We have studied the host galaxies of a sample of radio-loud AGN spanning more than four decades in the energy output of the nucleus .
The core sample includes 40 low-power sources ( BL Lac objects ) and 22 high-power sources ( radio-loud quasars ) spanning the redshift range 0.15 \lesssim z \lesssim 0.5 , all imaged with the high spatial resolution of HST .
All of the sources are found to lie in luminous elliptical galaxies , which follow the Kormendy relation for normal ellipticals .
A very shallow trend is detected between nuclear brightness ( corrected for beaming ) and host galaxy luminosity .
Black hole masses are estimated for the entire sample , using both the bulge luminosity–black hole mass and the velocity dispersion–black hole mass relations for local galaxies .
The latter involves a new method , using the host galaxy morphological parameters , \mu _ { e } and r _ { e } , to infer the velocity dispersion , \sigma , via the fundamental plane correlation .
Both methods indicate that the entire sample of radio-loud AGN are powered by very massive central black holes , with M _ { \bullet } \sim 10 ^ { 8 } to 10 ^ { 10 } M _ { \odot } .
Eddington ratios range from L / L _ { Edd } \sim 2 \times 10 ^ { -4 } to \sim 1 , with the high-power sources having higher Eddington ratios than the low-power sources .
Overall , radio-loud AGN appear to span a very large range in accretion efficiency , which is all but independent of the mass of the host galaxy .