Application of the mid-IR radio correlation to the Ĝ sample and the search for advanced extraterrestrial civilisations

Abstract

Wright et al. (2014, ApJ, 792, 26) have embarked on a search for advanced Karadashev Type III civilisations via the compilation of a sample of sources with extreme mid-IR emission and colours. The aim is to furnish a list of candidate galaxies that might harbour an advanced Kardashev Type III civilisation; in this scenario, the mid-IR emission is then primarily associated with waste heat energy byproducts. I apply the mid-IR radio correlation to this Glimpsing Heat from Alien Technology (Ĝ) sample, a catalogue of 93 candidate galaxies compiled by Griffith et al. (2015, ApJS, 217, 25). I demonstrate that the mid-IR and radio luminosities are correlated for the sample, determining a k-corrected value of q 22 = 1.35 ± 0.42. By comparison, a similar measurement for 124 galaxies drawn from the First Look Survey (FLS) has q 22 = 0.87 ± 0.27. The statistically significant difference of the mean value of q 22 for these two samples, taken together with their more comparable far-IR properties, suggests that theĜ sample shows excessive emission in the mid-IR. The fact that theĜ sample largely follows the mid-IR radio correlation strongly suggests that the vast majority of these sources are associated with galaxies in which natural astrophysical processes are dominant. This simple application of the mid-IR radio correlation can substantially reduce the number of false positives in theĜ catalogue since galaxies occupied by advanced Kardashev Type III civilisations would be expected to exhibit very high values of q. I identify nine outliers in the sample with q 22 > 2 of which at least three have properties that are relatively well explained via standard astrophysical interpretations e.g. dust emission associated with nascent star formation and/or nuclear activity from a heavily obscured AGN. The other outliers have not been studied in any great detail, and are deserving of further observation. I also note that the comparison of resolved mid-IR and radio images of galaxies on sub-galactic (kpc) scales can also be useful in identifying and recognising artificial mid-IR emission from less advanced intermediate Type II/III civilisations. Nevertheless, from the bulk properties of theĜ sample, I conclude that Kardashev Type III civilisations are either very rare or do not exist in the local Universe.

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