Recent work has shown that Milky Way-mass galaxies display an incredible range of stellar halo properties .
Yet , the origin of this diversity is unclear .
The nearby galaxy M81 — currently interacting with M82 and NGC 3077 — sheds unique light on this problem .
We present a Subaru Hyper Suprime-Cam survey of the resolved stellar populations in M81 ’ s halo .
Using a unique three-filter strategy , and numerous HST calibration fields , we reveal M81 ’ s halo in never-before-seen detail .
We resolve the halo to unprecedented V -band equivalent surface brightnesses of > 34 mag arcsec ^ { -2 } , and produce the first-ever global stellar mass density map for a Milky Way-mass stellar halo outside of the Local Group ( LG ) .
Using the minor axis , we confirm the previous assessment of M81 as one of the lowest mass and metal-poorest stellar halos known ( M _ { \star } \sim 10 ^ { 9 } M _ { \odot } , [ Fe/H ] \sim { - } 1.2 ) — indicating a relatively quiet prior accretion history .
Yet , in our global stellar mass census we find that tidally unbound material from M82 and NGC 3077 provides a substantial infusion of metal-rich material ( M _ { \star } \simeq 6 { \times } 10 ^ { 8 } M _ { \odot } , [ Fe/H ] \simeq { - } 0.9 ) .
We further show that , following the accretion of its massive satellite M82 ( and the LMC-like NGC 3077 ) , M81 will host one of the most massive and metal-rich stellar halos in the nearby universe .
Thus , the saga of M81 : following a relatively passive history , M81 ’ s merger with M82 will completely transform its halo from a low-mass , anemic halo rivaling the MW , to a metal-rich behemoth rivaled only by systems such as M31 .
This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced .