Reaching for the Edge I: Probing the Outskirts of Massive Galaxies with
  HSC, DECaLS, SDSS, and Dragonfly
  The outer light (stellar halos) of massive galaxies has recently emerged as a
possible low scatter tracer of dark matter halo mass. To test the robustness of
outer light measurements across different data sets, we compare the surface
brightness profiles of massive galaxies using four independent data sets: the
Hyper Suprime-Cam survey (HSC), the Dark Energy Camera Legacy Survey (DECaLS),
the Sloan Digital Sky Survey (SDSS), and the Dragonfly Wide Field Survey
(Dragonfly). We use customized pipelines for HSC and DECaLS to achieve better
sky background subtraction. For galaxies at $z<0.05$, Dragonfly has the best
control of systematics, reaching surface brightness levels of $\mu_r \sim 30$
mag/arcsec$^{2}$. At $0.19<z<0.50$, HSC can reliably recover surface brightness
profiles to $\mu_{r} \sim 28.5$ mag/arcsec$^{2}$ reaching $R=100 - 150$ kpc.
DECaLS surface brightness profiles show good agreement with HSC but are noisier
at large radii. The median profiles of galaxy ensembles in both HSC and DECaLS
reach $R > 200$ kpc without significant bias. At $0.19<z<0.50$, DECaLS and HSC
measurements of the stellar mass contained within 100 kpc agree within 0.05
dex. Finally, we use weak gravitational lensing to show that measurements of
outer light with DECaLS at $0.19<z<0.50$ show a similar promise as HSC as a low
scatter proxy of halo mass. The tests and results from this paper represent an
important step forward for accurate measurements of the outer light of massive
galaxies and demonstrate that outer light measurements from DECam imaging will
be a promising method for finding galaxy clusters for DES and DESI.
