Spectral energy distribution modelling of broad emission line quasars: From X-ray to radio wavelengths

Aims. We study differences in the physical properties of quasar host galaxies using an optically selected sample of radio-loud (RL) and radio-quiet quasars (in the redshift range 0.15 ≤ z ≤ 1.9) that we have further cross-matched with the VLA-FIRST survey catalogue. The sources in our sample have broad Hβ and Mg <ns0:sc>II</ns0:sc> emission lines (1000 km/s < FWHM < 15 000 km/s) with a sub-sample of high broad-line quasars (FWHM > 15 000 km/s). We constructed the broad-band spectral energy distribution (SED) of our broad-line quasars using multi-wavelength archival data and targeted observations with the AstroSat telescope. Methods. We used the state-of-the-art SED modelling code <ns0:monospace>CIGALE v2022.0</ns0:monospace> to model the SEDs and determine the best-fit physical parameters of the quasar host galaxies; namely, their star formation rate (SFR), main-sequence stellar mass, luminosity absorbed by dust, e-folding time, and stellar population age. Results. We find that the emission from the host galaxy of our sources is between 20% and 35% of the total luminosity, as they are mostly dominated by central quasars. Using the best-fit estimates, we reconstructed the optical spectra of our quasars, which show remarkable agreement in reproducing the observed SDSS spectra of the same sources. We plot the main-sequence relation for our quasars and note that they are significantly away from the main sequence of star-forming galaxies. Further, the main-sequence relation shows a bimodality for our RL quasars, indicating populations segregated by Eddington ratios. Conclusions. We conclude that RL quasars in our sample with lower Eddington ratios tend to have substantially lower SFRs for similar stellar mass. Our analyses thus provide a completely independent route to studying the host galaxies of quasars and addressing the radio dichotomy problem from the host galaxy angle.