Sunscreen: Photometric Signatures of Galaxies Partially Cloaked in Dyson
  Spheres
  The Search for Extraterrestrial Intelligence has so far come up negative for
Kardashev Type III societies that capture all starlight from a galaxy. One
possible reason is that shrouding a star in a megastructure is prohibitively
expensive. Most of a galaxy's starlight comes from bright stars, which would
require structures even larger than the classical Dyson sphere to enclose.
Using a custom spectral synthesis code, I calculate what happens to the
spectrum and colors of a galaxy when only stars below a luminosity L_min are
cloaked. I find the photometric signatures of galaxies with L_min <= 1 L_sun
are minor, especially for blue, galaxies with continuing star formation. Larger
luminosity thresholds (>~ 30 L_sun) result in galaxies with unnatural colors
and luminosities. Galaxies observed in NIR and galaxies without recent star
formation observed at UV-NIR wavelengths become redder than uncloaked galaxies
as L_min increases. Recently star-forming galaxies get bluer in UV and blue
light when they are cloaked, with colors similar to quasars but very low
luminosities. By selecting on color, we may find Type III societies in large
photometric surveys. I discuss how different metallicities, ages, and initial
mass functions affect the results.
