Tracing the anemic stellar halo of M101
  Models of galaxy formation in a cosmological context predict that massive
disk galaxies should have structured extended stellar halos. Recent studies in
integrated light, however, report a few galaxies, including the nearby disk
galaxy M101, that have no measurable stellar halos to the detection limit. We
aim to quantify the stellar content and structure of M101's outskirts by
resolving its stars. We present the photometry of its stars based on deep F606W
and F814W images taken with Hubble Space Telescope as part of the GHOSTS
survey. The constructed CMDs of stars reach down to two magnitudes below the
tip of the red giant branch. We derived radial number density profiles of the
bright red giant branch (RGB) stars. The mean color of the RGB stars at $R
\sim$ 40 -- 60 kpc is similar to those of metal-poor globular clusters in the
Milky Way. We also derived radial surface brightness profiles using the public
image data provided by the Dragonfly team. Both the radial number density and
surface brightness profiles were converted to radial mass density profiles and
combined. We find that the mass density profiles show a weak upturn at the very
outer region, where surface brightness is as faint as $\mu_g\approx 34$ mag
arcsec$^{-1}$. An exponential disk + power-law halo model on the mass density
profiles finds the total stellar halo mass of $M_{halo}=8.2_{-2.2}^{+3.5}\times
10^7M_\odot$. The total stellar halo mass does not exceed $M_{halo} = 3.2
\times 10^8$ $M_{\odot}$ when strongly truncated disk models are considered.
Combining the halo mass with the total stellar mass of M101, we obtain the
stellar halo mass fraction of $M_{halo}/M_{gal} = 0.20_{-0.08}^{+0.10}\%$ with
an upper limit of 0.78\%. We compare the halo properties of M101 with those of
six GHOSTS survey galaxies as well as the Milky Way and M31 and find that M101
has an anemic stellar halo.
