The low-mass and structured stellar halo of M83 argues against a merger origin for its starburst and extended neutral hydrogen disk

A merger origin has been suggested for M83’s massive, metal-rich extended HI 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]$\sim -1.15$ RGB stars with a mass between 15 and 40 kpc of $\log_{10}M_{,15-40,maj}/M_{\odot}=8.02\pm0.10$. 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 $\log_{10}M_{stream}/M_{\odot}=7.93\pm0.10$, with metallicity [M/H]$= -1.0\pm0.2$. The stream progenitor was only recently accreted, as its stellar populations suggest that it formed stars until $2.1\pm1.3$ 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 $\log_{10}M_{,accreted}/M_{\odot}=8.78^{+0.22}{-0.28}$, with the most massive past merger having $\log{10}M_{*,dom}/M_{\odot}=8.5\pm0.3$. 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 Hi disk. We conclude that other non-merger mechanisms, such as secular evolution or accretion of gas from the IGM, are likely to be responsible for M83’s remarkable properties.

💡 Research Summary

This paper investigates whether the spectacular nuclear starburst and the massive, metal‑rich extended H I disk of the nearby spiral galaxy M83 (NGC 5236) can be explained by a past major merger, or whether alternative mechanisms are required. To address this, the authors combine wide‑field ground‑based imaging from Subaru/Hyper Suprime‑Cam with deep, high‑resolution Hubble Space Telescope (HST) data from the GHOSTS survey. The HST data provide a clean training set for a nearest‑neighbor star‑galaxy classification algorithm that is then applied to the Subaru mosaic, enabling the authors to map resolved stellar populations out to ∼120 kpc from the galaxy centre despite M83’s nearly face‑on orientation and the presence of a bright, star‑forming H I disk.

The analysis isolates old, metal‑poor red‑giant‑branch (RGB) stars belonging to a smooth stellar halo. Between 15 and 40 kpc the halo has a stellar mass of log M_* = 8.02 ± 0.10