2024A&A...681A..15M

Almost dark galaxies are objects that have eluded detection by traditional surveys such as the Sloan Digital Sky Survey (SDSS). The low surface brightness of these galaxies (μ_r(0) > 26 mag arcsec^–2), and hence their low surface stellar mass density (a few solar masses per pc² or less), suggest that the energy density released by baryonic feedback mechanisms is inefficient in modifying the distribution of the dark matter halos they inhabit. For this reason, almost dark galaxies are particularly promising for probing the microphysical nature of dark matter. In this paper, we present the serendipitous discovery of Nube, an almost dark galaxy with ⟨μ_V⟩_e ∼ 26.7 mag arcsec^–2. The galaxy was identified using deep optical imaging from the IAC Stripe82 Legacy Project. Follow-up observations with the 100 m Green Bank Telescope strongly suggest that the galaxy is at a distance of 107 Mpc. Ultra-deep multi-band observations with the 10.4 m Gran Telescopio Canarias favour an age of ∼10 Gyr and a metallicity of [Fe/H] ∼ -1.1. With a stellar mass of ∼4 × 10⁸ M_⊙ and a half-mass radius of R_e = 6.9 kpc (corresponding to an effective surface density of ⟨Σ⟩_e ∼ 0.9 M_⊙ pc^–2), Nube is the most massive and extended object of its kind discovered so far. The galaxy is ten times fainter and has an effective radius three times larger than typical ultradiffuse galaxies with similar stellar masses. Galaxies with comparable effective surface brightness within the Local Group have very low mass (tens of 10⁵ M_⊙) and compact structures (effective radius R_e < 1 kpc). Current cosmological simulations within the cold dark matter scenario, including baryonic feedback, do not reproduce the structural properties of Nube. However, its highly extended and flattened structure is consistent with a scenario where the dark matter particles are ultralight axions with a mass of $ m_B = (0.8^{+0.4}_{-0.2}) \times 10^{-23} $ eV.