Abstract
We describe first results from the Dragonfly Telephoto Array, a robotic imaging system optimized for the detection of extended ultra low surface brightness structures. The imaging system is comprised of multiple commercial 400 mm \(f/2.8\) telephoto lenses which have high performance sub-wavelength nano-fabricated optical coatings designed to minimize scattered light and ghosting, resulting in wide-field low surface brightness imaging performance an order of magnitude better than that of the best reflectors optimized for wide-field imaging. The array is capable of directly imaging low surface brightness structures (such as streams and faint dwarf galaxies) that have hitherto only been detectable using star counts. Therefore a range of studies that have hitherto been restricted to the Local Group can now be undertaken using samples of hundreds of more distant galaxies. Harnessing this new capability, the Dragonfly Telephoto Array is now executing a fully-automated multi-year imaging survey of a sample of nearby galaxies in order to undertake the first census of ultra-faint substructures in galaxies in the nearby Universe. In this conference writeup, we report some early results from a single galaxy, M101, that we have used to help test the system. The radial surface brightness profile of this galaxy was measured down to \(\mu_g \sim 34\) mag/arcsec2, showing no significant upturn at large radius. The galaxy is well-approximated by a simple exponential disk model out to a radius of 70 kpc, corresponding to 18 disk scale lengths. The stellar halo mass fraction of this galaxy falls an order of magnitude below the expectations of theoretical predictions. Dragonfly data has also revealed the existence of seven large, low surface brightness objects in the field of M101, with effective radii of 10–30 arcsecond and central surface brightnesses of \(\mu_g \sim 25.5-27.5\) mag/arcsec2. Given their large apparent sizes and low surface brightnesses, these objects would likely be missed by standard galaxy searches in deep fields. Assuming these objects are companions to M101, the properties of these galaxies are similar to those of well-studied dwarf galaxies in the Local Group, such as Sextans I and Phoenix.
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Notes
- 1.
Interestingly, it appears that several of the basic design trades that make large telescopes possible (in particular, obstructed pupils and reflective surfaces) define the fundamental systematic errors that make pushing to very low surface brightnesses so difficult.
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Abraham, R., van Dokkum, P., Merritt, A., Zhang, J. (2015). First Results from Project Dragonfly. In: Freeman, K., Elmegreen, B., Block, D., Woolway, M. (eds) Lessons from the Local Group. Springer, Cham. https://doi.org/10.1007/978-3-319-10614-4_36
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