The Lack of Torus Emission from BL Lacertae Objects: An Infrared View of Unification with WISE

TL;DR

This study uses WISE infrared data to analyze BL Lac objects, finding no evidence of dusty torus emission, supporting the idea that accretion rate influences AGN unification beyond orientation effects.

Contribution

It provides the first large-scale infrared analysis of BL Lac objects, linking the absence of torus emission to radiatively inefficient accretion disks.

Findings

No observational signatures of dusty torus in weakly beamed BL Lacs

Supports radiatively inefficient accretion disks in BL Lacs

Highlights the role of accretion rate in AGN unification

Abstract

We use data from the Wide-Field Infrared Survey Explorer (WISE) to perform a statistical study on the mid-infrared (IR) properties of a large number ($\sim10^2$) of BL Lac objects --- low-luminosity Active Galactic Nuclei (AGN) with a jet beamed toward the Earth. As expected, many BL Lac objects are so highly beamed that their jet synchrotron emission dominates their IR spectral energy distributions. In other BL Lac objects, however, the jet is not strong enough to completely dilute the rest of the AGN emission. We do not see observational signatures of the dusty torus from these weakly beamed BL Lac objects. The lack of observable torus emission is consistent with suggestions that BL Lac objects are fed by radiatively inefficient accretion disks. Implications for the "nature vs. nurture" debate for FR I and FR II radio galaxies are briefly discussed. Our study supports the notion that, beyond orientation, accretion rate plays an important role in AGN unification.