Imaging the Cosmic Web with the Dragonfly Telephoto Array

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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 and faint galaxies by a factor of about 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 three 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 their existence is a major puzzle that has turned into a highly active area of investigation by the community. 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. The most exciting known ultra-low surface brightness structure is weak photoluminescence from the "cosmic web", a predicted large scale network of dark matter filaments believed to connect galaxies to each other. Observing the cosmic web directly would be a landmark observation of the largest coherent structure in the sky, and it would let us map out the distribution of most of the mass in the local Universe directly. With this long-term goal in mid, in this proposal we request funding to add a prototype narrow-band imaging capability to the Dragonfly Telephoto Array. Our design fully exploits the unique strengths of Dragonfly, but it relies on untested ideas, such as filtering and beam switching at the entrance pupil. The proposed prototype will let us determine us if our design for an ultra-sensitive narrow-band imager works as well in the real world as it does on paper. If it does, mapping the cosmic web would be within reach of a fully-upgraded Dragonfly Telephoto Array.