First results from the Dragonfly Telephoto Array: the apparent lack of a
  stellar halo in the massive spiral galaxy M101
  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 ~ 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 x 10^8 M_sun. The total
stellar mass of M101 is M_tot,* ~ 5.3 x 10^10 Msun, and we infer that the halo
mass fraction f_halo = M_halo / M_tot,* ~ 0.003. This mass fraction is lower
than that of the Milky Way (f_halo ~ 0.02) and M31 (f_halo ~ 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 ~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.
