Free-free emission fails its cross-check against its own tracer: matching radio brightness to dust-corrected H-alpha demands electron temperatures sometimes unphysically high, with latitude gradients photo-ionization equilibrium does not produce (Dickinson 2003; Planck 2016), while the flat spectrum degenerates with AME in joint fits, smearing residuals across components.
The model assumes one ionized-gas population in photo-ionization equilibrium, so H-alpha and radio free-free must reconcile through a physically reasonable electron temperature. Inferred temperatures that run unphysical or gradient-bearing say the sightlines hold ionized gas in states the single-population assumption excludes.
SCT predicts the single-population assumption fails, because the Galaxy's ionized medium is inherited as well as locally produced. The deposited material carries ionized components with diverse thermal histories (P25, P28), gas processed through prior structure before the Galaxy organized it, alongside the photo-ionized gas the H-alpha maps trace, and the two populations weight the observables differently: radio free-free integrates emission measure across all the ionized gas, while H-alpha selects the recombining photo-ionized component preferentially. Sightlines threading inherited ionized material then return radio-to-H-alpha ratios no single equilibrium temperature reconciles, with the latitude gradient tracking how the inherited component's scale height differs from the photo-ionized disk's.
This is the third reading of one diagnosis: the same inherited population diversity behind the plane's joint component failures (recid 202) and the Faraday-thick depolarization (recid 203), here caught in the cleanest cross-check the foreground sector owns. The registered expectation follows: the free-free anomaly map should correlate spatially with the depolarization and AME residual maps, symptoms of one heterogeneous medium, and the implied second population should appear in dispersion-measure data as ionized gas unaccounted by photo-ionization budgets. The inheritance mechanics are in Paper 4, From Chaos To Collisothermal Cosmogenesis.
Keystone economy: P25 supplies the second population; the impossible temperatures are two gases pretending to be one.
The registered kill: if refined H-alpha dust corrections and scattering treatments reconcile the radio and optical ledgers with one physically reasonable electron-temperature distribution, the second population is unnecessary. SCT separately requires the anomaly to be one symptom among correlated several; a free-free residual map statistically independent of the depolarization and AME residuals would break the one-medium diagnosis, and pulsar dispersion measures finding no ionized gas beyond the photo-ionized budget would remove the inherited component directly.