Ring or No Ring—Revisiting the Multiphase Nuclear Environment in M31

Abstract

Nuclear rings, prevalent in barred galaxies, are essential to understanding gas transport toward galactic nuclei. However, the peculiar nuclear ring in our neighboring galaxy M31 remains poorly understood. Here we present a comprehensive study of this multiphase gas structure, originally revealed by its dust emission, based on newly acquired CO mappings and archival spectroscopic imaging of atomic hydrogen and warm ionized gas, along with custom numerical simulations. These multiwavelength data offer an unprecedented view of the surface mass density and kinematics of the nuclear ring, challenging the notion of it being a single coherent structure. In particular, the ring shows significant asymmetry in its azimuthal mass distribution, with neutral gas concentrated in the northeast and ionized gas prominent in the southwest. The observed off-centered and lopsided morphology disfavors an interpretation as gas streamers or resonance rings driven solely by a barred potential known to exist in M31. Furthermore, the ring’s line-of-sight velocity distribution suggests circular motion in a plane inclined by ∼30° relative to M31’s outer disk, implying external torque likely from M32’s recent close-in passage. Our hydrodynamical simulations tracking the evolution of nuclear gas in M31 influenced by both a barred potential and an oblique collision with M32 reveal the natural formation of asymmetric spiral arms several hundred Myr after the collision, which could appear ringlike at appropriate viewing angles. Therefore, we suggest that M31’s nuclear gas structure, instead of being a persistent rotating ring, comprises recently formed, asymmetric spirals with a substantial tilt.