2021MNRAS.507.3761C

Kardashev Type III civilisations have by definition energy requirements that are likely to generate strong excess emission in the mid-infrared (MIR) that is associated with the waste heat they generate. For those civilisations that capture starlight via Dyson sphere like constructions, the Optical to MIR flux ratio of the host galaxies is also expected to be unusually low. Since a wide range of galaxy types adhere to the infrared-radio correlation (IRC), galaxies hosting Type III civilisations should also strongly deviate from this relation. Radio data can therefore play a crucial role in breaking the degeneracy between the effects of dust obscuration in a galaxy and the signature of an energy-intensive civilisation. We have used the newly released LoTSS-DR1 value-added catalogue to create a sample of 16 367 z < 0.2 sources with radio and MIR flux densities, optical photometry and (photometric) redshifts. We calculated the IRC parameter q^22µm^_150 MHz_ = log(S_150 MHz_/S22∼ µm_) and identified 21 sources with exceptionally high values of q^22 µm^_150 MHz_, an indication of MIR emission enhanced by a factor of ∼10. Out of the 21 high q-value sources, 4 sources have very red MIR colours, and appear to have relatively low optical/MIR ratios. Two of the four sources are not well known in the literature, they are considered as potential hosts of Type III civilisations. These sources deserve further study and investigation. Finally, we note that extending our analysis to the full LoTSS survey area can place very strong constraints on the incidence of Type III civilisations in the Universe.