Merger events are thought to be an important phase in the assembly of massive galaxies. At the same time, active galactic nuclei (AGN) play a fundamental role in the evolution of their star formation histories. Both phenomena can be observed at work in NGC 6240, a local prototypical merger classified as an ultraluminous infrared galaxy (ULIRG) thanks to its elevated infrared luminosity. Interestingly, NGC 6240 hosts two AGN separated by 1.5″(∼735 pc), detected in both X-ray and radio band. Taking advantage of the unprecedented sensitivity and wavelength coverage provided by the integral field unit (IFU) of the NIRSpec instrument on board JWST, we observed the nuclear region of NGC 6240 in a field of view of 3.7″ × 3.7″(1.9 × 1.9 kpc2) in order to investigate gas kinematics and interstellar medium (ISM) properties with a high spatial resolution of ∼0.1″ (or ∼50 pc). We characterized the 2D stellar kinematics, separated the different gas kinematic components through multi-Gaussian fitting, and studied the excitation properties of the ISM from the near-infrared diagnostic diagram based on the H2 1-0 S(1)/Brγ and [Fe II]λ1.257 μm/Paβ lines ratios. We isolated the ionization cones of the two nuclei and detected coronal line emission from both of them. Using H2 line ratios, we found that the molecular hydrogen gas is excited mostly by thermal processes. We computed a hot molecular gas mass of 1.3× 105 M⊙ and an ionized gas mass in the range of 105-107 M⊙, depending on assumptions. We studied with unprecedented spatial resolution and sensitivity the kinematics of the molecular and ionized gas phases, and we revealed the complex structure of the molecular gas and found a blueshifted outflow near the southern nucleus, together with filaments connecting a highly redshifted H2 cloud with the two nuclei. We speculate on the possible nature of this H2 cloud and propose two possible scenarios: outflowing gas or a tidal cloud falling onto the nuclei.
The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240 / Ceci, M.; Cresci, G.; Arribas, S.; Böker, T.; Bunker, A. J.; Charlot, S.; Fahrion, K.; Lamperti, I.; Marconi, A.; Perna, M.; Tozzi, G.; Ulivi, L.. - In: ASTRONOMY & ASTROPHYSICS. - ISSN 0004-6361. - ELETTRONICO. - 695:(2025), pp. A116.0-A116.0. [10.1051/0004-6361/202452207]
The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240
Ceci, M.;Cresci, G.;Lamperti, I.;Marconi, A.;Tozzi, G.;Ulivi, L.
2025
Abstract
Merger events are thought to be an important phase in the assembly of massive galaxies. At the same time, active galactic nuclei (AGN) play a fundamental role in the evolution of their star formation histories. Both phenomena can be observed at work in NGC 6240, a local prototypical merger classified as an ultraluminous infrared galaxy (ULIRG) thanks to its elevated infrared luminosity. Interestingly, NGC 6240 hosts two AGN separated by 1.5″(∼735 pc), detected in both X-ray and radio band. Taking advantage of the unprecedented sensitivity and wavelength coverage provided by the integral field unit (IFU) of the NIRSpec instrument on board JWST, we observed the nuclear region of NGC 6240 in a field of view of 3.7″ × 3.7″(1.9 × 1.9 kpc2) in order to investigate gas kinematics and interstellar medium (ISM) properties with a high spatial resolution of ∼0.1″ (or ∼50 pc). We characterized the 2D stellar kinematics, separated the different gas kinematic components through multi-Gaussian fitting, and studied the excitation properties of the ISM from the near-infrared diagnostic diagram based on the H2 1-0 S(1)/Brγ and [Fe II]λ1.257 μm/Paβ lines ratios. We isolated the ionization cones of the two nuclei and detected coronal line emission from both of them. Using H2 line ratios, we found that the molecular hydrogen gas is excited mostly by thermal processes. We computed a hot molecular gas mass of 1.3× 105 M⊙ and an ionized gas mass in the range of 105-107 M⊙, depending on assumptions. We studied with unprecedented spatial resolution and sensitivity the kinematics of the molecular and ionized gas phases, and we revealed the complex structure of the molecular gas and found a blueshifted outflow near the southern nucleus, together with filaments connecting a highly redshifted H2 cloud with the two nuclei. We speculate on the possible nature of this H2 cloud and propose two possible scenarios: outflowing gas or a tidal cloud falling onto the nuclei.
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