Origins of the Evil Eye: M64's Stellar Halo Reveals the Recent Accretion of an SMC-mass Satellite

Abstract

M64, often called the "Evil Eye" galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a "flyby" interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented $\sim$30${}^{\circ }$ relative to the major axis and along the rotation axis of the outer gas disk. The shell is $\sim$45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to $>$100 kpc. We estimate a stellar mass and metallicity for the southern shell of $M_{\star }=1.80\pm 0.54\times 10^8{M}_{\odot }$ and [M/H] $=$ $-$1.0, respectively, and a similar mass of $1.42\pm 0.71\times 10^8{M}_{\odot }$ for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite of $M_{\star ,\mathrm{prog}}\simeq 5\times 10^8{M}_{\odot }$. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.

12 pages, 5 figures, accepted for publication in ApJ Letters

References in corpus (14)

• The NumPy array: a structure for efficient numerical computation
• The Circumgalactic Medium
• Updated Nearby Galaxy Catalog
• Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
• The PHANGS-HST Survey: Physics at High Angular resolution in Nearby GalaxieS with the Hubble Space Telescope
• Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies
• On feathers, bifurcations and shells: the dynamics of tidal streams across the mass scale
• Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure and internal dynamics of galaxies with a large-scale counterrotation
• The Saga of M81: Global View of a Massive Stellar Halo in Formation
• d1005+68: A New Faint Dwarf Galaxy in the M81 Group
• The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging
• Hot and counter-rotating star-forming disk galaxies in IllustrisTNG and their real-world counterparts
• A New Metal-poor Globular Cluster and Resolved Stars in the Outer Disk of the Black Eye Galaxy M64: Implication for the Origin of the Type III Disk Break
• Multiple gas acquisition events in galaxies with dual misaligned gas disks

Cited by in corpus (2)

• Stellar tidal streams around nearby spiral galaxies with deep imaging from amateur telescopes
• Simulations of the accreted stellar halos of low-mass field galaxies