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.