A new constraint on mean-field galactic dynamo theory

Appealing to an analytical result from mean-field theory, we show, using a generic galaxy model, that galactic dynamo action can be suppressed by small-scale magnetic fluctuations. This is caused by the magnetic analogue of the R"{a}dler or $\mathrm{\Omega }\times J$ effect, where rotation-induced corrections to the mean-field turbulent transport result in what we interpret to be an effective reduction of the standard $\alpha$ effect in the presence of small-scale magnetic fields.