An XMM-Newton spectral survey of 12 micron selected galaxies. I. X-ray data

TL;DR

This study analyzes 126 12-micron selected galaxies using X-ray spectral data, focusing on Compton thick AGN, and demonstrates how geometry and obscuration affect X-ray observations and biases in surveys.

Contribution

Introduces new spectral fitting models based on Monte Carlo simulations for better characterization of Compton thick AGN in galaxy samples.

Findings

Maximum Fe Kα EW of ~150 eV for unobscured lines of sight.

Flux suppression >10 for N_H>10^25 cm^-2, indicating biases in X-ray surveys.

Compton thick fraction of 18% in the sample, higher than in hard X-ray selected samples.

Abstract

We present an X-ray spectral analysis of 126 galaxies of the 12 micron galaxy sample. We pay particular attention to Compton thick AGN with the help of new spectral fitting models that we have produced, which are based on Monte-Carlo simulations of X-ray radiative transfer, using both a spherical and torus geometry, and taking into account Compton scattering and Fe fluorescence. We use this data to show that with a torus geometry, unobscured sight lines can achieve a maximum EW of the Fe K\alpha line of ~150 eV, originally shown by Ghisellini, Haardt & Matt (1994). In order for this to be exceeded, the line of sight must be obscured with N_H>10^23 cm^-2, as we show for one case, NGC 3690. We also calculate flux suppression factors from the simulated data, the main conclusion from which is that for N_H>10^25 cm^-2, the X-ray flux is suppressed by a factor of >10 in all X-ray bands and at all redshifts, revealing the biases present against these extremely heavily obscured systems inherent in all X-ray surveys. Furthermore, we confirm previous results from Murphy & Yaqoob (2009) that show that the reflection fraction determined from slab geometries is underestimated with respect to toroidal geometries. For the 12 micron selected galaxies, we investigate the distribution of X-ray power-law indices, finding that the mean $<\Gamma>=1.90_{-0.07}^{+0.05}$ and $\sigma_\Gamma = 0.31_{-0.05}^{+0.05}$) is consistent with previous works, and that the distribution of \Gamma for obscured and unobscured sources is consistent with the source populations being the same, in general support of unification schemes. We determine a Compton thick fraction for the X-ray AGN in our sample to be 18+/-5% which is higher than the hard X-ray (>10 keV) selected samples. Finally we find that the obscured fraction for our sample is a strong function of X-ray luminosity, peaking at L_X~10^42-43 ergs s^-1.