We use a new telescope concept , the Dragonfly Telephoto Array , to study the low surface brightness outskirts of the spiral galaxy M101 .
The radial surface brightness profile is measured down to \mu _ { g } \sim 32 mag arcsec ^ { -2 } , a depth that approaches the sensitivity of star count studies in the Local Group .
We convert surface brightness to surface mass density using the radial g - r color profile .
The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R = 70 kpc , corresponding to 18 disk scale lengths .
Fitting a bulge + disk + halo model we find that the best-fitting halo mass M _ { halo } = 1.7 _ { -1.7 } ^ { +3.4 } \times 10 ^ { 8 } M _ { \odot } .
The total stellar mass of M101 is M _ { tot,* } = 5.3 _ { -1.3 } ^ { +1.7 } \times 10 ^ { 10 } M _ { \odot } , and we infer that the halo mass fraction f _ { halo } = M _ { halo } / M _ { tot,* } = 0.003 ^ { +0.006 } _ { -0.003 } .
This mass fraction is lower than that of the Milky Way ( f _ { halo } \sim 0.02 ) and M31 ( f _ { halo } \sim 0.04 ) .
All three galaxies fall below the f _ { halo } – M _ { tot,* } relation predicted by recent cosmological simulations that trace the light of disrupted satellites , with M101 ’ s halo mass a factor of \sim 10 below the median expectation .
However , the predicted scatter in this relation is large , and more galaxies are needed to better quantify this possible tension with galaxy formation models .
Dragonfly is well suited for this project : as integrated-light surface brightness is independent of distance , large numbers of galaxies can be studied in a uniform way .