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.

Dwarf satellite galaxies are a key probe of dark matter and of galaxy formation on small scales and of the dark matter halo masses of their central galaxies. They have very low surface brightness, which makes it difficult to identify and study them outside of the Local Group. We used a low surface brightness-optimized telescope, the Dragonfly Telephoto Array, to search for dwarf galaxies in the field of the massive spiral galaxy M101. We identify seven large, low surface brightness objects in this field, with effective radii of 10 − 30 arcseconds and central surface brightnesses of µ g ∼ 25.5 − 27.5 mag arcsec −2 . Given their large apparent sizes and low surface brightnesses, these objects would likely be missed by standard galaxy searches in deep fields. Assuming the galaxies are dwarf satellites of M101, their absolute magnitudes are in the range −11.6 M V −9.3 and their effective radii are 350 pc − 1.3 kpc. Their radial surface brightness profiles are well fit by Sersic profiles with a very low Sersic index (n ∼ 0.3 − 0.7). The properties of the sample are similar to those of well-studied dwarf galaxies in the Local Group, such as Sextans I and Phoenix. Distance measurements are required to determine whether these galaxies are in fact associated with M101 or are in its foreground or background.

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.

We use a new telescope concept, the Dragonfly Telephoto Array, to study the low surface brightness outskirts of the spiral galaxy M101. The radial surface brightness profile is measured down to µ g ∼ 32 mag arcsec −2 , a depth that approaches the sensitivity of star count studies in the Local Group. We convert surface brightness to surface mass density using the radial g − r color profile. The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R = 70 kpc, corresponding to 18 disk scale lengths. Fitting a bulge + disk + halo model we find that the best-fitting halo mass M halo = 1.7 +3.4 −1.7 × 10 8 M ⊙ . The total stellar mass of M101 is M tot, * = 5.3 +1.7 −1.3 × 10 10 M ⊙ , and we infer that the halo mass fraction f halo = M halo /M tot, * = 0.003 +0.006 −0.003 . This mass fraction is lower than that of the Milky Way ( f halo ∼ 0.02) and M31 ( f halo ∼ 0.04). All three galaxies fall below the f halo -M tot, * relation predicted by recent cosmological simulations that trace the light of disrupted satellites, with M101's halo mass a factor of ∼ 10 below the median expectation. However, the predicted scatter in this relation is large, and more galaxies are needed to better quantify this possible tension with galaxy formation models. Dragonfly is well suited for this project: as integrated-light surface brightness is independent of distance, large numbers of galaxies can be studied in a uniform way. 3 This is only true for distances where the (1 + z) 4 cosmological surface brightness dimming is unimportant.

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.