Central massive object

{{Short description|Central object of a galaxy}}

{{Infobox

| name = NGC 4244

| image = 250px

| caption = The star-like nucleus at the centre of this edge-on spiral galaxy, NGC 4244, is a central massive object.

}}

A central massive object (CMO) is a high mass object or cluster of objects at the centre of a large star system, such as a galaxy or globular cluster. In the case of the former, the CMO may be a supermassive black hole, a nuclear star cluster, or even both together.{{sfn|Böker|2010a|p=61}}

The most massive galaxies are thought to always contain a supermassive black hole (SBH); these galaxies do not contain nuclear star clusters, and the CMO is identified with the SBH. Fainter galaxies usually contain a nuclear star cluster (NSC). In most of these galaxies, it is not known whether a supermassive black hole is present, and the CMO is identified with the NSC.{{cite book|last=Merritt|first=David|author-link=David Merritt|title=Dynamics and Evolution of Galactic Nuclei|year=2013|publisher=Princeton University Press|location=Princeton, NJ|url=https://openlibrary.org/works/OL16802359W/Dynamics_and_Evolution_of_Galactic_Nuclei}} A few galaxies, for instance the Milky Way and NGC 4395, are known to contain both a SBH and a NSC.{{cite journal|author1=Nishiyama, S.|author2=Schödel, R.|title=Young, Massive Star Candidates Detected throughout the Nuclear Star Cluster of the Milky Way|year=2012|journal= Astronomy & Astrophysics|volume=549|pages=A57|arxiv=1210.6125|bibcode=2012yCat..35490057N|doi=10.1051/0004-6361/201219773|s2cid=17865037 }}

Although this is suggestive that all galaxies have CMOs, and that a common mechanism of galaxy formation causes both, ESA MIRI scientist Torsten Böker observes that some galaxies appear to have neither SBHs nor NSCs.{{sfn|Böker|2010b|p=103}}

The mass associated with CMOs is roughly 0.1–0.3% times the total mass of the galactic bulge.