2025ApJ...990L..40C

We present long-baseline observations of the Fomalhaut outer debris disk at 223 GHz (1.3 mm) from Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 5, which we use along with archival short-baseline observations to produce a 0$\mathop{.}\limits{^{\prime\prime} }$57 resolution mosaic of the disk at a sensitivity of 7 μJy bm^–1. We use radial profiles to measure the disk at the ansae and find that the southeast (SE) side of the disk is 4 au wider than the northwest (NW) side as observed by ALMA. We also find that the peak brightness of the NW ansa is 21% ± 1% brighter than the SE ansa. We perform Markov Chain Monte Carlo fits of the ALMA visibilities using two analytical, eccentric disk models. Our results suggest that the model including a dispersion parameter for the proper eccentricity (${\sigma }_{{e}_{p}}$), which accounts for additional scatter in the eccentricity of individual orbits, is preferred over the model without one. Such a model implies that self-gravitation, particle collisions, and close-packing could play a role in shaping the overall structure of the Fomalhaut disk, as is seen in eccentric planetary rings. Crucially, neither model can reproduce the brightness or width asymmetry near the NW ansa. No emission from the intermediate belt is detected, allowing us to place a 3σ upper limit of 396 μJy at 1.3 mm. We also discover a spectral line in archival Cycle 3 data centered at ν_obs ≈ 230.25 GHz at the location of the “Great Dust Cloud,” whose redshift from the expected CO line for Fomalhaut confirms the source is a background galaxy.