We report the discovery of 47 low surface brightness objects in deep images of a 3 • × 3 • field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness µ(g, 0) ranging from 24 -26 mag arcsec −2 and effective radii r eff = 3 ′′ -10 ′′ , as measured from archival Canada France Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r eff = 1.5 kpc -4.6 kpc their sizes are similar to those of L * galaxies even though their median stellar mass is only ∼ 6 × 10 7 M ⊙ . The galaxies are relatively red and round, with g − i = 0.8 and b/a = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope ACS observation. The ACS imaging shows a large spheroidal object with a central surface brightness µ 475 = 25.8 mag arcsec −2 , a Sersic index n = 0.6, and an effective radius of 7 ′′ , corresponding to 3.4 kpc at the distance of Coma. The galaxy is not resolved into stars, consistent with expectations for a Coma cluster object. We speculate that these "ultra-diffuse galaxies" (UDGs) may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.

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.

Galaxies are thought to grow through accretion; as less massive galaxies are disrupted and merge over time, their debris results in diffuse, clumpy stellar halos enveloping the central galaxy. Here we present a study of the variation in the stellar halos of galaxies, using data from the Dragonfly Nearby Galaxies Survey (DNGS). The survey consists of wide field, deep (µ g > 31 mag arcsec −2 ) optical imaging of nearby galaxies using the Dragonfly Telephoto Array. Our sample includes eight spiral galaxies with stellar masses similar to that of the Milky Way, inclinations of 16 − 90 degrees and distances between 7 − 18 Mpc. We construct stellar mass surface density profiles from the observed g-band surface brightness in combination with the g − r color as a function of radius, and compute the halo fractions from the excess stellar mass (relative to a disk+bulge fit) beyond 5 half-mass radii. We find a mean halo fraction of 0.009 ± 0.005 and a large RMS scatter of 1.01 +0.9 −0.26 dex. The peak-to-peak scatter is a factor of > 100 -while some galaxies feature strongly structured halos resembling that of M31, three of the eight have halos that are completely undetected in our data. We conclude that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. We find no convincing evidence for an environmental or stellar mass dependence of the halo fraction in the sample.

We present the unexpected discovery of four ultra diffuse galaxies (UDGs) in a group environment. We recently identified seven extremely low surface brightness galaxies in the vicinity of the spiral galaxy M101, using data from the Dragonfly Telephoto Array. The galaxies have effective radii of 10 − 38 and central surface brightnesses of 25.6 − 27.7 mag arcsec −2 in g-band. We subsequently obtained follow-up observations with HST to constrain the distances to these galaxies. Four remain persistently unresolved even with the spatial resolution of HST /ACS, which implies distances of D > 17.5 Mpc. We show that the galaxies are most likely associated with a background group at ∼ 27 Mpc containing the massive ellipticals NGC 5485 and NGC 5473. At this distance, the galaxies have sizes of 2.6 − 4.9 kpc, and are classified as UDGs, similar to the populations that have been revealed in clusters such as Coma, Virgo and Fornax, yet even more diffuse. The discovery of four UDGs in a galaxy group demonstrates that the UDG phenomenon is not exclusive to cluster environments. Furthermore, their morphologies seem less regular than those of the cluster populations, which may suggest a different formation mechanism or be indicative of a threshold in surface density below which UDGs are unable to maintain stability.