Probing the Low Surface Brightness Universe with Dragonfly

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Over the past 40 years the cutting edge of astronomical imaging has transitioned from photographic plates on ground based telescopes to CCD detectors on space telescopes. This has improved our ability to detect small faint galaxies by about seven magnitudes (over a factor of 600). Over this same period of time there has been no improvement in our ability to detect large but faint (low surface brightness) structures in the Universe. This is because the limiting factor in this case is not photon statistics or image resolution, it is control of systematic errors, such as the scattering of light and sky variability. Working with a colleague at Yale, I recently developed a novel new telescope, the Dragonfly Telephoto Array, which has succeeded in breaking through this technical barrier and which is revealing the composition of the Universe at very faint surface brightness levels (Abraham & van Dokkum 2014). This is an area of astronomy that is almost totally unexplored, and in the last two years we have used this telescope (constructed from an array of high-end commercial telephoto lenses) to probe the Universe down to hitherto impossibly faint surface brightness levels. A number of remarkable discoveries have been made, most notably the existence of a new population of giant but faint galaxies that preferentially inhabit rich galaxy clusters. These "ultra-diffuse galaxies" are as big as the Milky Way but have only 1/100 – 1/1000 of its mass, and explaining how they survive in the very hostile environment of a galaxy cluster is a major puzzle that has become a highly active area of investigation in the last year. But these early results from Dragonfly are only the tip of the iceberg. It is now clear that the low surface brightness Universe is a treasure trove of almost totally unexplored astrophysical phenomena, and the race is now on to study these. In this proposal I describe a program of investigation whose goals are to: (1) explore the nature of dark matter in ultra-diffuse galaxies by determining their masses in galactic stellar halos; (2) test fundamental predictions of models for galaxy assembly by undertaking an ambitious wide-area survey of the low surface brightness sky; (3) determine the ultimate limits for the performance of the Dragonfly concept, in order to see if we can grow the system to the point that we can directly explore the "cosmic web"– a predicted large-scale network of dark matter filaments believed to connect galaxies to each other.