2012MNRAS.422.1586G

Four new nuclear star cluster masses, M_nc, plus seven upper limits, are provided for galaxies with previously determined black hole masses, M_bh. Together with a sample of 64 galaxies with direct M_bh measurements, 13 of which additionally now have M_ncmeasurements rather than only upper limits, plus an additional 29 dwarf galaxies with available M_nc measurements and velocity dispersions σ, an {img} diagram is constructed. Given that major dry galaxy merger events preserve the M_bh/L ratio, and given that L∝σ⁵ for luminous galaxies, it is first noted that the observation M_bh∝σ⁵ is consistent with expectations. For the fainter elliptical galaxies it is known that L∝σ², and assuming a constant M_nc/L ratio, the expectation that M_nc∝σ² is in broad agreement with our new observational result that M_nc∝σ^1.57 ±0.24^. This exponent is however in contrast to the value of ∼ 4 which has been reported previously and interpreted in terms of a regulating feedback mechanism from stellar winds.

Finally, it is predicted that host galaxies fainter than M_B ∼ -20.5 mag (i.e. those not formed in dry merger events) which follow the relation M_bh∝σ⁵, and are thus not `pseudo-bulges', should not have a constant M_bh/M_host ratio but instead have {img}. It is argued that the previous near-linear {img} and {img} relations have been biased by the sample selection of luminous galaxies, and as such should not be used to constrain the co-evolution of supermassive black holes in galaxies other than those luminous few built by major dry merger events.