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 .