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

TL;DR

This paper expands the M_bh-sigma diagram by including nuclear star cluster masses, revealing a different scaling relation for faint galaxies than previously thought, and providing new observational data for galaxy nuclei.

Contribution

It introduces new measurements of nuclear star cluster masses and integrates them into the M_bh-sigma diagram, challenging previous feedback-based interpretations.

Findings

M_bh ~ sigma^5 for luminous galaxies

M_nc ~ sigma^{1.57±0.24} for faint galaxies

Contrasts with previous M_nc ~ sigma^4 results

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.