Ultra-compact dwarf galaxies are among the densest stellar systems in the Universe. These systems have masses of up to 2 × 108 solar masses, but half-light radii of just 3–50 parsecs1. Dynamical mass estimates show that many such dwarfs are more massive than expected from their luminosity2. 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 expected3,4. 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 × 107 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 galaxies2.
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This work was based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). Work on this paper by A.C.S. was supported by NSF CAREER grant AST-1350389. J.L.W. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award number 1102845. J.B. is supported by NSF grant AST-1109878. M.J.F. is supported by German Research Foundation grant Ko4161/1. I.C. acknowledges support from the Russian Science Foundation grant 14-22-00041.
The authors declare no competing financial interests.
Extended data figures and tables
Residuals for both spectra are shown in green. The top spectrum is from one of the central-most pixels and is the spectrum from a single 0.05′′ × 0.05′′ pixel. The bottom spectrum is at a radius of 0.4′′ and is the sum of 17 spatial pixels. Signal-to-noise ratios are given per resolution element. The contrast in dispersion is seen very clearly, with broad smooth lines in the top spectrum and sharper lines in the bottom. Both spectra were normalized to one; the central spectrum was then offset by +1 for visibility. The residuals were offset by 0.5 and 1.5.
a, Radial velocity; b, dispersion; c, the skewness h3; d, kurtosis h4. Black contours show the K-band continuum at intervals of 1 mag arcsec−2. The median 1σ errors are 5.8 km s−1 for the velocities, 6.8 km s−1 for dispersion, 0.06 for h3 and 0.07 for h4. The skewness clearly shows the commonly seen anti-correlation with the velocity49.
Extended Data Figure 3 Distribution of mass as function of spin and average radius of the orbits as inferred from the dynamical model.
The average spin is defined as , where is the average angular momentum along the z-direction, is the average radius, and is the average second moment of the orbit. Several distinct components are visible. Although 70% of the mass is on co-rotating orbits with (shown as red and blue lines), there is also a significant amount of mass in components without rotation and counter-rotation.
a, β and βz shown as solid and dashed lines, respectively. Anisotropy indicates the relative size of the velocity ellipsoid in spherical coordinates and it is relatively constant over the radii probed by the kinematics. On the other hand (in cylindrical units) gradually declines. We note, however, that the velocity ellipsoid cannot be aligned with the cylindrical coordinates throughout a stellar system, and thus a physical interpretation of βz is not straightforward. b, The relative orbit fraction as function of radius.
a, The solid line shows the best-fitting g-band mass-to-light ratio (M/Lg) for a model that includes an M/L gradient but no black hole. The maximum M/Lg expected for a normal stellar population (assuming a standard IMF and age of 13 Gyr) is ∼5.1; the central M/Lg is about three times this value. Grey lines indicate the range of M/Lg values for gradient models within 1σ of the best fit. The dashed line shows the best-fitting constant-M/L model for a model including a supermassive black hole. b, The enclosed mass as a function of radius. The variable M/L fit with no black hole is shown as the solid line with uncertainties in grey. The black and red dashed lines show the enclosed stellar mass and the mass including the black hole from the constant-M/L + black hole fit.
a, The evolution of mass bound to the progenitor as it is being stripped to form a UCD. The progenitor properties were based on estimates from M60-UCD1, while the stripping galaxy is based on the potential of M60. b, The density profile evolution of a tidally stripped galaxy that results in a final object similar to that of M60-UCD1.
The Supplementary Information contains further details related to interpretation of our results. The numbering follows the sections in the methods section. In section 1 we discuss our calculation of the number density of UCDs with black holes, while in section 2 we show it is feasible for M60 to disrupt a ~1010 M⊙ galaxy to form an object like M60-UCD1. We also discuss alternative formation scenarios. (PDF 150 kb)
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Seth, A., van den Bosch, R., Mieske, S. et al. A supermassive black hole in an ultra-compact dwarf galaxy. Nature 513, 398–400 (2014). https://doi.org/10.1038/nature13762
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