| Advanced civilisations harnessing energies on galactic scales (so-called Kardashev Type III civilisations) are expected to be detectable in the mid-Infrared part of the spectrum via the emission of significant waste heat products. A team of astronomers led by Dr. Jason Wright (Penn State University, USA) have used a mid-IR all-sky survey (conducted by the WISE satellite) to draw up a list of interesting candidate galaxies (the so-called Ĝ [Glimpsing Heat from Alien Technologies] sample) that show the kind of mid-IR features that might be expected of a galaxy hosting an advanced civilisation. One of the images above, presents an advanced civilisation harnessing power from stars via Dyson swarms.
One potential problem with this approach is that some astrophysical processes can also produce similar features via natural processes associated with thermal emission from dust. One way to weed out these "false-postives" is to apply the well known infrared/radio correlation to the Ĝ sample. It turns out that the vast majority of the galaxies in the Ĝ sample do indeed follow the mid-IR/radio correlation - this implies that their mid-IR emission almost certainly arises from natural astrophysical processes. The presence of radio emission at the levels expected from the correlation, suggests that the mid-IR emission is not heat from alien factories but more likely emission from dust – for example, dust generated and heated by regions of massive star formation or a central AGN.
There are a handful of candidates with very high values of q (log L_MIR/L_Radio) as might be expected if a advanced civilisation was present in these particular galaxies (see one of the images above). However, these systems also seem best explained via a standard astrophysical explanation e.g. nascent star formation. Nevertheless, there are few objects that have not yet been studied in any detail, and these require further multi-wavelength observations to be made.
In short, out of a list of 100000 resolved objects in the WISE survey, only a handful of galaxies appear to remain as viable Kardashev Type III candidates, and even these are most likely to be best explained via natural astrophysical interpretations. The conclusion drawn is that advanced civilisations are rare or entirely absent in the local Universe. Why this should be is not at all clear - there are several possibilities - perhaps technological civilisations are very short lived, perhaps advanced civilisations have found ways of reducing their waste heat emissions, or perhaps we really are alone in the Milky Way and the rest of the Universe. In any case, and at the risk of tempting fate, alien invasion seems unlikely - good night, sleep tight!
These results will be presented this week as a letter in the European journal Astronomy & Astrophysics - Garrett 2015, A&A 581, L5 - see also: http://arxiv.org/abs/1508.02624