The obscured nucleus of NGC3256 S. I. Anisotropic AGN near-IR radiation field traced by CO v=1-0 with JWST/NIRSpec

Abstract

The southern nucleus of NGC3256, a nearby luminous infrared (IR) interacting system (d=40 Mpc; L(IR)=10^11.6 Lsun), hosts a deeply obscured low-luminosity active galactic nucleus (AGN) associated with a massive and fast bipolar collimated molecular outflow. JWST/NIRSpec observations revealed bright spatially resolved ro-vibrational CO v=1-0 4.67um emission at the base of this molecular outflow extending out to ~300pc, which is excited by the ~4.7um IR radiation from the southern dusty core. Using Monte Carlo radiative transfer (MCRT) models, we analyzed the CO v=1-0 band emission to determine the intensity of the ~4.7um IR radiation field across the outflow base. We find that the core spectral luminosity density at ~4.7um, as seen by the material in the outflow base, is ~2-4 times higher than that inferred along our line of sight, indicating an anisotropic near-IR radiation field. Moreover, the slower than d^-2 decrease of the near-IR flux density across the outflow base suggests a collimated radiation field, for which we estimate a small half-opening angle (15-20deg) and a diameter of the collimating structure of 15 +- 5 pc. This is slightly smaller than the diameter of the obscuring envelope (25 +- 5 pc). Combined with the expansion of this obscuring envelope, traced by the blueshifted CO v=1-0 absorption toward the nucleus, this suggests that the AGN is beginning to clear its environment. We estimate N_H~10^22-23 cm^-2 for NGC 3256 S, which is similar to that of Type 2 AGN. However, in contrast to Type 2 AGN, a high covering factor of the obscuring material, possibly sustained by inflows linked to the galaxy merger, and similar to that expected in deeply obscured compact obscured nuclei (CONs), is needed to explain the absence of extended high-ionization emission. Therefore, NGC 3256 S shares properties with CONs and Type 2 AGN.