An almost dark galaxy with the mass of the Small Magellanic Cloud

Vol. 680
4. Extragalactic astronomy

An almost dark galaxy with the mass of the Small Magellanic Cloud

by M. Montes, I. Trujillo, A. Karunakaran, R. Infante-Sainz, K. Spekkens, G. Golini, M. Beasley, M. Cebrián, N. Chamba, M. D'Onofrio, L. Kelvin, J. Román 2023, A&A, 680, A15 alt

Some low surface brightness galaxies are so weak that they elude detection in the Sloan Digital Sky Survey. These “almost dark galaxies” have a red surface brightness at the center of more than 26 mag/arcsec2, corresponding to a stellar surface density of less than a few solar masses per pc^2. This small amount of stars implies that no stellar feedback has modified its dark matter halo distribution. These objects thus offer an interesting possibility to constrain the microphysical properties of dark matter. The authors report on the serendipitous discovery with deep imaging of Nube, an almost dark galaxy located at a distance of 107 Mpc. From observations with the 100m Green Bank Telescope, they derive a dark halo mass of 2.6 10^{10} Msun. Ultra-deep multi-band observations with the 10.4m Gran Telescopio Canarias favor an age of ∼ 10 Gyr and a metallicity of [Fe/H]∼ −1.1. With a stellar mass of ∼ 4×10^8 Msun and a half-mass radius of Re = 6.9 kpc, 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. Current cosmological simulations within the cold dark matter scenario, including baryonic feedback, do not reproduce the structural properties of Nube. They are, however, compatible with a fuzzy dark matter scenario in which the dark matter particles are ultra-light axions with a mass of 0.8 10^{-23} eV, in agreement with other astrophysical constraints.