title: A supermassive black hole in an ultra-compact dwarf galaxy.
abstract: Ultra-compact dwarf galaxies are among the densest stellar systems in the Universe. These systems have masses of up to 2 × 10(8) solar masses, but half-light radii of just 3-50 parsecs. Dynamical mass estimates show that many such dwarfs are more massive than expected from their luminosity. It remains unclear whether these high dynamical mass estimates arise because of the presence of supermassive black holes or result from a non-standard stellar initial mass function that causes the average stellar mass to be higher than expected. Here we report adaptive optics kinematic data of the ultra-compact dwarf galaxy M60-UCD1 that show a central velocity dispersion peak exceeding 100 kilometres per second and modest rotation. Dynamical modelling of these data reveals the presence of a supermassive black hole with a mass of 2.1 × 10(7) solar masses. This is 15 per cent of the object's total mass. The high black hole mass and mass fraction suggest that M60-UCD1 is the stripped nucleus of a galaxy. Our analysis also shows that M60-UCD1's stellar mass is consistent with its luminosity, implying a large population of previously unrecognized supermassive black holes in other ultra-compact dwarf galaxies.
authors.affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
authors.forename: Anil C
authors.initials: AC
authors.lastname: Seth
authors.affiliation: Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany.
authors.forename: Remco
authors.initials: R
authors.lastname: van den Bosch
authors.affiliation: European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, 7630355, Chile.
authors.forename: Steffen
authors.initials: S
authors.lastname: Mieske
authors.affiliation: School of Mathematics and Physics, University of Queensland, St Lucia, Queensland 4072, Australia.
authors.forename: Holger
authors.initials: H
authors.lastname: Baumgardt
authors.affiliation: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: den  Brok
authors.affiliation: Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: Jay
authors.initials: J
authors.lastname: Strader
authors.affiliation: 1] Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany [2] European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany.
authors.forename: Nadine
authors.initials: N
authors.lastname: Neumayer
authors.affiliation: 1] Smithsonian Astrophysical Observatory, 60 Garden Street MS09, Cambridge, Massachusetts 02138, USA [2] Sternberg Astronomical Institute, Moscow State University, 13 Universitetski prospect, Moscow 119992, Russia.
authors.forename: Igor
authors.initials: I
authors.lastname: Chilingarian
authors.affiliation: European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Hilker
authors.affiliation: 1] Australian Astronomical Observatory, 105 Delhi Road, Sydney, New South Wales 2113, Australia [2] Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Richard
authors.initials: R
authors.lastname: McDermid
authors.affiliation: 1] Australian Astronomical Observatory, 105 Delhi Road, Sydney, New South Wales 2113, Australia [2] Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Lee
authors.initials: L
authors.lastname: Spitler
authors.affiliation: University of California Observatories and Department of Astronomy and Astrophysics, University of California, Santa Cruz, California 95064, USA.
authors.forename: Jean
authors.initials: J
authors.lastname: Brodie
authors.affiliation: Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königsstuhl 12, D-69117 Heidelberg, Germany.
authors.forename: Matthias J
authors.initials: MJ
authors.lastname: Frank
authors.affiliation: Department of Astronomy, The University of Texas at Austin, 1 University Station C1400, Austin, Texas 78712, USA.
authors.forename: Jonelle L
authors.initials: JL
authors.lastname: Walsh