Radio spectral index distribution of SDSS-FIRST sources across optical
  diagnostic diagrams
  A detailed understanding of how the activity of a galactic nucleus regulates
the growth of its host is still missing. To understand the activity and the
types of accretion of supermassive black holes in different hosts, it is
essential to study radio-optical properties of a large sample of extragalactic
sources. In particular, we aim at studying the radio spectral index trends
across the optical emission line diagnostic diagrams to search for potential
(anti)correlations. To this goal, we combine flux densities from the radio
FIRST survey at $1.4\,{\rm GHz}$ (with the flux density range $10\,{\rm mJy}
\leq F_{1.4} \leq 1000\,{\rm mJy}$) for 209 SDSS sources at intermediate
redshift $(0.04\leq z \leq 0.4)$ with the Effelsberg radiotelescope
measurements at $4.85\,{\rm GHz}$ and $10.45\,{\rm GHz}$. The information about
the optical emission-line ratios is obtained from the SDSS-DR7 catalogue. Using
the Effelsberg data, we were able to infer the two-point radio spectral index
distributions for star-forming galaxies, composite galaxies (with a combined
contribution to the line emission from the star-formation and AGN activity),
Seyferts, and low ionization narrow emission region (LINER) galaxies. While
studying the distribution of steep, flat, and inverted sources across optical
diagnostic diagrams, we found three distinct classes of radio emitters for our
sample: (i) sources with steep radio index, high ionization ratio and high
radio loudness, (ii) sources with flat radio index, lower ionization ratio and
intermediate radio loudness, (iii) sources with inverted radio index, low
ionization ratio and low radio loudness. The classes (i), (ii), (iii) cluster
mainly along the transition from Seyfert to LINER sources in the BPT diagram.
We interpret these groups as a result of the recurrent nuclear-jet activity.
