The Dragonfly Nearby Galaxies Survey (DNGS)

The Dragonfly Telephoto Array is a robotic, refracting multi-lens array of Canon telephoto lenses operating out of the New Mexico Skies Observatory. Dragonfly is optimized for low surface brightness optical imaging, and features fast optics, a large (~2x3 degrees) field of view, and nano-fabricated sub-wavelength coatings on the optical elements which efficiently suppress scattered starlight and internal reflections. Commissioned in 2013 by Pieter van Dokkum and Roberto Abraham, Dragonfly has grown over the past three years from an 8-lens system to a 10-lens, then 24-lens, and finally, as of the spring of 2016, a 48-lens system.

For my thesis I have designed and carried out the Dragonfly Nearby Galaxies Survey (DNGS), a photometric survey of nearby galaxies using Dragonfly with the primary goal of characterizing the stellar halos. We have compiled a sample of galaxies using only luminosity, proximate amounts of cirrus, and observability from New Mexico Skies as selection criteria. The sample is volume-complete as a function of luminosity. By intentionally avoiding selecting galaxies based on intrinsic physical characteristics (in particular, known tidal streams), we hope to use our objective sample to explore trends between galaxy properties and their accreted fraction.

Additionally, previously published results from Team Dragonfly include the identification of low surface brightness satellites around nearby galaxies, the apparent lack of stellar halo around the spiral galaxy M101, and the discovery of ultra-diffuse galaxies (UDGs) in the Coma Cluster.

Discovery Space: Low Surface Brightness Galaxies

Abstract: 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 (MV) are in the range -11.6 to -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.

Paper: The Discovery of Seven Extremely Low Surface Brightness Galaxies in the Field of the Nearby Spiral Galaxy M101.

Substantial Variation in the Stellar Halos of Spiral Galaxies

Abstract: 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 ($\mu_{g} > 31$ 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 $0.009 \pm 0.005$ 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.

Paper: The Dragonfly Nearby Galaxies Survey. I. Substantial Variation in the Stellar Halos of Spiral Galaxies.

Discovery Space: Ultra diffuse galaxies

Abstract: 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 $\sim 27$ 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.

Paper: The Dragonfly Nearby Galaxies Survey. II. Ultra Diffuse Galaxies Near the Elliptical Galaxy NGC 5485.