Radio-loud and Radio-quiet QSOs

We discuss 6 GHz JVLA observations covering a volume-limited sample of 178 low-redshift (0.2< z< 0.3) optically selected quasi-stellar objects (QSOs). Our 176 radio detections fall into two clear categories: (1) about 20% are radio-loud QSOs (RLQs) with spectral luminosities of {L}₆≳ {10}^23.2 {{W}} {{Hz}}^-1 that are primarily generated in the active galactic nucleus (AGN) responsible for the excess optical luminosity that defines a bona fide QSO; and (2) the remaining 80% that are radio-quiet QSOs (RQQs) that have {10}²¹≲ {L}₆≲ {10}^23.2 {{W}} {{Hz}}^-1 and radio sizes ≲ 10 {kpc}, and we suggest that the bulk of their radio emission is powered by star formation in their host galaxies. “Radio-silent” QSOs ({L}₆≲ {10}²¹ {{W}} {{Hz}}^-1) are rare, so most RQQ host galaxies form stars faster than the Milky Way; they are not “red and dead” ellipticals. Earlier radio observations did not have the luminosity sensitivity of {L}₆≲ {10}²¹ {{W}} {{Hz}}^-1 that is needed to distinguish between such RLQs and RQQs. Strong, generally double-sided radio emission spanning \gg 10 {kpc} was found to be associated with 13 of the 18 RLQ cores with peak flux densities of {S}_{{p}}> 5 {mJy} {{beam}}^-1 ({log}(L)≳ 24). The radio luminosity function of optically selected QSOs and the extended radio emission associated with RLQs are both inconsistent with simple “unified” models that invoke relativistic beaming from randomly oriented QSOs to explain the difference between RLQs and RQQs. Some intrinsic property of the AGNs or their host galaxies must also determine whether or not a QSO appears radio-loud.