Re-evaluating the Local Compton-thick AGN Fraction

TL;DR

This study analyzes the distribution of absorption in AGN using INTEGRAL data, revealing that selection biases affect observed fractions and estimating a true Compton-thick AGN fraction exceeding 24%.

Contribution

It re-evaluates the local Compton-thick AGN fraction accounting for selection biases in high-energy surveys.

Findings

Absorbed AGN constitute 43% of the sample.

Compton thick AGN are 7% of the sample.

The true Compton thick fraction is estimated to be over 24%.

Abstract

We study the NH distribution in a complete sample of 88 AGN selected in the 20-40 keV band from INTEGRAL/IBIS observations. We find that the fraction of absorbed (NH > 10^22 cm^-2) sources is 43% while Compton thick AGN comprise 7% of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported from an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore we show that the measured fraction of absorbed sources in our sample decreases from 80% to ~20-30% as a function of redshift with all Compton thick AGN having z<0.015. We conclude that in the low redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in type 1 and 2 AGN, we estimate the fraction of Compton thick objects to be >24%