Near- to Mid- Infrared Imaging and Spectroscopy of Two Buried AGNs of the Nearby Merging Galaxy NGC 6240 with Subaru/IRCS+AO and GTC/CanariCam

TL;DR

This study uses high-resolution infrared imaging and spectroscopy to analyze the buried AGNs in NGC 6240, revealing complex nuclear structures and estimating the AGN's luminosity with advanced modeling.

Contribution

It provides new high-resolution infrared observations and applies clumpy torus models with Bayesian fitting to characterize the buried AGNs in NGC 6240.

Findings

The two nuclei show complex geometry with double peaks in K' band and single compact structures in others.

The southern nucleus's AGN has an estimated bolometric luminosity of about 10^45 erg/s.

The high covering factor torus is obscured by foreground dust, affecting the observed emission.

Abstract

We report near-infrared K', L', and M' band imaging observations of the nearby merging galaxy NGC 6240 with the Infrared Camera and Spectrograph on the Subaru telescope. The observations were performed with the assistance of the Subaru Adaptive Optics System, and the achieved spatial resolutions were around 0.10--0.20$^{\prime\prime}$. We also obtained new mid-infrared imaging in the Si-2 filter band (8.7$\mu$m) and N-band (7.5--13$\mu$m) spectroscopy of this galaxy with the CanariCam on the Gran Telescopio Canarias with a spatial resolution of 0.4--0.5$^{\prime\prime}$. In the K' band image the two nuclei of the galaxy each show a double-peak suggesting the complex geometry of the source, while the L', M', and Si-2 band images show single compact structures in each of the two nuclei. Assuming that the center core observed at wavelengths longer than 3.8$\mu$m is associated with dust heated by the buried AGN, we re-evaluated the spectral energy distributions (SEDs) of the southern nucleus from 2 to 30$\mu$m with the additional literature values, and performed the SED+spectroscopy fitting using the clumpy torus models of Nenkova et al. (2008) and a Bayesian fitting approach. The model fit suggests that the high covering factor torus emission in the southern nucleus is also obscured by foreground dust in the host galaxy. The estimated AGN bolometric luminosity of the southern nucleus, $L_{\rm bol}({\rm AGN})\sim1\times10^{45}$ [erg$\cdot$s$^{-1}$], accounts for approximately 40% of the whole luminosity of the system.