The Dragonfly Nearby Galaxies Survey. I. Substantial variation in the
  diffuse stellar halos around spiral galaxies
  Galaxies are thought to grow through accretion; as less massive galaxies are
disrupted and merge over time, their debris results in diffuse, clumpy stellar
halos enveloping the central galaxy. Here we present a study of the variation
in the stellar halos of galaxies, using data from the Dragonfly Nearby Galaxies
Survey (DNGS). The survey consists of wide field, deep ($\mu_{g} > 31$ mag
arcsec$^{-2}$) optical imaging of nearby galaxies using the Dragonfly Telephoto
Array. Our sample includes eight spiral galaxies with stellar masses similar to
that of the Milky Way, inclinations of $16-90$ degrees and distances between
$7-18$ Mpc. We construct stellar mass surface density profiles from the
observed $g$-band surface brightness in combination with the $g-r$ color as a
function of radius, and compute the halo fractions from the excess stellar mass
(relative to a disk$+$bulge fit) beyond $5$ half-mass radii. We find a mean
halo fraction of $0.009 \pm 0.005$ and a large RMS scatter of
$1.01^{+0.9}_{-0.26}$ dex. The peak-to-peak scatter is a factor of $>100$ --
while some galaxies feature strongly structured halos resembling that of M31,
three of the eight have halos that are completely undetected in our data. We
conclude that spiral galaxies as a class exhibit a rich variety in stellar halo
properties, implying that their assembly histories have been highly
non-uniform. We find no convincing evidence for an environmental or stellar
mass dependence of the halo fraction in the sample.
