Extending the M_(bh)-sigma diagram with dense nuclear star clusters

Abstract

Abridged: 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 ~ sigma^5 for luminous galaxies, it is first noted that the observation M_bh ~ sigma^5 is consistent with expectations. For the fainter elliptical galaxies it is known that L ~ sigma^2, and assuming a constant M_nc/L ratio (Ferrarese et al.), the expectation that M_nc ~ sigma^2 is in broad agreement with our new observational result that M_nc ~ sigma^{1.57\pm0.24}. 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.Comment: 6 pages, 2 figures. Submitted 08/08/2011 to MNRAS, first referee report received 19/01/2012, accepted 10/02/201