ON THE ORIGIN OF THE FIR-RADIO RELATION IN ACTIVE GALAXIES

The origin of the far-infrared (FIR)-radio relation observed in active galaxies is investigated. Models developed to calculate the FIR-radio relation expected in starbursts are presented, and their results are compared with samples of radio quiet (RQAG) and radio loud (RLAG) acti ve galaxies. The presence of an active galactic nucleus (AGN) is enter ed, in bolometric terms, as a function of the energy output generated by a starburst. Pure starbursts, modelled by a continuous star formati on process lasting for, at least, 15 million yr, cover the range 6.6 g reater than or equal to log(L(FIR)/L(rad)) greater than or equal to 5. 3. Star formation processes characterized by different IMF slopes and mass limits do not greatly affect the value of the FIR-radio relation. The major effect is due to the fraction of energy absorbed and re-emi tted by dust. These results would explain in a natural way why spirals follow a universal FIR-radio relation while the observed scatter woul d be due to variations on the internal extinction from galaxy to galax y. Pure starburst models reproduce the FIR-radio relation observed in a large fraction of RQAG including starbursts, Seyferts, luminous infr ared galaxies and radio quiet quasars (QSOs). However, pure starbursts cannot explain RQAG characterized by log(L(FIR)/L(rad)) values in the range 5.3 to 4.5. For these RQAG, the contribution of a radio loud AG N (alpha(AGN)=L(FIR)(AGN)/L(rad)(AGN)greater than or equal to 10(-4)), dominating the radio emission output, is required.No pure starburst m odels can explain the FIR-radio relation observed in radio galaxies an d radio loud quasars. The FIR and radio luminosity in RLAG must be ori ginated in a radio loud AGN (10(-3.25)less than or equal to alpha(AGN) less than or equal to 10(-1.5)). Luminous circumnuclear star-forming r egions must be present in many of the brightest RQAG, i.e. QSOs. Detec tion of such circumnuclear star-forming rings in low redshift QSOs wit h HST would confirm empirically the conclusions presented here.