Extending the Mbh-σ diagram with dense nuclear star clusters

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-nc measurements rather than only upper limits, plus an additional 29 dwarf galaxies with available M-nc measurements and velocity dispersions sigma, an (M-bh + M-nc)-sigma diagram is constructed. Given that major dry galaxy merger events preserve the M-bh/L ratio, and given that L proportional to sigma(5) for luminous galaxies, it is first noted that the observation M-bh proportional to sigma(5) is consistent with expectations. For the fainter elliptical galaxies it is known that L proportional to sigma(2), and assuming a constant M-nc/L ratio, the expectation that M-nc proportional to sigma(2) is in broad agreement with our new observational result that M-nc proportional to sigma(1.57 +/- 0.24). This exponent is however in contrast to the value of similar to 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 similar to -20.5 mag (i.e. those not formed in dry merger events) which follow the relation M-bh proportional to sigma(5), and are thus not 'pseudo-bulges', should not have a constant M-bh/M-host ratio but instead have M-bh proportional to L-host(5/2). It is argued that the previous near-linear M-bh-L and M-bh-M-spheroid 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.