"Hidden" Seyfert 2 Galaxies in the Chandra Deep Field North

TL;DR

This study compares local Seyfert 2 galaxies with distant absorbed AGNs in the Chandra Deep Field North, finding their properties consistent and suggesting many distant sources are obscured Seyfert 2s due to observational effects.

Contribution

It demonstrates that local Seyfert 2 galaxies can account for the properties of distant absorbed AGNs, supporting the idea that many are hidden by observational limitations.

Findings

Excellent agreement between simulations and observed flux ratios.

No significant physical differences between local Seyfert 2s and distant absorbed AGNs.

Supports the hypothesis that many distant AGNs are obscured Seyfert 2s.

Abstract

We have compared the X-ray--to--optical flux ratios (F_x/F_opt) of absorbed active galactic nuclei (AGNs) in the Chandra Deep Field North (CDF-N) with those of nearby, optically classified Seyfert 2 galaxies. The comparison provides an opportunity to explore the extent to which the local population of absorbed AGNs can account for the properties of the distant, spectroscopically ambiguous sources that produce the hard X-ray background. Our nearby sample consists of 38 objects that well represent the local Seyfert 2 luminosity function. Integrated UBVRI photometry and broadband X-ray observations are presented. Using these data, we have simulated the F_x/F_opt ratios that local Seyfert 2s would exhibit if they were observed in the redshift range 0.2 < z < 1.3 as part of the CDF-N. In the simulations we account for the effects of redshift on flux measurements in fixed observed-frame bands, and the way the luminosity function of a given population is sampled in a flux-limited survey like the CDF-N. Overall, we find excellent agreement between our simulations and the observed distribution of F_x/F_opt ratios for absorbed AGNs in the CDF-N. Our analysis has thus failed to reveal any physical differences between the local population of Seyfert 2s and CDF-N sources with similar X-ray properties. These results support the hypothesis that the nuclear emission lines of many distant hard X-ray galaxies are hidden in ground-based spectra due to a combination of observational effects: signal-to-noise ratio, wavelength coverage, and dilution by host-galaxy light.