The Saga of M81: Global View of a Massive Stellar Halo in Formation

Abstract

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 around M81, revealing M81's stellar halo in never-before-seen detail. We resolve the halo to unprecedented V-band equivalent surface brightnesses of 33 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. Using the minor axis, we confirm M81's halo as one of the lowest mass and metal poorest known (M ⋆ ≃ 1.16 × 109 M ⊙, [Fe/H] ≃ −1.2)—indicating a relatively quiet prior accretion history. Yet, our global halo census finds that tidally unbound material from M82 and NGC 3077 provides a substantial infusion of metal-rich material (M ⋆ ≃ 5.4 × 108 M ⊙, [Fe/H] ≃−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 passive history, M81's merger with M82 will completely transform its halo from a low-mass, anemic halo rivaling the Milky Way, 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.

Relating the Diverse Merger Histories and Satellite Populations of Nearby Galaxies

“…Further, models predict that the measurable properties of these accreted stellar populations are often dominated by the most massive merger the central has experienced (e.g., Deason et al 2015b;D'Souza & Bell 2018a;Monachesi et al 2019). Recent detailed studies of the halos of M31 (D'Souza & Bell 2018b) and M81 (Smercina et al 2020) support this picture. Since the stellar mass present in the stellar halo is primarily accreted from the largest merger partner, the measured accreted mass is approximately equivalent to the stellar mass of the dominant progenitor galaxy.…”

“…Theoretical modeling has long predicted that MW-mass galaxies should have a wide range in their merger histories, and particularly in the largest mergers that they have experienced (e.g., Purcell et al 2010;Deason et al 2015a), with many galaxies experiencing mergers with progenitors more massive than the Large Magellanic Cloud (LMC; Stewart et al 2008). Study of their stellar halos has largely confirmed this predicted diversity by observing a wide range of stellar halo masses and metallicities around galaxies with masses comparable to the MW, with correlations between them showing that stars accreted from the largest past merger dominate the properties of the halo stellar populations (Harmsen et al 2017;Bell et al 2017;D'Souza & Bell 2018a;Smercina et al 2020). Meanwhile, recent evidence suggests that the 'classical' satellite populations of nearby MW-analogs are equally diverse (Geha et al 2017;Smercina et al 2018;Bennet et al 2019;Carlsten et al 2021).…”

The infall of dwarf satellite galaxies are influenced by their host's massive accretions

“…The last two galaxies are motivated by the fact that we some prior knowledge about their accretion history. M81 has a small (Harmsen et al 2017;Smercina et al 2020) and old stellar halo (Durrell et al 2010), but is currently undergoing a massive accretion of two large galaxies (M82 and NGC3077). Though the orbital properties of M82 and NGC 3077 are unclear, it is believed that these progenitors have recently been accreted (e.g.…”

Introducing the LBT Imaging of Galactic Halos and Tidal Structures (LIGHTS) survey. A preview of the low surface brightness Universe to be unveiled by LSST

“…The star counting technique (either with space-or ground-based telescopes) has been used to explore the stellar properties of haloes belonging to the closest galaxies to a high level of detail (see e.g. Rejkuba et al 2005;Harris et al 2007;Ibata et al 2009;McConnachie et al 2009;Durrell et al 2010;Tanaka et al 2011;Monachesi et al 2013;Ibata et al 2014;Okamoto et al 2015;Crnojević et al 2016;Harmsen et al 2017;Carlin et al 2019;Smercina et al 2020). However, this technique remains mostly limited to galaxies where individual stars can be resolved (i.e.…”