The Low-mass and Structured Stellar Halo of M83 Argues Against a Merger Origin for Its Starburst and Extended Neutral Hydrogen Disk*

Abstract A merger origin has been suggested for M83’s massive, metal-rich extended H i disk and nuclear starburst. We observe M83’s stellar halo to test this idea. We train nearest-neighbor star–galaxy separation on wide-area Subaru imaging with Hubble Space Telescope data to map M83’s halo in resolved stars. We find that M83 has an extended, very low-density smooth stellar halo of old and metal-poor [M/H] ∼ −1.15 RGB stars with a mass between 15 and 40 kpc of log 10 M ∗ , 15 − 40 , maj / M ⊙ = 8.02 ± 0.10 . In addition to M83’s well-known Northern Stream, our ground-based Subaru imaging reveals a new stream to M83’s south, which modeling suggests could be its trailing arm. The combined stream masses are log 10 M stream / M ⊙ = 7.93 ± 0.10 , with metallicity [M/H] = −1.0 ± 0.2. The stream progenitor was only recently accreted, as its stellar populations suggest that it formed stars until 2.1 ± 1.3 Gyr ago. M83 lies on the stellar halo mass–metallicity correlation seen for other Milky Way–mass galaxies, albeit with low stellar halo mass. We infer a total accreted mass of log 10 M * , accreted / M ⊙ = 8.7 8 − 0.28 + 0.22 , with the most massive past merger having log 10 M * , dom / M ⊙ = 8.5 ± 0.3 . We identify plausible M83 analogs in TNG-50 with similar stellar halos, finding that while a recent accretion can create a prominent stellar stream, such accretions do not trigger starburst activity, nor do they deliver enough gas to form M83’s extended H i disk. We conclude that other nonmerger mechanisms, such as secular evolution or accretion of gas from the intergalactic medium, are likely to be responsible for M83’s remarkable properties.