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We use a new telescope concept, the Dragonfly Telephoto Array, to study thelow surface brightness outskirts of the spiral galaxy M101. The radial surfacebrightness profile is measured down to mu_g ~ 32 mag/arcsec^2, a depth thatapproaches the sensitivity of star count studies in the Local Group. We convertsurface brightness to surface mass density using the radial g-r color profile.The mass density profile shows no significant upturn at large radius and iswell-approximated by a simple bulge + disk model out to R = 70 kpc,corresponding to 18 disk scale lengths. Fitting a bulge + disk + halo model wefind that the best-fitting halo mass M_halo ~ 1.7 x 10^8 M_sun. The totalstellar mass of M101 is M_tot,* ~ 5.3 x 10^10 Msun, and we infer that the halomass fraction f_halo = M_halo / M_tot,* ~ 0.003. This mass fraction is lowerthan that of the Milky Way (f_halo ~ 0.02) and M31 (f_halo ~ 0.04). All threegalaxies fall below the f_halo - M_tot,* relation predicted by recentcosmological simulations that trace the light of disrupted satellites, withM101's halo mass a factor of ~10 below the median expectation. However, thepredicted scatter in this relation is large, and more galaxies are needed tobetter quantify this possible tension with galaxy formation models. Dragonflyis well suited for this project: as integrated-light surface brightness isindependent of distance, large numbers of galaxies can be studied in a uniformway.

First results from the Dragonfly Telephoto Array: the apparent lack of a stellar halo in the massive spiral galaxy M101