Extragalactic SETI: The Tully-Fisher relation as a probe of Dysonian
  astroengineering in disk galaxies
  If advanced extraterrestrial civilizations choose to construct vast numbers
of Dyson spheres to harvest radiation energy, this could affect the
characteristics of their host galaxies. Potential signatures of such
astroengineering projects include reduced optical luminosity, boosted infrared
luminosity and morphological anomalies. Here, we apply a technique pioneered by
Annis (1999) to search for Kardashev type III civilizations in disk galaxies,
based on the predicted offset of these galaxies from the optical Tully-Fisher
relation. By analyzing a sample of 1359 disk galaxies, we are able to set a
conservative upper limit at 3% on the fraction of local disks subject to
Dysonian astroengineering on galaxy-wide scales. However, the available data
suggests that a small subset of disk galaxies actually may be underluminous
with respect to the Tully-Fisher relation in the way expected for Kardashev
type III objects. Based on the optical morphologies and infrared-to-optical
luminosity ratios of such galaxies in our sample, we conclude that none of them
stand out as strong Kardashev type III candidates and that their inferred
properties likely have mundane explanations. This allows us to set a tentative
upper limit at 0.3% on the fraction of Karashev type III disk galaxies in the
local Universe.
