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.