Comparison between the Luminosity functions of X-ray and [OIII] selected AGN

TL;DR

This study compares the luminosity functions of X-ray and [OIII] selected AGN, finding that both methods are similarly effective in detecting powerful AGN, with optical selection capturing more low-luminosity Seyferts.

Contribution

It constructs and compares predicted and observed X-ray luminosity functions for Seyfert galaxies using combined optical and X-ray data, clarifying selection biases.

Findings

Optical and X-ray luminosity functions show reasonable agreement in common luminosity ranges.

Optical selection probes less luminous Seyferts than X-ray surveys.

No evidence that [OIII] selection is more robust than X-ray in detecting powerful AGN.

Abstract

We investigate claims according to which the X-ray selection of AGN is not as efficient compared to that based on [OIII] selection because of the effects of X-ray absorption.We construct the predicted X-ray luminosity function both for all Seyferts as well as separately for Seyfert-1 and Seyfert-2 type galaxies, by combining the optical AGN [OIII] luminosity functions derived in SDSS with the corresponding L_X-L_[OIII] relations. These relations are derived from XMM-Newton observations of all Seyfert galaxies in the Palomar spectroscopic sample of nearby galaxies after correction for X-ray absorption and optical reddening. We compare the predicted X-ray luminosity functions with those actually observed in the local Universe by HEAO-1, RXTE as well as INTEGRAL. The last luminosity function is derived in the 17-60 keV region and thus is not affected by absorption even in the case of Compton-thick sources. In the common luminosity regions, the optically and X-ray selected Seyfert galaxies show reasonable agreement. We thus find no evidence that the [OIII] selection provides a more robust tracer of powerful AGN compared to the X-ray. Still, the optical selection probes less luminous Seyferts compared to the current X-ray surveys. These low luminosity levels, are populated by a large number of X-ray unobscured Seyfert-2 galaxies.