas ionized by the radiation from the AGN.

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We thank the referee for his/her thorough review, comments and suggestions, which helped us to significantly improve this paper. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil) and south-eastCYT (Argentina). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The authors acknowledges support from FAPERGS (project No. 2366-2551/14-0) and CNPq (project Nos. 470090/2013-8 and 302683/2013-5).

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Correspondence to Gabriele da Silva Ilha.

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Ilha, G.d.S., Bianchin, M. & Riffel, R.A. The origin of the near-IR line emission from molecular, low and high ionization gas in the inner kiloparsec of NGC 6240. Astrophys Space Sci 361, 178 (2016).

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