| Abstract : | This thesis attempts to address the long-standing open question about the radio-loud (RL)/radio-quiet (RQ) divide in AGN and the reasons behind the short/stunted radio outflows in RQ AGN. Our GMRT - EVLA radio polarimetric study of the 20 Palomar-Green (PG) RQ quasars as well as 2 intermediate AGN - III Zw 2 and Mrk 231, suggests they are likely powered by multi-component (jet + ‘wind’) radio outflows; signatures of possible outflow stratification emerge from the different inferred magnetic field structures in the polarization observations. In addition, there is a contribution from stellar-related activity to the total radio emission, as is evident from the radio spectral indices. The jets are mostly small-scaled, low-powered, weakly collimated, and bent, which could either arise from stellar-mass loading or Kelvin-Helmholtz instabilities triggered by recollimation shocks/jet-medium interactions. The ‘wind’ component could either be an accretion disk wind or the outer layers of a widened jet (like a “jet sheath”) or both. Our findings are consistent with the idea that the radio-loudness of a source is a function of its epoch of observation. An EVLA polarization - HST [O III] emission-line study of 5 RQ quasars from the Quasar Feedback Survey reveals that the polarized emission more often does not spatially overlap with the [O III] emission. This suggests that the radio emission is likely to be depolarized by the emission line gas. A close interaction between the different emission components themselves as well as with the local host environments may be the reason for the stunted nature of the radio outflows in RQ AGN. Based on correlations between radio data, ALMA CO(2-1) data, and KPNO [O III] data, the presence of “AGN feedback” cannot be unambiguously ruled out in these RQ sources. Nevertheless, the merger scenario can also influence our interpretation of “AGN feedback” in these sources. |
