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We present a morphological and spectral study of a sample of 99 BL Lacs usingthe LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emissionhas been identified at gigahertz frequencies around BL Lacs, but with LDR2 itis now possible to systematically study their morphologies at 144 MHz, wheremore diffuse emission is expected. LDR2 reveals the presence of extended radiostructures around 66/99 of the BL Lac nuclei, with angular extents ranging upto 115 arcseconds, corresponding to spatial extents of 410 kpc. The extendedemission is likely to be both unbeamed diffuse emission and beamed emissionassociated with relativistic bulk motion in jets. The spatial extents andluminosities of the extended emission are consistent with the AGN unificationscheme where BL Lacs correspond to low-excitation radio galaxies with the jetaxis aligned along the line-of-sight. While extended emission is detectedaround the majority of BL Lacs, the median 144-1400 MHz spectral index and coredominance at 144 MHz indicate that the core component contributes ~42% onaverage to the total low-frequency flux density. A stronger correlation wasfound 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 photonflux (r = 0.42). This suggests that the radio-to-gamma-ray connection weakensat low radio frequencies because the population of particles that give rise tothe gamma-ray flux are distinct from the electrons producing the diffusesynchrotron emission associated with spatially-extended features.

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