The black hole mass of low redshift radiogalaxies

Abstract

We make use of two empirical relations between the black hole mass and the global properties (bulge luminosity and stellar velocity dispersion) of nearby elliptical galaxies, to infer the mass of the central black hole (MBH) in low redshift radiogalaxies. Using the most recent determinations of black hole masses for inactive early type galaxies we show that the bulge luminosity and the central velocity dispersion are almost equally correlated (similar scatter) with the central black-hole mass. Applying these relations to two large and homogeneous datasets of radiogalaxies we find that they host black-holes whose mass ranges from ∼5 × 107 to ∼6 × 109 M⊙ (average < LogMBH > ∼ 8.9). MBH is found to be proportional to the mass of the bulge (Mbulge). The distribution of the ratio MBH/Mbulge has a mean value of 8 × 10-4 and shows a scatter that is consistent with that expected from the associated errors. At variance with previous claims no significant correlation is instead found between MBH (or Mbulge) and the radio power at 5 GHz.