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Extending the Mbhσ diagram with dense nuclear star clusters


  • Alister W. Graham

    Corresponding author
    1. Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
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Four new nuclear star cluster masses, Mnc, plus seven upper limits, are provided for galaxies with previously determined black hole masses, Mbh. Together with a sample of 64 galaxies with direct Mbh measurements, 13 of which additionally now have Mnc measurements rather than only upper limits, plus an additional 29 dwarf galaxies with available Mnc measurements and velocity dispersions σ, an inline image diagram is constructed. Given that major dry galaxy merger events preserve the Mbh/L ratio, and given that L∝σ5 for luminous galaxies, it is first noted that the observation Mbh∝σ5 is consistent with expectations. For the fainter elliptical galaxies it is known that L∝σ2, and assuming a constant Mnc/L ratio, the expectation that Mnc∝σ2 is in broad agreement with our new observational result that Mnc∝σ1.57 ± 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 MB∼−20.5 mag (i.e. those not formed in dry merger events) which follow the relation Mbh∝σ5, and are thus not ‘pseudo-bulges’, should not have a constant Mbh/Mhost ratio but instead have inline image. It is argued that the previous near-linear inline image and inline image 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.