Disentangling AGN and Star Formation in Soft X-rays

TL;DR

This study disentangles the contributions of star formation and AGN activity to soft X-ray emission in Seyfert 2 galaxies using spectral modeling, imaging analysis, and infrared line proxies, providing a method for cases without resolvable nuclear emission.

Contribution

It introduces a combined spectral and infrared line analysis approach to separate star formation and AGN contributions in soft X-ray emissions of Seyfert 2 galaxies, including a validation with Monte Carlo simulations.

Findings

Spectral modeling and imaging analysis yield consistent luminosity estimates within 3σ.

Infrared line proxies [NeII] and [OIV] effectively disentangle star formation and AGN contributions.

The method provides reasonable estimates of star formation emission even without nuclear resolution.

Abstract

We have explored the interplay of star formation and AGN activity in soft X-rays (0.5-2 keV) in two samples of Seyfert 2 galaxies (Sy2s). Using a combination of low resolution CCD spectra from Chandra and XMM-Newton, we modeled the soft emission of 34 Sy2s using power law and thermal models. For the 11 sources with high signal-to-noise Chandra imaging of the diffuse host galaxy emission, we estimate the luminosity due to star formation by removing the AGN, fitting the residual emission. The AGN and star formation contributions to the soft X-ray luminosity (i.e. L$_{x,AGN}$ and L$_{x,SF}$) for the remaining 24 Sy2s were estimated from the power law and thermal luminosities derived from spectral fitting. These luminosities were scaled based on a template derived from XSINGS analysis of normal star forming galaxies. To account for errors in the luminosities derived from spectral fitting and the spread in the scaling factor, we estimated L$_{x,AGN}$ and L$_{x,SF}$ from Monte Carlo simulations. These simulated luminosities agree with L$_{x,AGN}$ and L$_{x,SF}$ derived from Chandra imaging analysis within a 3\sigma\ confidence level. Using the infrared [NeII]12.8\mu m and [OIV]26\mu m lines as a proxy of star formation and AGN activity, respectively, we independently disentangle the contributions of these two processes to the total soft X-ray emission. This decomposition generally agrees with L$_{x,SF}$ and L$_{x,AGN}$ at the 3\sigma\ level. In the absence of resolvable nuclear emission, our decomposition method provides a reasonable estimate of emission due to star formation in galaxies hosting type 2 AGN.