Metallicity in the merger Seyfert galaxy NGC 6240

TL;DR

This study models the physical conditions of the narrow-line region in NGC 6240, revealing the roles of AGN and starburst activity, shock velocities, and elemental abundances, with implications for understanding galaxy merger environments.

Contribution

It provides a detailed analysis of the ionization sources, shock conditions, and elemental abundances in NGC 6240, highlighting the interplay between AGN activity and starburst-driven processes.

Findings

Optical spectra originate from AGN-photoionized clouds heated by shocks.

Infrared line ratios come from starburst-ejected clouds with black-body radiation.

Elemental abundances are higher than solar, likely due to H2 trapping.

Abstract

We have calculated the physical conditions throughout the NLR of the merger Seyfert galaxy NGC 6240 by modelling the observed optical and infrared line ratios. We have found that the optical spectra are emitted by clouds photoionised by the power-law radiation flux from the AGN (or AGNs), and heated mainly by the shock accompanying the propagation of the clouds outwards. The infrared line ratios are emitted from clouds ejected from a starburst which photoionises the gas by the black-body radiation flux corresponding to a stellar colour temperature of about 50,000 K. Both the flux from the AGN and the ionization parameters are low. The most characteristic physical parameters are the relatively high shock velocities (>400 km/s) and low preshock densities (about 40-60 cm-3) of the gas. The C/H, N/H, O/H relative abundances are higher than solar by a factor lower or about 1.5. We suggest that those high relative abundances indicate trapping of H into H2 molecules rather than high metallicities. Adopting an initial grain radius of 1 micron, the dust temperatures calculated in the clouds reached by the power-law radiation flux and by the black-body radiation flux are 81 K and 68 K, respectively.