The Contact Frontier in Galactic Civilization Networks: Radial Structure and the Galactic Habitable Zone

The probability of interstellar contact depends not only on the abundance of technological civilizations but also on their spatial connectivity. Treating civilizations as nodes linked whenever their separation is smaller than the distance a signal travels within a communicative lifetime, we determine the radial structure of Galactic civilization networks. An exponential stellar disc replaces the uniform disc of earlier work, and the fraction of stars hosting civilizations may vary with radius as a Galactic Habitable Zone (GHZ). The isolation, contact and network transitions are frontiers several kiloparsecs wide, not boundaries: the contact frontier is set entirely by the disc scale length, and the network frontier, broadened by gradient percolation, stays near 1.5 scale lengths. The two frontiers overlap over a band containing the Sun, where both transitions proceed at once. Within this factorized GHZ model, the radius of peak civilization density depends only logarithmically on the strength of supernova sterilization, and the wide range of published peak radii, from the Galactic centre to 12 kpc, is reproduced primarily by varying that one parameter. Under the present normalization, the location of the zone has only a modest effect on the local contact probability, whereas the modelled Galactic connectivity is minimized when the civilization-density peak lies near the solar Galactocentric radius, where current determinations place the peak.