Galaxies that feed on other galaxies

An international team of astronomers led by Giuseppina Battaglia, researcher at the Instituto de Astrofísica de Canarias (IAC), finds signs that the outer halo of the Milky Way contains stellar remains of massive dwarf galaxies that were devoured by our own.

Most of the information we have about the Milky Way stellar halo comes from its inner region, which we can observe close to the solar neighbourhood. However, for the first time, the chemical properties of the external regions of the halo of our galaxy were explored with high-resolution spectroscopy in the optical of a sample of 28 red giant stars at large distances from the sun. Spectroscopic analysis consists of separating the light of the stars into its individual frequencies in order to obtain information on the star's chemical composition. The analysis of the chemical properties of the stars can provide information on the characteristics of the environment in which they were born.

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Credit: Giuseppina Battaglia

Distant Milky Way halo giants marked on a Pan-STARRS1 map. Location of our targets overlaid on a RGB rendering of the distribution of Milky Way halo stars.

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Peering deep into the vast stellar halo that envelops our Milky Way galaxy, astronomers using NASA's Hubble Space Telescope have uncovered tantalizing evidence for the possible existence of a shell of stars that are a relic of cannibalism by our Milky Way.

Hubble was used to precisely measure, for the first time ever, the sideways motions of a small sample of stars located far from the Milky Way galaxy's center. Their unusual lateral motion is circumstantial evidence that the stars may be the remnants of a shredded galaxy that was gravitationally ripped apart by the Milky Way billions of years ago.

These stars support the idea that the Milky Way grew, in part, through the accretion of smaller galaxies.