Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses1,2. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies3,4. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy5 NGC1052–DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected2. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.
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A.J.R. was supported by National Science Foundation grant AST-1616710 and as a Research Corporation for Science Advancement Cottrell Scholar. Results are based on observations obtained with the W. M. Keck Observatory on Mauna Kea, Hawaii. We are grateful for the opportunity to conduct observations from this mountain and wish to acknowledge its important cultural role within the indigenous Hawaiian community.
The authors declare no competing financial interests.
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Extended data figures and tables
The full Dragonfly field, approximately 11 degree2, centred on NGC 1052. The zoom-in shows the immediate surroundings of NGC 1052, with NGC1052–DF2 highlighted in the inset.
Open symbols are galaxies from the Nearby Dwarf Galaxies catalogue15 and the solid symbol with error bars is NGC1052–DF2. The size of each symbol indicates the logarithm of the stellar mass, as shown in the key. There are no galaxies in the Local Group that are similar to NGC1052–DF2. Galaxies with a similar velocity dispersion are a factor of about 10 smaller and have stellar masses that are a factor of about 100 larger. The object labelled And XIX is an Andromeda satellite that is thought to be in the process of tidal disruption55.
We use the surface brightness fluctuation (SBF) signal in the HST I814 band to constrain the distance to NGC1052–DF2. a, The galaxy after subtracting a smooth model and masking background galaxies and globular clusters. The image spans 33″ × 33″. b, The azimuthally averaged power spectrum. Following previous studies9,46,47, the power spectrum is fitted by a combination of a constant (dotted line) and an expectation power spectrum E(k) (dashed line). From the normalization of E(k) we find that the SBF magnitude m814 = 29.45 ± 0.10. The implied distance is DSBF = 19.0 ± 1.7 Mpc, consistent with the 20 Mpc distance of the luminous elliptical galaxy NGC 1052.
a, The sum of g and r images taken with the Dragonfly Telephoto Array. The image was smoothed by a 10″ × 10″ median filter to bring out faint emission. The lowest surface brightness levels visible in the image are about 29 mag arcsec−2. b, Sum of SDSS g, r and i images. In SDSS, the overdensity of compact objects stands out. c, The Gemini-North i-band image of NGC1052–DF2, which provides the best information on the morphology of the galaxy. Black ellipses mark R = Re and R = 2Re. White arrows mark the most obvious reduction artefacts. The galaxy is regular out to at least R ≈ 2Re, with a well-defined centre and a position angle and axis ratio that do not vary strongly with radius.
a, b, Globular cluster velocities as a function of projected position along the major axis (a) and the minor axis (b). There is no evidence for any trends. For reference, a Gaussian with σ = 8.4 km s−1 is shown in b.
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van Dokkum, P., Danieli, S., Cohen, Y. et al. A galaxy lacking dark matter. Nature 555, 629–632 (2018). https://doi.org/10.1038/nature25767
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