Speaker
Prof.
Melvyn Davies
(Lund University)
Description
We review the ecology of galactic nuclei considering how supermassive
black holes may form and grow in galactic nuclei and also how massive
stars may be produced from low-mass stars via Bondi-Hoyle accretion
within gaseous accretion discs. We suggest this is how the massive
stars seen in the centre of the Milky Way were produced.
Supermassive black holes are found in most galactic nuclei. A large
fraction of these nuclei also contain a nuclear stellar cluster
surrounding the black hole. Here we consider the idea that the nuclear
stellar cluster formed first and that the supermassive black hole grew
later. In particular we consider the merger of stellar clusters
to form a nuclear stellar cluster, where some of these clusters contain
a single intermediate-mass black hole (IMBH). In the cases where
multiple clusters contain IMBHs, we discuss whether the black holes are
likely to merge and whether such mergers are likely to result in the
ejection of the merged black hole from the nuclear stellar cluster. In
some cases, no supermassive black hole will form as any merger product
is not retained. This is a natural pathway to explain those galactic
nuclei that contain a nuclear stellar cluster but apparently lack a
supermassive black hole; M33 being a nearby example. Alternatively, if
an IMBH merger product is retained within the nuclear stellar cluster,
it may subsequently grow, e.g. via the tidal disruption of stars, to
form a supermassive black hole.
Gaseous accretion discs may be produced in galactic nuclei for example
by the tidal shredding of giant molecular clouds by supermassive black holes.
We discuss how low-mass stars which find themselves inside such an accretion
disc will grow by Bondi-Hoyle accretion producing massive stars, many of which
will ultimately produce stellar-mass black holes. This process may explain the
observed massive stars in the centre of the Milky Way. In addition this channel
enhances enormously the number of stellar-mass black holes found in the
very centres of galactic nuclei and thus will affect the EMRI rate.
Primary author
Prof.
Melvyn Davies
(Lund University)
Co-authors
Dr
Abbas Askar
(Lund University)
Prof.
Doug Lin
(UCSC)
Dr
Ross Church
(Lund Observatory)
