BPT diagrams for sources with SDSS spectra showing the main source of ionization. Green and black markers correspond to sources with projected separation of less than or greater than (2 + ϵ)″, respectively. Gray circles are all “compact radio source-dwarf galaxy” matches with spectral line information in SDSS. Markers with a colored inner circle (colors other than black) occupy interesting parts of parameter space and are discussed in Sect. 4. The galaxy marked with a white inner circle lies above the gray line in the S II/Hα panel, while being well within the star formation range in the [N II]/Hα panel. Blue and pink inner circles fall within the composite region in the [N II]/Hα panel, where we can expect a contribution from both star formation (H II regions) and AGN activity. Finally, the galaxy marked by a cyan inner circle was classified as an AGN candidate by Truebenbach & Darling (2017) by selecting sources detected in the AllWISE and FIRST catalogs but not detected in 2MASS or SDSS DR7 and DR9.