Journal Article

Extending the <i>M</i><sub>bh</sub>–<i>σ</i> diagram with dense nuclear star clusters

Alister W. Graham

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 422, issue 2, pages 1586-1591
Published in print May 2012 | ISSN: 0035-8711
Published online April 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20734.x
Extending the Mbh–σ diagram with dense nuclear star clusters

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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 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 . It is argued that the previous near-linear and 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.

Keywords: galaxies: dwarf; galaxies: fundamental parameters; galaxies: kinematics and dynamics; galaxies: nuclei; galaxies: star clusters: general; galaxies: structure

Journal Article.  4231 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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