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 M/M has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 10 solar masses) and increases both towards lower masses and towards higher masses. 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 galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2 × 10 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 × 10 solar masses. This implies that the ratio M/M is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

We present COSMOS-Drift And SHift (DASH), a Hubble Space Telescope WFC3 imaging survey of the COSMOS field in the H 160 filter. The survey comprises 456 individual WFC3 pointings corresponding to an area of 0.49 deg 2 (0.66 deg 2 when including archival data) and reaches a 5σ point-source limit of H 160 = 25.1 (0. 3 aperture). COSMOS-DASH is the widest HST/WFC3 imaging survey in H 160 filter, tripling the extragalactic survey area in the near-infrared at HST resolution. We make the reduced H 160 mosaic available to the community. We use this dataset to measure the sizes of 162 galaxies with log(M /M ) > 11.3 at 1.5 < z < 3.0, and augment this sample with 748 galaxies at 0.1 < z < 1.5 using archival ACS imaging. We find that the median size of galaxies in this mass range changes with redshift as r eff = (10.4 ± 0.4) × (1 + z) (−0.65±0.05) kpc. Separating the galaxies into star forming and quiescent galaxies using their restframe U −V and V − J colors, we find no statistical difference between the median sizes of the most massive star-forming and quiescent galaxies at z = 2.5: they are 4.9 ± 0.9 kpc and 4.3 ± 0.3 kpc respectively. However, we do find a significant difference in the Sèrsic index between the two samples, such that massive quiescent galaxies have higher central densities than star forming galaxies. We extend the size-mass analysis to lower masses by combining it with the 3D-HST/CANDELS sample of van der Wel et al. (2014), and derive empirical relations between size, mass, and redshift. Fitting a relation of the form r eff = A × m α , with m = M /5 × 10 10 M and r eff in kpc, we find log A = −0.25 log (1 + z) + 0.79 and α = −0.13 log(1 + z) + 0.27. We also provide relations for the subsamples of star forming and quiescent galaxies. Our results confirm previous studies that were based on smaller samples or ground-based imaging.

We recently found an ultra diffuse galaxy (UDG) with a half-light radius of R e =2.2 kpc and little or no dark matter. The total mass of NGC1052-DF2 was measured from the radial velocities of bright compact objects that are associated with the galaxy. Here, we analyze these objects using a combination of Hubble Space Telescope (HST) imaging and Keck spectroscopy. Their average size is r 6.2 0.5 h á ñ =  pc and their average ellipticity is 0.18 0.02From a stacked Keck spectrum we derive an age of 9 Gyr and a metallicity of [Fe/H]=−1.35±0.12. Their properties are similar to ω Centauri, the brightest and largest globular cluster in the Milky Way, and our results demonstrate that the luminosity function of metal-poor globular clusters is not universal. The fraction of the total stellar mass that is in the globular cluster system is similar to that in other UDGs, and consistent with "failed galaxy" scenarios, where star formation terminated shortly after the clusters were formed. However, the galaxy is a factor of ∼1000 removed from the relation between globular cluster mass and total galaxy mass that has been found for other galaxies, including other UDGs. We infer that a dark matter halo is not a prerequisite for the formation of metal-poor globular cluster-like objects in high-redshift galaxies.

Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these ultra-diffuse galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here, we present the stellar kinematics of Dragonfly44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of s = - . The mass-to-light ratio is, and the dark matter fraction is 98% within r 1 2 . The high mass of Dragonfly44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in  0. 4 seeing we infer that Dragonfly44 has -+ 94 20 25 globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly44 by comparing the amount of dark matter within = r 4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of~10 12  M , similar to the mass of the Milky Way. The existence of nearly dark objects with this mass is unexpected, as galaxy formation is thought to be maximally efficient in this regime.

We present Hubble Space Telescope (HST) imaging of two ultra diffuse galaxies (UDGs) with measured stellar velocity dispersions in the Coma cluster. The galaxies, Dragonfly 44 and DFX1, have effective radii of 4.7 kpc and 3.5 kpc and velocity dispersions of 47 6 8 -+ km s −1 and 30 7 7 -+ km s −1 , respectively. Both galaxies are associated with a striking number of compact objects, tentatively identified as globular clusters: N 74 18 gc =  for Dragonfly 44 and N 62 17 gc =  for DFX1. The number of globular clusters is much higher than expected from the luminosities of the galaxies but is consistent with expectations from the empirical relation between dynamical mass and globular cluster count defined by other galaxies. Combining our data with previous HST observations of Coma UDGs we find that UDGs have a factor of 6.9 2.4 1.0 -+ more globular clusters than other galaxies of the same luminosity, in contrast to a recent study of a similar sample by Amorisco et al., but consistent with earlier results for individual galaxies. The Harris et al. relation between globular cluster count and dark matter halo mass implies a median halo mass of M 1.5 10 halo 11´M for the sixteen Coma UDGs that have been observed with HST so far, with the largest and brightest having M 5 10 halo 11´M .