Distribution of spectral indices of cores, ${\alpha }_{1400\mathrm{MHz}}^{144\mathrm{MHz}}$ of the core-D AGNs sample (black), and the 15 candidate restarted galaxies selected based on SSC criteria presented in Sect. 3.2 (blue) in the final sample of 69 candidate restarted galaxies presented in Sect. 3.5. The dashed red line marks the value of SSC = −0.65, and all sources to the left of the dashed line have SSC ≤ −0.65.