Galactic scale IA updated March 9, 2016  D. Carrigan carrigan@fnal.gov (subject line must be sensible)

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Galactic scale artifacts introduction

A possible model is a galaxy filled with Dyson spheres. After one gets over the problem of  building a Dyson sphere filling a galaxy is only a matter of slow replication. Two things are required, relatively slow interstellar travel and replication at ten to a hundred times the scale of the automobile output of Toyota over several decades.

Freeman Dyson notes "a type III (Kardashev civilization) in our own galaxy would change the appearance of the sky so drastically that it could hardly have escaped our attention," while Annis observes "It is quite clear that the Galaxy itself has not transformed into a type III civilization based on starlight, nor have M31 or M33, our two large neighbors."

Fermi paradox
More than half a century ago Fermi noted that space travel is possible.  It just requires patience. With only a factor of ten increase in speed compared to that already obtained one can cross the galaxy in sixty million years. For example Voyager 1 is travelling fast enough to escape the sun. If 100 times the energy had been available it would have been able to cross the galaxy in sixty million years as it rotates.  Fermi supposedly next asked why we didn't see space men on earth. This turns out to be a hard paradox to dodge.

FermiDyson
Whirlpool galaxy

Fermi bubble

If a civilization was busily turning stars into Dyson spheres it could create a "Fermi bubble" or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light.
Finding bubbles in a spiral galaxy is challenging because the structure pattern itself is based on voids of a sort. The M51 Whirlpool galaxy seen face on in this Spitzer image shows the challenges. A rough estimate suggests that there are no unexplained voids at the level of 5%.
Annis has suggested that elliptical galaxies might be a better place to look for Fermi voids since ellipticals exhibit little structure. The problem is that they can be thick so interior voids are difficult to identify. M87 in Virgo 55 mly away is a nice illustration of an elliptical. (Note - the designation Fermi bubbles has also been used for galactic center observations with the Fermi Large area gamma telescope.)
Elliptical galaxy