Diffuse Light in the M81 Group, the Centaurus A Group, and HCG 79: Fractions Set by Encounters, Not Mass

Diffuse light is not only a cluster phenomenon, and the group regime is where its production can be watched at the level of individual encounters. This paper assembles adopted diffuse-light budgets for three named groups: the M81 group, whose tidal debris from the M81-M82-NGC 3077 encounter is the nearest intragroup light known; the Centaurus A group, with its shells and diffuse envelope; and the compact group HCG 79, whose common halo holds an exceptional share of the system's light. The diffuse fractions are 0.032 for M81, 0.050 for Centaurus A, and 0.30 for HCG 79, an unweighted mean of 0.127 that conceals a factor of 9.4 between the compact group and the loose group. Averaged over a 150 kpc radius, the M81 group's diffuse light sits at 29.67 magnitudes per square arcsecond, but concentrated in its streams, covering some 2500 square kiloparsecs, the same 3.5e+09 solar luminosities shine at 26.04. Group diffuse light is therefore a phenomenon of bright, thin structures rather than a smooth faint glow, which is why groups show so little diffuse light in integrated fractions while displaying it so vividly in images.