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The correlation between radio spectral index and redshift has long been usedto identify high redshift radio galaxies, but its cause is unknown. Traditionalexplanations invoke either $(i)$ intrinsic relations between spectral index andpower, $(ii)$ environmental differences at high redshift, or $(iii)$ higherinverse Compton losses due to the increased photon energy density of the cosmicmicrowave background. In this paper we investigate whether the increasedinverse Compton losses alone can cause the observed spectral index - redshiftcorrelation by using spectral modelling of nearby radio galaxies to simulatehigh redshift equivalents. We then apply selection effects and directly comparethe simulated radio galaxy sample with an observed sample with sufficientredshift coverage. We find excellent agreement between the two, implying thatinverse Compton losses and selection effects alone can largely reproduce theobserved spectral index - redshift correlation.

Investigating the cause of the $\alpha\,$-$\,z$ relation