Nube dark galaxy HiPERCAM urgiz images

Follow-up observations with the 100m Green Bank Telescope strongly suggest that the galaxy is at a distance of 107Mpc. Ultra-deep multi-band observations with the 10.4m Gran Telescopio Canarias favour an age of ~10Gyr and a metallicity of [Fe/H]~-1.1. With a stellar mass of ~4x10^8^M_{sun}) and a half-mass radius of Re=6.9kpc (corresponding to an effective surface density of <{SIGMA}>_e_~0.9 M_{sun}_/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 ultra-diffuse galaxies with similar stellar masses. Galaxies with comparable effective surface brightness within the Local Group have very low mass (tens of 10^5^M_{sun}_) and compact structures (effective radius Re<1kpc). 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 ultra-light axions with a mass of mB=(0.8^+0.4^_-0.2_)x10^-23^eV.