The relative growth of Optical and Radio Quasars in SDSS

TL;DR

This study compares the evolution of optical and radio quasars using SDSS data, revealing distinct growth patterns, accretion rates, and black hole masses over cosmic time, and suggesting that radio activity may be driven by properties like black hole spin.

Contribution

It provides the first detailed comparison of accretion histories and black hole growth between optical and radio quasars across different redshifts.

Findings

Radio quasars have higher mean Eddington ratios at z > 2.

Radio quasars are less massive than optical quasars at high redshift but become more massive at lower redshift.

Radio quasars grow only modestly since z ~ 4, with lower mass BHs less likely to be radio loud.

Abstract

We cross-correlate the SDSS DR3 quasar sample with FIRST and the Vestergaard et al. black hole (BH) mass sample to compare the mean accretion histories of optical and radio quasars. We find significant statistical evidence that radio quasars have a higher mean Eddington ratio Lambda at z > 2 with respect to optical quasars, while the situation is clearly reverse at z < 1. At z > 2 radio quasars happen to be less massive than optical quasars; however, as redshift decreases radio quasars appear in increasingly more massive BHs with respect to optical quasars. These two trends imply that radio sources are not a mere random subsample of optical quasars. No clear correlation between radio activity and BH mass and/or accretion rate is evident from our data, pointing to other BH properties, possibly the spin, as the driver of radio activity. We have checked that our main results do not depend on any evident bias. We perform detailed modelling of reasonable accretion histories for optical and radio quasars, finding that radio quasars grow by a factor of a few, at the most, since z ~ 4. The comparison between the predicted mass function of active radio quasars and the observed optical luminosity function of radio quasars, implies a significantly lower probability for lower mass BHs to be radio loud at all epochs, in agreement with what is observed in the local universe.