Ultra-diffuse galaxies, an extreme type of dwarf galaxy, have been the focus of extensive observational and theoretical studies over the past decade. With stellar masses comparable to dwarf galaxies (between 10⁷ and 10⁹ solar masses) but much larger in size (as defined by their effective radius), they exhibit an extremely low surface brightness. These galaxies display highly diverse properties: some have large dark matter halos, others lack them, and their number of globular clusters varies widely. Studies of their kinematics and stellar populations have shown that these extreme galaxies consist of at least two distinct classes, following different evolutionary paths.

The study of variations of the main properties within these galaxies (e.g. ages and metallicities) is crucial for understanding their formation. However, their low brightness has severely limited the amount of spectroscopic observations that can allow for such analysis, with data available for only one UDG, DF44. Surprisingly, its metallicity profile is completely opposite to what has been shown for classical dwarf galaxies (both in observations and simulations), and for simulated studies of ultra-diffuse galaxies. While the latter typically exhibit clearly decreasing profiles, DF44 presents a nearly flat or even slightly increasing profile.

In this letter, published in A&A, we confirm, using a larger sample of dwarf low surface brightness galaxies, that this result holds for all types of such galaxies (“everything”). This suggests that the entire galaxy shares similar properties (“everywhere”) and that its structure formed simultaneously (“all at once”). Altogether, this points to a completely different evolutionary path for low surface brightness dwarf galaxies compared to what was previously known for traditional dwarf galaxies.