Characterising the extended morphologies of BL Lacs at 144 MHz with LOFAR

TL;DR

This study uses LOFAR data to systematically analyze the extended radio structures of 99 BL Lac objects at 144 MHz, revealing their morphology, spectral properties, and connection to gamma-ray emission, supporting AGN unification models.

Contribution

First systematic low-frequency morphological study of BL Lacs at 144 MHz, revealing extended structures and their spectral and gamma-ray correlations.

Findings

66 out of 99 BL Lacs show extended emission at 144 MHz.

Extended emission spans up to 410 kpc, consistent with AGN unification.

Weak correlation between extended radio flux and gamma-ray flux.

Abstract

We present a morphological and spectral study of a sample of 99 BL Lacs using the LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies around BL Lacs, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, where more diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BL Lac nuclei, with angular extents ranging up to 115 arcseconds, corresponding to spatial extents of 410 kpc. The extended emission is likely to be both unbeamed diffuse emission and beamed emission associated with relativistic bulk motion in jets. The spatial extents and luminosities of the extended emission are consistent with the AGN unification scheme where BL Lacs correspond to low-excitation radio galaxies with the jet axis aligned along the line-of-sight. While extended emission is detected around the majority of BL Lacs, the median 144-1400 MHz spectral index and core dominance at 144 MHz indicate that the core component contributes ~42% on average to the total low-frequency flux density. A stronger correlation was found between the 144 MHz core flux density and the gamma-ray photon flux (r = 0.69) compared to the 144 MHz extended flux density and the gamma-ray photon flux (r = 0.42). This suggests that the radio-to-gamma-ray connection weakens at low radio frequencies because the population of particles that give rise to the gamma-ray flux are distinct from the electrons producing the diffuse synchrotron emission associated with spatially-extended features.