Authors: Alister W. Graham
Abstract:
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 (Mbh + Mnc)–σ
diagram is constructed.
Given that major dry galaxy merger events should 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 (Ferrarese et al.),
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 spheroids 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 `pseudobulges',
should not have a constant Mbh/Mhost
ratio but instead have Mbh ∝ L5/2host.
It is argued that the previous log-linear Mbh–L and Mbh–Mspheroid
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.