Faint galaxies scattered across the Perseus cluster are like ghosts among their bright neighbors. They are so diffuse that distant objects shine through them, and they contain hundreds of times fewer stars than our Milky Way. Astronomers pointed a spectrograph (an instrument that splits starlight into colors, like a prism splitting a rainbow) at six such objects and peered into their past.
The light revealed that the stars in these galaxies are not young: on average, they are about seven billion years old, more than half the age of the universe. They are poor in heavy elements — iron, magnesium, and others that are forged in stellar interiors and spread by explosions. However, they have more magnesium relative to iron than our Sun does. This chemical fingerprint suggests that the stars formed quickly and early, from gas that had not yet accumulated many metals.
For three galaxies, astronomers measured how the stellar composition changes from the edge to the center. Two have a flat profile, like typical dwarf galaxies, while one shows increasing metallicity toward the center. This unusual behavior might be linked to globular clusters: the galaxy's tidal forces can tear them apart, and the freed stars, rich in metals, settle closer to the center. The more globular clusters a galaxy has, the stronger the effect. It's like dissolving more bouillon cubes in soup — the flavor becomes richer toward the middle of the bowl.
The study found no link between the properties of the stars and how the galaxy fell into the cluster: external conditions seem to have little influence on the internal life of these faint objects. Now astronomers plan to study more such galaxies to understand how they survive in dense environments and why their stars are so old.
One of the studied galaxies, DF11 in the Coma cluster, has a chemical composition reminiscent of galaxies as they were two billion years after the Big Bang, even though it exists in the present-day universe.